964/993 Oil Cooler Fan Operation & Troubleshooting

Contributed by Randall Granaars

Stuff You’ll Find:

Component Location/Part Number/Cost
Basic System Operation
Troubleshooting Techniques
Normal Operating Temperatures
Component Removal/Replacement
Systems Known to be—or Previously—Operating Incorrectly
Known Successful Repairs
More Gory Details

AC Fan Resistor Replacement DIY

NOTE:  Whenever “??” appears, it means that the information is unknown, in question, or was not clear as provided.

Component Location/Part Number/Cost:

Relay:  Inside fuse box in (front) trunk.  Relay is the same as that used for the A/C blower motor and the rear heater blower.  Part #811 951 253.  Approximately $28 from a “discount” Porsche dealer.  Roy Eames reports the relay is “generic,” and can be purchased from Pelican Parts for $15.80.

Oil cooler and fan: In right front (passenger side) fender, behind mud guard.  Note that with a flashlight, you can see the cooler/fan by looking through the horizontal grill at the right-front corner of the car.  You will have to lie on the ground and look up to do this.  A new oil cooler fan (a.k.a. blower) costs approximately $100.

Oil cooler temperature sensor (a.k.a. thermosender, thermoresistor or thermistor):  Behind passenger-side mudguard—or behind the passenger side headlight on a 993—on the top outer-most part of the oil cooler unit.  Part #964 624 110 00.  Approximately $16.

2-speed or ballast resistor:  Behind the oil cooler, mounted to the sheet metal.  0.45 Ohm resistor.  PITA to replace.  Part #993 616 521 01.  Approximately $45.

Climate control unit: Mounted in dash. Part # varies by year & model  Around $100 on Ebay (depends on how lucky you get), around $250-$450 used parts dealer, $900 new.

Basic System Operation:

A reference photo of the oil temperature gauge, with the white lines given a corresponding temperature, can be found here (credit to Robin Sun and Viken):

At 87c/188F (per Bruce Anderson's hi-performance book) for a 964, 83c/181F for a 993 (per manual), the oil cooler thermostat opens and oil starts to flow to the cooler.

At 212For 230F (993), the thermosender—via the control unit—starts the oil cooler fan in slow speed.
At 239F (964), the oil cooler fan switches to fast speed.  The 993’s fast-speed set-point is unknown.

Troubleshooting Techniques:

To verify that the oil cooler thermostat is passing oil to the cooler, simply feel the right-front fender (or wing) after the engine is warmed up.  The fender should be noticeably warm to the touch.  You may also hear a "gurgling" sound coming from the right-front fender, as oil flows through the cooler. 

To verify your fan is operating with the engine at operating temperature (or warmer):

-Place your hand under the horizontal grill at the right-front corner of the car and feel for air. 
-As a reference, you can run your A/C, and check for airflow out of the horizontal grill on the left-front corner of the car.

Rough (faxed and subsequently scanned) wiring schematics:
(Click in the picture for a larger view)

Oil Cooler Schematic1
Oil Cooler Schematic 2
Oil Cooler Schematic 3 
Oil Cooler Schematic 4 

Before going further, you’ll want to check the 30A oil cooler blower fuse in the luggage compartment fuse box.
To verify that the fan will physically operate in slow or fast speed:

Caution:  Keep in mind the relay terminals are energized (12V). 
-Remove relay R04 (oil cooler blower).
-Jump terminals 3(30) and 7(87c) for slow speed.  Fan should run, even with the ignition off.  If the fan runs in slow speed, your ballast resistor should be okay. 
-Jump terminals 3(30) and 5(87) for fast speed.  Fan should run, even with the ignition off.

You can check the A/C condenser fan similarly, removing relay R14.
To force the fan to run in fast speed when the ignition is on:

--On a 993, remove the passenger side headlight, and unplug the cable shown here (credit to E.J.):

--On a 964, the mudguard must be removed, and the cable above the fan unit unplugged.  The connector looks just as it does for a 993.  After unplugging the cable, you'll probably need to secure the cable with a tie-wrap--lest it fall into the fan's operating space (could be messy!).

A photo of the 964 temperature sensor and oil cooler fan can be found here:

And here is the temperature sensor unplugged, and the cable tie-wrapped:

-After starting the engine, the fan may not start running in fast speed until the engine has been running for a minute or so.

