(Home) Viscous coupling

i have had my 1990 4wd townace 2ltr turbo diesel now for nearly two weeks, and i know that this is a faq from lots of new owners but i would like to be sure that i don’t have a big cooling problem developing i have trawled through the site and i'm still none the wiser.

i think my viscous coupling has long since expired and is now more of a friction coupling. i say this because when i start off in the morning with a cold engine, i can here the rush of air produced by the main engine fan, however this only lasts whilst its in first gear. as soon as second engages, the noise stops.

on my way home this evening, i ran it with the seat up and checked the fan speed as best as i could for the 9 mile journey. as i suspected, initially the fan was spinning very quickly whilst in first, then slowed to almost free wheeling for the rest of the way.
as far as the temp gauge is concerned, it is rock steady at dead centre of the gauge. not just to the left as many other people report.

i have looked around the radiator area and there are signs of previous overheating but nothing recent, as most of the hoses and other bits are quite clean. i have also noticed that on the front radiator grill either side of the number plate, there is what looks like signs of vented, cooked cooling fluid.

i have also noticed that the power such as it is, is a lot less with a cold engine than it is after a few minutes of running.

any ideas?



- (#5742) dave Bright, 8 Oct 02

Normal for the engine to be lethargic when cold. Unfortunate side effect of the post-heat cycle of the plugs.

Sounds like your viscous coupling is duff, though. Get it sorted asap! Or it'll cost a lot more.

No ideas on your bumper stains tho.

- (#5743) david miller, 8 Oct 02

Dave. I agree with David that you should check the viscous fan thoroughly. However, when the engine is running at normal, or slightly cooler temperatures, the fan SHOULD freewheel to some degree. Unless it has failed completely, it will only start to "lock up" (and, it never totally locks up) when the radiator temperature is high enough to operate the built in thermo valve which changes it's function from a freewheel fan to a driven fan. Was your journey home in the cool of the evening ? The "seat up" check, excellent though it is to prove that the fan is turning, does help the air flow through the radiator and is not an entirely reliable guide to the fan operation. There is not much "ram" cooling on the Town/Masterace but, a low ambient temperature, and good air flow through the radiator, will help to keep the fan from driving at engine speed so, don't yet write off the viscous coupling entirely. Your initial check " the fan was spinning quickly in first", is quite normal but, with a cool engine, the fan should not be "positively" moving much air. Try the "thread" test as a next step. You will see a reference to this test in the Ace Answers archives.
"The rush of air, on start up" ? Are you sure that this is the fan and not some other noise. The turbo can be heard, even over the noise of a diesel and it could mislead you re the fan operation.

- (#5744) John Davis, 8 Oct 02

Thanks David, bizarrely its comforting to know that cold engine lethargy is quite normal.
I mentioned the stains on the bumper because I think the heater matrix is in this location? There does seem to be some sort of overflow, or vent hose hanging down behind the bumper. I shall have to clean the stains off and keep an eye on it
- (#5745) dave Bright, 8 Oct 02

Prior to aquiring my Townie I had several Volvos which also us a viscous coupling (these are bomb-proof compared to Toyota's) and EVERY one the couplings where engaged for the first 30-60 seconds. I spoke to a friend of mine early doors about this (he was a Volvo technician, mechanic to you and me) and he informed me it was perfectly normal.

- (#5746) Dave B, 8 Oct 02

Thanks John,
Its not so much a rush of air on start up, its an increasing whoosh as the revs increase once mobile, then once second gear engages The whoosh is gone for good. Although my knowledge of turbo’s is limited, its not the usual whine or whistle associated with turbo chargers. Hence my theory that if the fan is binding when cold, i.e. first thing in the morning, it will cause the fan to rev up with the engine, then when the engine revs die on engaging second gear, the momentum of the fan is sufficient to overcome the binding.

It did seem to be freewheeling most of the time, and yes it was in the cool of the evening.
Its interesting to hear that they rarely lock up, and mostly do so when hot. I will check the fan in the morning to see if I can feel any binding before starting the engine, and I will try the thread test as well.
- (#5747) dave Bright, 8 Oct 02

The vent hose hanging down is out of the bottom of the heater box, it is the drain for the air con evaporator, this is needed because as the evaporator cools the air down, it condenses moisture out of the atmosphere (like the water that condenses on the mirror when you are in the shower)this needs to go somewhere, so it exits via that pipe onto the ground, run the A/C on a hot humid day & see how bigger puddle of water collects on the ground! Ours also had the stain on the front bumper, I think it is where water gets rusty on the inside edge of the bumper, then drains forward when you stop, thus leaving the mark.HTH Rob.

