Faulty fan clutch?

[millo777]

Another viscous clutch problem I'm afraid...

Having replaced the viscous clutch last august, the w124 decided to get a bit hot and bothered again after sitting at idle with the AC on. The Temp got precariously close to 120c before I noticed and switched the heater on full to avert an over heat.

Back home I tried to test the clutch and with the engine was off the fan can be spun about 1/4 of a revolution so is quite draggy. This is with a cold engine having left it over night.

I tried to replicate the problem today by letting the car idle to bring the temp up and noticed that increasing the revs actually brought the temp down rather than raising it. So it would seem the fan is partially engaged all the time.

There is certainly not the expected whoosh and roar of a fan coming on at 100c (I assume the marking between 80c and 120c on the scale is 100c?, or is foolish to assume it's a linear scale) and is roughly when the fan should kick in.

I'm intending to take it off and return it to GSF for a refund, but wanted to sanity check I'm not missing anything before I waste a morning fitting a new fan.

Dumb question I know, but is 500 rpm at idle fast enough for a fan to cool the engine anyway?

 

[martyz]

The fan is switched on via the temperature sensor,bridge the connector from it with a piece
of wire,the clutch should energise and rotate the fan.

 

[grober]

On engine idle the viscous fan will "idle" also so there will be a slight movement of air thro the radiator-- this is important as its this air temperature that impinges on the bimetal strip in the centre of the viscous coupling to cause it to engage. Normally on idle the engine + radiator temperature will rise as the " idling" fan will not shift sufficient air to cool the car.-- the viscous fan should then engage.
HOWEVER you mention your car has A/C which imposes more thermal loading on the engine cooling and normally there are 1 or 2 auxilliary electric fans at the front of the radiator which should also cut in- so before you condemn the viscous coupling make sure the auxilliary electrical cooling fan/s are working correctly.

 

[millo777]

Thanks Graeme, would I be right in thinking the aux fans need 2 inputs to switch on, from a temp sensor on the thermostat and from the AC pressure switch?

I'm not entirely sure the AC is functional, it's one of those special AC systems that needs regassing every year but nobody can find a leak on, so I guess I'll need to short both sets of contacts to get the aux fans working?

Regardless of the aux fans, the viscous clutch should still come on with a audible roar at around 105c though right?

The other issue is whether or not the clutch should have any drag when the engine is cold.

Apologies for the draft questions. I'm just sanity checking myself to prevent buying and replacing parts needlessly which has become something of a lait motif with this car.

 

[grober]

You should hear a notable increase in fan noise as the viscous clutch engages correct. There is always a small degree of drive to the fan but you should be able to physically stop its rotation when cold.The gold standard is the so called carrot test. When completely coupled up it should not be possible to stop the fan rotating. ITS ADVISED TO USE SOMETHING SOFT OR PLIABLE FOR THIS ! This is to stop the fan blades breaking or people getting their hands /face damaged

as to the electrics maybe this will help?

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1995/E320/SYSTEM WIRING DIAGRAMS/5408.pdf

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1995/E320/SYSTEM WIRING DIAGRAMS/5407.pdf

 

[kth286]

do not discount the fact you may have a coolant pump problem.

I can let my same engine stand running all day and the viscous fan would never come on.

A badly maintained cooling system will boil at lower temp, and missed coolant changes will eat pump impellar blades, so the coolant may not be running
at correct rate through the system, especially if there is sludge in the sytem also.

Missed coolant changes mean the coolant gets very acidic and eats into
internal components.

Only use Merc coolant additive which is properly pH neutral when new.

Also de ionised water is best.

 

[millo777]

I managed to do the carrot test, well, a rolled newspaper test in reality. From a cold start in the morning the fan managed to comprehensively shred the sunday times culture section.

After a long drive I let the car idle for a while and the temp remained static at a whisker over 80. I then put the AC on at the coldest setting with the fan on full and the engine temp started climbing. It got to just below 120c without any change of noise in the fan, and without either the viscous clutch or the aux fans coming on.

The aux fans I'll test at the weekend when I have time to source some small crocodile clips and make a test cable.

The viscous fan I'll just replace, but a decent one this time not a cheapy Turkish jobby from GSF, though in my defence they had promised it would be Sachs when I ordered it.

@kth286, you're quite right of course, this is treating the symptomn rather than the cause. The water pump looks quite new, but I'll flush the coolant system properly and reaplce with the good stuff before I start changing that as well.

I've just ordered a block tester and compression tester to double check the head gasket isn't leaking combustion gas into the coolant and will do that before chucking any more money at it.