If your A/C blower is operating, you can perform a "rough-check" of the oil cooler fan relay by swapping the two identical relays.  If your A/C blower no longer operates, it's safe to say your relay is bad.

Resistance readings for the thermosender.  With the exception of the room temperature reading, all readings are taken across G12 & G18.  G12 & G18 can be accessed from the back of the climate control unit. 

Climate Control Unit Removal

You’ll need four, skinny objects to unlock the CCU—unless you happen to have the special tool.  This tool can be purchased at Kragens, Pep Boys, or equivalent, about $5.

· If you don’t have the special tool, some custom tools used in the past include thin allen wrenches and “tweeker” screwdrivers from a tweeker screwdriver set. 

· Your tool of choice gets inserted in the holes in each corner of the CCU. 

· You’ll want to push the CCU out from behind the dash (lesson learned!).  You may damage the plastic face or pull out buttons if you pull from the front.

· Once you have unlocked the CCU, push the CCU out from behind the dash and through the turn-signal & cruise control stalks. 

· Once the CCU is pulled out, you may want to put a rag beneath the unit to keep it from scratching the dash as you work off the connectors.

· Removing the two connectors (“G” & “K”) is a PITA, but self-explanatory. 

· The connectors are labeled on the back of the CCU, as are the pins.

· When taking continuity readings, you may find that you have to press firmly against the “G” connector pins, lest you get a false open reading.

Or, the equivalent readings can be obtained at 14-pin connector T-34 in the luggage compartment, between pins #1 and #10.  T-34 is almost directly below one of the screws holding the long, rectangular shaped plastic cover in place, between the fuse box and firewall, on the passenger side.  In particular, the connector is visible with the rectangular cover in place, but you may not have enough space to obtain resistance readings with the cover in place.

If you take readings across connector T-34, you’ll want to take the readings from the upper (female) part of the connector.  Readings taken across the male portion—with the CCU still connected—will measure resistance of the CCU (not what you want).

Expected readings:

-13.6-19.6 K Ohm when at room temperature, sensor removed from system (will vary widely with room temp.).
-3.6-4.0 K Ohm at 60C (sensor installed, across G12 & G18).
-1.4-1.6 K Ohm at 85C (sensor installed, across G12 & G18).
-0.9-1.0 K Ohm at 100C (sensor installed, across G12 & G18).

Expected behavior for a normally operating system:

-Oil cooler thermostat opens at about the first mark past warm-up (at approximately 188F). 
-At approximately the 9:00 position (probably a little bit above the 9:00 position), the fan will start in slow-speed, corresponding to approximately 212F (964) or 230F (993).  This pic shows a typical start temperature:

-Very little information on when the fan switches to fast speed, but the manual specifies 239F (964 only, 993 unknown).
-This pic shows a typical temperature at which the fan (having cooled the oil) shuts itself off:


Normal Operating Temperatures:

Varies from car to car, but 122F - 194F when running on the freeway is normal.  As is reaching the 9:00 position or a bit higher when stuck in traffic on a warm day.  Reading at the 10:00 (248F??) position is not unusual when tracking a car. 

Owners that have just restored automatic oil cooler fan operation have reported that their engine does not runner hotter or much hotter than the 9:00 position, even when stuck in stop-and-go traffic. 

It is safe to assume that routinely running at the 10:00 or 248F position is abnormal, as is running above the 248F mark under any circumstances.

In summary, “by the book,” an engine should probably run no hotter than the 194F mark during freeway driving, and no higher than the slow-speed fan starting temperature when stuck in traffic.

NOTE:  R.G. & Sean S. have demonstrated that—when stuck in traffic—their cars went from running halfway between the 9:00 and the 248F position with the fan OOS, to not getting any warmer than the 194F mark with the fan locked into fast-speed. 

Component Removal/Replacement:

-Relay: Self-explanatory, located inside the luggage compartment fuse-box.
-Climate Control Unit Removal
-Oil Cooler Temperature Sensor Replacement
-Oil Cooler Fan Resistor Replacement(for a 993)
-Replacing AC condenser resistor 

And some informative Rennlist threads, with respect to resistor replacement:

Systems Known to be—or Previously—Operating Incorrectly: 

-Roy Eames had a problem with his fan's operation.  Basically, he thought his engine was running too warm at times (e.g., when stuck in traffic).  It's likely his fan wasn't running at all, and he was getting cooling purely from flow through the oil cooler.

