Cooling Fan Clutch Rehab
The 928 (up thru MY86) uses a thermostatically-controlled silicone fluid cooling fan clutch. By failing to move enough air through the radiator, this clutch may well be responsible for many of the high-temperature problems that we experience. Since a replacement clutch lists for about $400 and sells for about $300, not many of us will replace it on suspicion alone, and there isn't a test that tell us whether it is slipping due to a loss of silicone fluid.
There is, however, a repair procedure for the clutch. This procedure is not Porsche-approved, and will not guarantee original performance of the clutch, but it will greatly improve the performance of a faulty clutch. I would expect this to take you one or two hours to complete. Try the procedure at your own risk. I don't know of any way that this could damage your car, but if you find a way to do so, don't blame me! YMMV!
1. Toyota uses a similar fan clutch, but they also sell silicone fluid to refill their clutches. Go to your Toyota dealer, and buy two bottles of this silicone oil, at a cost of $6 or $7 for a very small bottle. You will also eed a small quantity of silicone sealer (RTV), and a heat source. The best is a flameless hot air gun, but you might be able to use a propane torch if you are VERY careful.
2. Remove your cooling fan, complete with fan, clutch, bracket and pulley. No disassembly of the fan unit is required except as detailed in Steps 4 and 5.
3. Carefully clean the fan unit, especially the front face of the clutch, which faces the radiator. Clamp the fan bracket to a work surface with the fan face-up in a horizontal position, leaving the fan free to rotate.
4. There should be a 1" x 3" piece of sheet metal across the center of the clutch. This is a bimetallic element. DON'T BEND IT! The strip is retained by slots cut into the ribs on the fan, and is kept from rotating out of the slots by small dabs of silicone sealer (RTV). Remove the RTV, and CAREFULLY slide the strip slightly to free it from the ribs. Remember which side is up.
5. Under the strip should be a silicone rubber grommet, with a small metal rod in the center of the grommet. Remove the rod, being careful to not lose it. Remove the grommet, being careful not to tear it. Cut the tip off the bottle of silicone oil at the very end.
6. Heat the fan clutch while rotating the fan and clutch at a moderate speed. (Be careful not to overheat and damage the fan.) This expands the air in the clutch, and moves the oil in the clutch to the outer edges.
7. Remove the heat source. Continue to rotate the fan and clutch, and drip the oil into the grommet hole. The combination of cooling and rotation will pull the oil in and distribute it to the outer edges. When oil stops going into the hole, repeat the cycle of heating and cooling. I suggest using two bottles, since there is no way of knowing how much oil is really needed.
8. Replace the grommet, and insert the metal rod. Carefully replace the metal strip, being certain to not bend it, and to get the right side up. Apply small dabs of RTV to keep it from moving.
9. Reinstall the fan unit. Let me know whether or not the procedure helped your overheating problems.
Toyota dealer says there's three types of viscous fan clutch silicone. They are rated at 3000 CST, 4000 CST, and 10,000 CST. All priced the same. Parts jock didn't know what the difference was, or even what CSTmeans. Any guess as to which to use?
What I used was the 3000 CST - Toyota Part number 08816-03001. I did not know there was a choice. This is what the counter man ordered up. Get two bottles - mine took over one and a half.
I would go with the 10,000 CST, as it would be the thickest and would transfer more power to the fan.
I'll probably regret this but I went with the 3,000 Cst.
The only reference to this stuff that the Toyota parts jock could find, was a TSB from 1975 specifying the fill procedure for the clutches of that year, which evidently arrived empty.
The 3,000 Cst. was for all Toyotas except for the Celica, which got 6,000 Cst. The 10,000 Cst. was not assigned an application by the TSB. He guessed maybe the Land Cruiser but didn't really know.
Anyway, the part numbers are 08816-03001, 08816-06001, and 08816-10001, respectively.
A small update for Wally's procedure, in the interest of speed, and reduced pain.
I thought I would try pushing the stuff in with a small hypodermic, which we had from one of the pets getting something. The idea was to avoid the slow dropping process, and waiting for air bubbles to emerge. The silicon however is so thick it is even slower than drops trying to push it through the needle. So then I took the needle off - the end of the syringe is about 3mm dia, so I just pushed it in the little centre hole, and pushed the plunger -imagine my surprise when I met almost no resistance, and the whole 3ml went straight in! On removing the syringe from the hole some silicon bubbled back out, so I had to suck it back up. Next time, I kept the syringe end in the hole for a minute or so, kept topping it up in place, pushing more in (until I ran out!). If you wait a bit after pushing the last in, watch for any pushing back out, you will have minimal losses pushing back out. Took about 2 minutes to push 18ml in - back to Toyota for more on Saturday, to make sure it is FULL!
83 Euro S A/T White/white 41K Mile
I found that Dow corning sell silicone oil, grade DC 200/10,000 fluid is fine, it's about 22 UK pounds. I bought it from Merck, a UK based laboratory supplies company who my company has an account with. They are the UK Dow silicones agent. I tried the Toyota parts network, but they did not recognise the listed parts numbers. The following link may be helpful for sourcing equivalents in the US:
I tried the various ideas for refilling the viscous coupling. Having removed the bimetallic strip and the push rod, the syringe method worked well, but the oil ran out rather easily again, so I plan to make a slightly oversized pushrod as the rubber bush is clearly a nonstandard item and not that easy to source a replacement. I was at least pleased to note that the input shaft oil seal was not leaking, though that looks like a standard industrial part. Silicone oils are notorious for leaking and getting everywhere, so sealing is clearly going to be a pain. The reason for using a silicone oil is that they do not change viscosity anything like as much as regular hydrocarbon oils. For my first go, I refilled with the 50 cSt ( centistokes) but that is too thin, and the drag too weak to move much air.
Hope this helps someone.
928 S2 1984
I've just finished reading with great interest a procdeure for rebuilding a 928 fan clutch on your 928 website. There were several questions thate I may be of assistance in answering.
The biggest problem with the procedure described is getting the correct amount of fluid back into the clutch. The best way to determine this is to add about 5 cc of fluid with a syringe as described, and then test to see if the clutch functions. If it does not, add another 5 cc & repeat.
The basic test for function is to shut the engine down at idle & observe the fan. It should stop within 3- 5 revolutions of the engine stopping. This process is labor intensive, but it is the only way to get the correct amount of fluid back into the clutch.
The caveat here is that if the clutch was low on fluid, the ceals are probably bad & the fluid will leak back out in a short time.
We specialize in the rebuild fan clutches of this type as well as other old units. The rebuild consists of a complete mechanical rebuild using new high quality seals.
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