I've been told that the efans should kick in on high when the engine is at 102C. i have never heard them come on by themselves. Today i had to do a lot of stop and go driving and the temp got way up there. still no fans.

What i know:
If you pull the 2 pole harness off of what i am pretty sure is the fan switch, and short the harness, the fans come on high right away.

If the a/c is on, the fans are on, just on low speed.

if the engine temp gets to where they should be coming on and i leave the engine running and pull said 2 pole harness off the switch, the fans go to high right away. when i hook it back up, they turn off.

what i have done,
replaced the 2 pole fan switch, and no change.

any idea what's next on troubleshooting this?

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__________________
'92 300D 2.5
'99 E300TD

Check the resistor behind the headlight on drivers side... also the temp switch may be damaged or the relay.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

i haven't checked the giant resistor because i thought it was only "in play" when the a/c was on to get the low speed which i have.

do you happen to know which relay in particular i need to check?

__________________
'92 300D 2.5
'99 E300TD

OK here. If you short the harness. The fan should comes on as you ground the pins and trigger the relay. No issue with relay.

OK here. The big resister R14, I think is working to give slow speed.

This is the part I do NOT understand. The fan control is really simple. How can the fan goes to high speed when you PULL the harness and stop when you plug it back in? It does not make sense as the logic is completely reverse, contrary to your first step. Unless it has been butchered up but I doubt it.

Try this. Measure continuity between the 2 poles when engine is cold. It should be open. Measure the continuity when engine is showing > 100C. It should be closed. The thermo switch is red in color? It is then rated at 100C to close, different color has higher temperature. The other test is to use an infra red gauge to measure the temp of the coolant to see the dash temp matches the real temp.

Good luck.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

https://www.ecklersmbzparts.com/mercedes-switch-auxiliary-fan-switch-on-engine-block-blue-2-pole.html

this is the one i replaced the original one with, it was also blue, if you are referring to the color plastic on the switch.

i would agree it is odd that pulling the harness makes the fans come on high. but it is only when the engine is already really hot, (above 100). i just thought it was a fail safe of some kind.

i will measure the resistance at cold tomorrow, and at temp when i get to work.

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'92 300D 2.5
'99 E300TD

First and second gen 124s use K10 (low speed, via refrigerant pressure) and K9 (high speed, via head temperature) to control the fans.

K9 feeds power direct to the motors.
K10 feeds to the motors through R14.

First gen 124s use S25/5 (binary temperature switch) to control K9's coil.

Second gen 124s use the climate control PBU to control K9.

The B10/8 analog temperature sensor in the head goes into the PBU, and the PBU decides to turn K9 on.

This means that second gen 124s use a different head sensor than 1st gen. On 1st gen it is a switch. On 2nd gen it is a thermistor.

Edit: I would definitely get an IR thermometer and see if you are really running that hot. That gauge reading is way too high for my liking.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

thanks for the detailed response. do you have a schematic that illustrates these different sensors/switches?

__________________
'92 300D 2.5
'99 E300TD

Go to w124-zone.com and register then navigate to Factory Service Manuals.

Their copy is cleaner than the one at startek. Mercedes had a fire in the archives back a few years ago and the scan on startek is very hard to read. Somehow the guy running w124-zone.com has access to a clearer scan.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

your engine mechanical fan clutch is dead, you need to check that. Also make sure the radiator/condenser fins are clean of dirt/bugs/grass etc. If they are clogged - your engine will overheat even if its getting air from a jet thruster.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

zulfiqar, i had suspected that all along, any way to troubleshoot the fan clutch?

would the fan clutch not engaging at the right temp cause the efans to not come on when they should?

and finally, i had read on another thread that the design of the efan shrouds acts as a bug and leaf catcher. mine is no different, even with "only" 132k. i looked in the bottom of the shroud with a mirror and it's pretty packed along the bottom inch or so.

__________________
'92 300D 2.5
'99 E300TD

The elec fans should be on high speed with the needle that high. They are controlled by the climate control unit.

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

You have a problem with both the clutch and the electric fan circuit. For the clutch, check to see if the fan is locked when it hits that temperature. My bet is no.

It's possible that the sensor was bad out of the box, it's been happening quite a bit lately. Even the best companies are having problems with thermosensors. If a second sensor doesn't fix it, it's the controller.

greythunder,

Sumpin' blowed. Good to keep your eye on the temperature gauge. Glad it seems to be workin'! Would suggest you NOT run the AC under those engine coolant temperatures.

funny thing is, when the a/c is on, it doesn't get that hot. it took about 1 hour of very heavy traffic in a construction zone then sitting in a drive through to get it that hot with the a/c.

then i turned the a/c on, the temp dropped about 10 degrees. my guess, is the mech fan is completely fuct, and since the aux fans come on low when the ac is on that is SOME air moving across the radiator.

mxfrank, where is the controller?

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'92 300D 2.5
'99 E300TD

Even with a missing engine fan - the electric fans should kick on to full speed at that engine temperature. I am suspecting that your engine does not have the correct temperature sensor on the front of the cylinder head or the PBU is bad or the wiring from the climate control Push button Unit is bad.

The engine fan test is easy - does your car sound like a garbage truck when it reaches 100C? - If not - then fan clutch is shot or there is no airflow from the radiator to the clutch sense element (its the copper strip in the middle of it)

Your fan should also roar like a garbage truck for the first minute of startup in the morning - no roar = dead clutch due to missing clutch blood (silicone oil)

Two choices depending on how good you are in DIY, One is to buy a bottle of 7000 wt difflock fluid from an RC hobby store and a syringe - fill the clutch with 5cc and retest. If you see some difference then you can decide to leave it be or add about 2 or 3cc more and retest.

It depends on how much oil is missing - some clutches fail to work even if they are short 5cc, some still chug along

The other choice is to buy a new clutch from MB (buy the 1999 E300 Turbodiesel fan clutch - its the best design) - pricey though. And please do not settle for brands like URO etc etc.

Yes the electric fan shroud acts a bug catcher - you can remove it if you like - MB installed it so that the lower end of the condenser is somewhat protected from road salt.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)