-E.J.'s 993 has a fan that will not operate automatically.   He unplugs the oil temperature sensor (forcing fast-speed fan operation) when running on the track.  His fan runs both in slow and fast-speed when jumpered.

-Patrick's 993 has a fan that will not operate in slow speed.  The fan runs fine in fast speed, but not in slow-speed when jumpered. 

-R.G.'s '91 C2 fan will not operate automatically in any speed.  Both speeds operate when jumpered, and the fan runs in fast speed when the oil cooler temperature sensor is unplugged.  R.G.'s car runs no hotter than halfway between the 9:00 position and the 248F mark (no fan operation).  R.G. suspects his fan has been O.O.S. for a long time, and is getting cooling purely from flow through the cooler.  He knows he is getting flow through the cooler, because he can hear a gurgling sound, and the right-front fender gets very warm.

Another R.G. observation.  With the oil cooler fan locked into fast speed, the engine’s temperature barely goes above the 194F mark—runs much cooler than it did with no fan operation.  Sequence of operation with the oil cooler fan locked in fast speed:

a) Engine warms to 194F, thermostat opens.
b) Engine temperature drops quickly to between the 122F and 194F mark and remains there—even in stop and go traffic. 

-Sean S.’s 993 was running hot when first purchased, and the problem was corrected by replacing the thermostat.  About 4 months later, he noticed his engine running hot again—averaging between the 9:00 and 10:00 positions.  He found his oil cooler fan was not running at all.

Sean also observed that with the oil cooler fan locked in fast-speed (oil temperature sensor disconnected), engine temperature did not climb above 194F.

-Jai’s ‘95 993 had a slow-seed fan that would not operate.  He has verified that the 2-speed resistor is faulty through the relay jumper test. 

Interestingly, Jai’s A/C condenser fan 2-speed resistor was also faulty, and was replaced by the dealer.  The dealer verified the resistor was faulty using the relay jumper test.

-Even more interesting, Larry N. has a ’95 with the exact same problem as Jai.  His car will be going into the shop soon for resistor (x2) replacement.

-MikeF’s ’94 had an oil cooler fan that would not operate automatically, and an A/C condenser fan that would cycle on and off in fast-speed.  He verified his A/C condenser resistor as faulty by the relay jumper test and resistance readings.  His oil cooler fan seemed to check out okay, but still would not operate automatically.

-Multiple cases of failed resistors—too many to name.

Known Successful Repairs:

-After troubleshooting, E.J.’s problem is likely the $CCU$.  This has not been verified by use of a donor CCU.  In the interim, E.J. has installed a slow-speed jumper (see below), which runs his fan continuously in slow-speed.

-Roy Eames' fan works great now.  He replaced the temperature sensor and relay at the same time, so it's not possible to tell which was at fault.  However, based on the resistance readings Roy took on his old temperature sensor, he suspects the relay was at fault.  Roy’s fan starts in slow-speed at » the 9:00 position.

-R.G. traced his problem to a defective CCU.  The CCU being defective was verified by temporarily installing a substitute CCU (which started the fan automatically).  Considering the high-cost of a replacement CCU—and the advantages of cooler oil temperatures—R.G. designed a jumper which runs the fan continuously in slow-speed.  This keeps the oil temperature <194F:

Update: Junly, 2002

After running with the slow-speed jumper installed for a few months, R.G replaced his $CCU$, restoring automatic operation of the oil cooler fan. 

-Sean S. learned his relay was defective by swapping the A/C and oil cooler blower relays.  After swapping, the oil cooler fan worked, the A/C condenser blower did not.  Even though Sean has found the problem, he has elected to run with the temperature sensor disconnected (i.e., continuous fast-speed operation).

-After replacing the resistor, Jai’s fan now works automatically in slow-speed.  The fan starts at just below the 9:00 position, and his engine doesn’t run any hotter than the 9:00 position—even when stuck in traffic.

-Larry N. had both the A/C condenser and oil cooler fan resistors replaced.  His car now runs much cooler.  He took resistance readings on the old resistors, and found one reading about 700 ohms, the other around 7k ohms.