- (#5748) Rob Drinkwater, 8 Oct 02

OK I am really impressed, I started off this evening with a few issues that were troubling me. Now it seems that it all appears to be normal.
Was I worried? yes. Am I worried now? no. Will I keep checking It? yes, just to be sure.
Tonight I shall sleep easy for this weekend I shall head for the hills.
Thank heavens I didn’t buy an ordinary car. You wouldn’t find a site like this for a Ford escort.
Great site! keep it up.
Thanks everyone,
Dave Bright

- (#5749) dave Bright, 8 Oct 02

Dave.
"Its interesting to hear that they rarely lock up, and mostly do so when hot"
Yes, locking up or "drag" when the engine/radiator is hot, is exactly the function of a viscous fan.
I hope that this does not sound as if I am an expert on these matters (which I am not) but, I have had some experience with viscous fans and their operation. Just in case you do not know how it operates (and it is quite an ingenious device)here's a brief explanation.
There are two chambers in the coupling body. A viscous silicone fluid is contained in the "slipping" chamber when the engine/radiator/ air flow is cool. Relative to engine speed, the fan speed is much less. As the radiator temperature rises, the air, passing through the rad and through the fan, also increases in temperature. This rise in air temperature allows the viscous fluid, via a thermostatically controlled valve in the coupling body, to pass from the slipping chamber into the "driving" chamber where it's "drag, on a system of internal vanes or baffles, turns the fan at a faster speed, ie, with little or no slippage. The coupling is "tuned" to the engine temperature requirements and, theoretically, should provide more cooling air through the radiator in hot conditions, and little or no air when the engine/rad etc is cold. This is a simple explanation and some v/fans have more sophisticated arrangements but, basically, this is how your fan operates.
A good test is to run the vehicle for a few miles and get the engine & radiator quite hot. Stop the car, switch off, raise the passenger seat and check that there is a fair amount of "drag" when you try to turn the fan manually. If the fan is very free, with little or no resistance, then the coupling is likely to need replacing.

- (#5750) John Davis, 9 Oct 02

John

Thanks for your reply on this, I am getting a much better understanding of how this VC works.

I performed the thread test yesterday morning and it broke on start up. Then I selected first gear only and drove off, keeping the revs at a steady level. After a short distance the viscous fan disengaged.

The temp gauge is fairly quick to rise to dead centre, around 2 to 3 miles, then it sits steady for the remaining 5 miles. During this time I cannot hear the same whoosh so I am attempting to devise a sensor that will measure the speed of the main engine fan. If it works it will indicate when the main fan is freewheeling or when it is engaged. Ill let you know if it’s successful.

dave Bright

- (#5751) dave Bright, 10 Oct 02

had my townie 12 months now. always makes the whoosh when starting from cold, until change up to 2nd gear. sounds a bit like a bus pulling away.
no heating probs even on long motorway run in summer, (4 hours)
inclined to think this may be normal?
nb. can't think now if it does it when engine is hot, don't think it does.
ed.

- (#6066) eddie arrowsmith, 8 Nov 02


Can anyone assist with advice on how to service the viscous coupling and fan ? I have notice that the fan on my 1989 Super Extra is stiff and spins on start up even when stone cold. When warmed up the fan appears to remain at a constant speed despite how fast I am driving.Is there anything I can do or is it leccy fan time ? I have read that you can buy a silicone refill for the coupling. Would this help and how do I apply it ? Any advice greatly received form a overheating-obsessed Towny owner.

Tom Mairs

- (#8026) Tom Mairs, 12 Apr 03 4:53

Tom. The fan coupling will always "drive", even with the engine stone cold and, at this low radiator temperature, the silicon fluid, inside the "slipping" chamber of the coupling will still exert some effort to the fan but, as engine speed rises, the fan speed will be much less. As the radiator heats up, and the warmed air passed over the coupling, the bi-metallic coil, on the front of the coupling, will open the internal valve,(between the "slipping" and "driving" chambers) allowing the silicone fluid to be "scooped" into the drive chamber therebye bringing the fan speed up. Even with a very warm engine, and, presumably,most of the silicone fluid in the drive chamber, there will still be some slippage. Without putting a rev counter on the fan, it's difficult to estimate how fast the fan is running. I replaced my coupling and the characteristics were still as you describe. You can check that the valve is working by heating the bi-metallic coil with, say, a hair dryer but, this will not cause the coupling to "stiffen" up because the coupling needs to be rotating, at some speed, for the transfer, or "scooping" of the silicone fluid to take place.
I am not sure if the silicone fluid degrades or not but I have heard that it can be replaced. My thoughts are that any malfunction of the coupling is, partly, due to the valve/bi-metallic coil. I am shortly to dismantle my old, spare, coupling and I will post any results if I find that the valve is not operating or that the fluid has disappeared.

- (#8027) John Davis, 12 Apr 03 9:25

Old fashioned test was to tie a peice of cotton thread to the fan (outer edge of blade) and something immovable. Once it gets up to temp it should snap the thread.
regards Jim

- (#8028) J Adgo, 12 Apr 03 11:19


On a previous posting, I mentioned that I would be dismantling my old, spare, viscous coupling and, hopefully, would post the results, with some images, onto a webpage. I have now done this and this is the link for anyone who might be interested.

http://www.marengo.dnsalias.com/coupling/

If you do take a look, please bear in mind that my descriptions of the ports, the valve assembly etc, are just my initial conclusions on it's operation and others might be able to add further information on some of the finer points. However, I do think that there would be benefits to owners who suspect that their couplings are not operating correctly, hence my comments on the testing and adjustment of the bi-metallic valve operation.

- (#8136) John Davis, 19 Apr 03 9:00