-MikeF. replaced the A/C condenser fan resistor himself, finding it in horrible shape—nearly 1/3 crumbled away.  Resistor replacement restored normal operation of the A/C condenser fan—i.e., operating with the A/C with the ignition on.

Replacing the temperature sensor restored automatic oil cooler fan operation.  Mike’s fan now starts just above the 9:00 position.

More Gory Details:

The slow-speed fan jumper test already verifies the resistor is intact, but an additional check is to measure resistance across the resistor:

· Measure resistance across terminals #5 & #7 of the relay plug.  Values obtained to date are 0.8 ohms, 0.7 ohms and 0.6 ohms.  This applies to both the oil cooler and A/C condenser fans.  Of course, an open circuit indicates a failed resistor.

Yet another verification that the resistor is intact:

· Remove the relay, measure voltage between terminals #2 & #5.  Should read around 12V.
· Jumper the fan in slow-speed (across terminals #3 & #7).
· Measure voltage across terminals #2 & #5 again.  The reading should have dropped to around 4.2V …. the voltage drop across the resistor. 

More continuity tests, to verify your wiring is 100% okay:

· From CCU G-12 to temp. sensor terminal #2 = continuity.
· From CCU G-18 to temp. sensor terminal #1, continuity.
· Temp sensor installed, between G-12 & G-18 = some value depending on hot warm the temperature sensor is. 
· Between K/22 & oil cooler relay terminal #1 (fast-speed control), continuity.
· Between K/10 & oil cooler relay terminal #6 (slow-speed control), continuity. 
· Between G/9 (voltage feedback) and oil cooler relay terminal #5, continuity (you may not have the voltage feedback on a 993).
· Between K/23 & A/C condenser terminal #6 (slow-speed control), continuity.
· Between G-7 (voltage feedback) & A/C condenser terminal #5, continuity (you may not have the voltage feedback on a 993).

How the relay appears to work:

· Terminal 1 = CCU control for oil cooler blower fast-speed-at 12V when fan is off, »0.86V when fan is on. 
· Terminal 1 = 3-Level pressure switch control for A/C condenser fast-speed-at 12V when fan is off, »0.86V when fan is on??
· Terminal 2 = Always "Hot" (12V) supply for slow-speed fan.  Shorted to terminal 3.
· Terminal 3 = Always "Hot" (12V) supply for fast-speed fan.
· Terminal 4 = Not used.
· Terminal 5 = 12V, relayed power for fast-speed fan—not powered when fan is off
· Terminal 6 = CCU control for slow-speed fan—at 12V when fan is off, »0.86V when fan is on
· Terminal 7 = 12V, relayed power for slow-speed fan—not powered when fan is off
· Terminal 8 = Only "Hot" with accessory position, relay control power??
· There is a 75 ohm resistor between 86 (terminal #8) and 85 (terminal #1) of the relay itself.
· There is also a 75 ohm resistor between 86 and 85c (terminal #6)

To operate slow-speed fan:

· CCU shorts terminal 6 through a diode??, which gives the »0.86V.  (The 0.86V is verified.  The existence of a diode within the CCU is not.) 
· Relay no longer sees 12V at terminal #6, closes contact for slow-speed fan.
· Terminal #7 now powered through terminal #2 (12V), fan starts in slow-speed.

 To operate fast-speed fan:

· CCU (or 3-Level pressure switch for the A/C condenser fan) shorts terminal #1 through a diode (which gives the »0.86V).
· Relay no longer sees 12V at terminal #1, closes contact for fast-speed fan.
· Terminal #5 now powered through terminal #3 (12V), fan starts in fast-speed.

The current through terminal #8—when the fan is running—is approximately 12V/75 ohms = » 0.15A. 

Oil cooler temperature sensor replacement: 

· On a 964, you’ll certainly have to remove the passenger-side mudguard.  You’ll probably have to do this for a 993, as well.

· You’ll probably want to perform the replacement when the engine is cool (the thermostat closed).

· Your new sensor should come with a fresh metal gasket.

· Your Porsche toolkit includes the correct sized wrench to fit the sensor.

· Expect a small amount of oil after the sensor is removed.  Nothing a few paper towels cannot handle.

· The sensor simply threads out and in.

· After replacing, check for oil leaks after the thermostat has opened.


Oil Cooler Fan Resistor Replacement

SUBJECT: Oil Cooler
Date:Thu, 11 Oct 2001 13:18:11 -0700
From: "Choi, Jai (JHCH)" <JHCH@chevrontexaco.com>
To:  "'Randall Granaas'" <rgranaas@flash.net>

Hi Randall, how have you been ? .....thought that I gave you an update on my oil cooler fan resistor situation.....well, let's see...where do I start ..

I spent last 2 days (at least a couple of hours after work) try to replace the darn thing on my own......finally gave up and took it to my Porsche mechanic at the dealership this morning. He did exactly what I did as I watched him (no other dealerships that I know would allow their customers watch mechanics work on cars except this one....don't ask me why....no complaints from me !!) working on it, basically loosen up all the bolts that the cooler's attached to the body of the car. The cooler still wouldn't move and could see from his face expression that he was running out of patience......well to make the long story short, I learned a trick today that may help you guys a lot if you plan to do this on your own.....it basically boils down to one bolt (located at not so obviously place that is at least to me) that has be removed. 

YOU HAVE TO REMOVE THE HEADLIGHT ASSEMBLY FIRST. UNDERNEATH IT, YOU WILL SEE A BOLT THAT HOLDS THE SHEET METAL (which is attached to the cooler) IN PLACE......once he removed it,  he was able to slide out the cooler w/ no effort. From that point on, all you need is a 4 mm allan wrench to take out the old resistor and put the new one back in place.....maybe 2 minutes job.

Now the fan comes on at the low speed. It started coming on when the temp needle was approaching 9 o'clock position. As I were driving the car to home even in heavy traffic, it never exceeded 9 o'clock position. Although I am $150 out of pocket, I am happy to know that my car has no mechanical flaws now.  I am also happy that I can share this w/ the fellow rennlisters. 

Let's keep in touch. 

> -----Original Message-----

> From: Randall Granaas [SMTP:rgranaas@flash.net]
> Sent: Monday, October 08, 2001 10:52 AM
> To:   Choi, Jai (JHCH)
> Subject:      Re: oil cooler

> Hi Jai,

> This sounds encouraging.  I have forwarded this information onto E.J. and Patrick (both own '95 993s).

> Look forward to hearing how your resistor replacement goes.  If it was even close to a reasonable driving distance, I would be more than happy to come over and give you a hand.  I'm in Dana Point, just south of Laguna Beach.

> Good luck with the resistor replacement!

> Randall

> "Choi, Jai (JHCH)" wrote:

> > Hi randall, just talked to Pete at Andial. He mentioned that no oil cooler lines have to come out. He said all I have to do is to loosen up the 10 mm bolts (2) holding the cooler in place on a mounting bracket....the rest is self explanatory (he said).

> I think I will give it a shot this evening again.

> > P.S. yes, I live in Southern California (Bakersfield)....how about you ?

AC Ballast Resistor Replacement
Date: Thu, 27 Dec 2001 19:37:24 -0500

From: "Mike Feinstein" <mikncath@bellsouth.net>

To: "Randall Granaas" <rgranaas@flash.net>


Thanks again for all your help with diagnosis, and encouragement with the repair.  As you know, my oil cooler sending unit was the culprit and a new one solved that problem.  Also, thanks to your directions for diagnosing fan problems, I was able to pinpoint the A/C ballast resistor as the problem in the other front fender.  Besides having no low speed fan when using a jumper as you describe, when the AC system was turned on, the high speed fan would cycle on and off at 15 to 20 second increments...also a tell-tale sign that the ballast resistor is gone.  Pulling the fender liner of my 94 964 confirmed this diagnosis.  The old ballast resistor was literally missing a third of the ceramic material that holds it together.  It is suspicious that the batter overflow tube is located directly above the resistor.  This explains alot.  I'm amazed that Porsche would design it this way.

First thing I did was add a length of plastic tubing and attach it to the "spigot" that protudes through the inner fender wall to drain any battery overflow to the ground.  Be sure to tie wrap the tube to ensure no interference with fan operation.

Under normal circumstances, the resistor is easy to reach and remove...there is an allen screw that attaches the resistor to the top of the condensor shroud and it threads into a nut that is welded in place on the inside (unreachable) of the shroud.  The allen screw uses a 4mm allen wrench.  Unfortunately, the allen screw on mine was rusted solid and the head broke off when I attempted to unscrew it, leaving a permanent stud mounted to the top of my condensor shroud.  I suspect that a bit of battery acid overflow caused this corrosion and ate my resistor at the same time.  Suspisciously though, there is no other evidence of battery acid leakage...strange.

I started the car and turned on the AC with the new resistor plugged in, but unmounted....the AC fan came on low speed and stayed that way.......a beautiful thing.  I agonized for some time about a suitable location to mount the new resistor.  Since the resistor wire is fairly short, I decided to mount it to the inside fender wall, just above its original location (see pics).  To do this, I had to drill a small hole in the fender wall to accomodate a single 6mmx30mm stainless hex head bolt.  I placed a lock nut in the hex opening in the resistor (it was clearly made for this) fastened it in  place with only  the screw head and a washer showing under the front carpeting against the fender liner.  Be careful not to over tighten the resistor because it appears to be made of a ceramic type of material and seems like it could break with too much torque on the screw.  I snugged it up until I could no longer rotate the resistor by hand.

Although I removed the mudguard and the lower plastic fan cover, only the mudguard was necessary for this repair.  The finished product looks clean and professional.  I suspect that the only reason the resistor comes mounted on the condensor shroud from the factory is that it made it easier to install the fan/condensor as one complete unit with a minimum of external connections.  The new resistor is well away from the newly extended battery overflow and if the resistor ever dies again, replacing it will take 10 minutes, tops.

Thanks again for all your help Randall.  You saved me a bundle and I had fun doing the repair.  Just wish I had access to a hammer to clear the old fault codes.



Original Relay Position
Battery Drain with Hose Added
Battery drain-tube outlet
New Resistor Mounted in New Location
Old and New Resistors

The above information were obtained from Randall's web site, click here to visit his web site

Below DIY is contributed by Dan 96C2 St.Louis 
(Posted on Rennlist Discussion board 7/22/02)

AC Fan Resistor Replacement 

By following the diagnostic steps from p-car.com, I determined that both my AC condenser & oil cooler fan series resistors were defective.
I planned to replace both today, but due to unexpected complications & 100F temps only did the AC side. The job is technically easy (maybe a 3 on a 10 pt scale), but the location of the resistor makes it a frustrating, cursing, sweaty, pita project.

It is not necessary to remove the bumper to get to the AC resistor & unless the Turbo nose & front end internals are far different than my Carrera, it wouldn't help anyway. It is only necessary to remove the front most fender liner.

Tools needed:
- #2 phillips screwdriver.
- 4mm hex wrench (a 1/4 inch drive fine tooth ratchet wrench w/4mm shortened shank hex socket makes this project much easier - see below).
- very small hands with very long fingers.
- infinite patience.

The steps are:
1. Jack up the front end, secure w/jack stand & remove the wheel.
2. Remove the front section of the fender liner - rather intuitive after removing 7 or so screws.
3. Disconnect the resistor plug electrical connection (just pulls apart) after pulling the connector from the plastic clips on the inside fender wall.
4. The resistor is located at the top of the interior side of the condenser/cooler. It is secured with one (1) 4mm hex head (internal - "allen") screw through the center of the resistor.
5. Now comes the pita part:
A. the screw is not visible & must be removed & re-installed by feel - it's difficult to get the hex wrench mated to the screw,
B. the space is so limited that my avg size hand could barely fit, precluding turning the screw by hand,
C. the arc for turning a fixed hex wrench or a ratchet w/hex socket is, perhaps, 20-30 degrees, so it takes many partial turns (wish I had a fine tooth ratchet), and
D. the clearance above the screw head is insufficient to fit the tools I had. I ended up using a 1/4" ratchet wrench w/4mm socket & a very short piece of 4mm hex wrench cut to size w/a Dremel (or use a hack saw) & temporarily glued into the socket w/just 1/8" exposed. 
6. Removal takes time, but once I made the tool, wasn't too bad.
7. Re-installation of the new resistor is the reverse of the above, but since the screw hole is not visible & the screw has a fine thread, it took me many, many, many tries during which I had nothing good to say about our marque. The only good thing is there's no place for a dropped socket to go.

Perhaps someone has an easier way I overlooked to get to the AC resistor. It looked to me that the only alternative is to drop the AC fan/condenser which seemed at least as much trouble.



E-mail Randall

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