Which Coolant Temp sensor? 124.128 with 124.133 engine
 

Hi,

Temperatures have gone up and I realized that my aux fans are dead.
Apart from that, I've noticed another issue: there's no voltage to the fans when AC is on. The voltage appears when I jump the pigtails going to pressure switch on drier. That leads me to thinking that it's either bad K9 relay or...the coolant temperature sensor is a switch and not a thermistor. The car is 1992 300D with 1987 300D engine (om603 turbo). I'm not entirely sure if the thermostat neck was replaced. The "sensor" is three prong, gray.

If it's the 87 switch, this will affect CCU readout of temperature and makes sense that the fans don't get voltage at idle.
If it's a proper sensor, it may be bad. Which replacement is ok? 006-545-42-24? Aren't the 3-prongs all switches?

When I jump the cables to 2-prongs on that "sensor", the AC cuts off. Which is correct AFAIK. In heavy traffic my AC was cut off when temperature climbed to 100...so something works.

Please help me clarify this. Yes, I used the search function.

Thanks

The temp switch on the filter/dryer requires that the refrigerant pressure be high enough to activate it. If you're low on refrigerant or in a low ambient condition, it's normal that the fans won't be running. Since they run when you short the wires together, that proves the fans, the resistor, and the relay that controls them functions as designed.

The electric fans are not there to cool your engine, they're there to improve performance of the A/C and to serve as a backup if the engine is dramatically overheating.

The grey temp switch doesn't enable the high fan speed until something like 115˚C.

Make sure the condenser coil and the radiator are clean and not full of dirt or debris. If you haven't done it yet, or have no record of it being done, it's time for a fan clutch. The engine driven fan is your primary cooling air flow. A weak or failed clutch will dramatically reduce the cooling ability, especially in city or stop/go traffic.

Well, according to the forums, fans should be running as soon as AC is turned on.
Additionally there's a preglow cable from that gray sensor to glowplug relay and my glow plug light is always about 20s despite engine temp, so that also makes me think the sensor is bad. Maybe then I should just order 008-542-45-17 which is for 1992 300D and leave preglow wire unconnected? Since it doesn't seem to adjust the preglow timing anyway...

The fans should only be running when the AC pressure comes up high enough to trip the switch. That's why when you short the wires together, the fans come on. That means all the gear controlling the fans works but your refrigerant charge is low or the switch itself is faulty. Think about it....

If you have the 1987 engine in a 1992 car, you have the wrong sensor setup for the afterglow relay. The 1986/87 engines had no afterglow function and used a different relay. The 90s cars use an afterglow relay and have an additional sensor in the head to tell the relay how hot the water jacket is to adjust its glow time and afterglow time.

Ideally you should change out that grey combination sensor to the correct one for your car. The high aux fan speed (overheating engine) is controlled by the climate control unit, and supposedly the later sensors are thermistors, not bimetallic switches like the grey sensor is.

The bottom line still applies here: The aux fans aren't there to cool your engine. That's what the engine driven fan is for. Unless it's >100˚ outside with the A/C on and you're sitting in traffic, the Aux fans will be on low speed and are triggered by the A/C pressure/temperature.

Fix your fan clutch. Check your refrigerant charge. You should be above 200-225PSI on the high side for the fans to engage. If you're lower than that, they shouldn't be on. Before adding refrigerant consider the ambient conditions - if it's in the 70s like it's been lately and there's low load on the A/C system, it's normal for the fans not to run since the head pressure will be less.

Red: 006-545-42-24 (100/110)
Green: 006-545-45-24 (105/115)
Grey: 006-545-61-24 (105/120)
Blue: 006-545-64-24 (105/128)

These are the 3 prongs sensor you can use in your car. I changed out to red for safety. The aux fan only runs at low speed if contact is made at the high pressure sensor in the recv/drier.

It runs at high speed when the first temp number above is met. The second temp number will kill the AC. The prong basically grounds the control relay. So if you measure the pins statically, I think 2 of the pins should give 12v ( through the relay coil ) and one should measure ground.

Good luck.

How much did you pay for that info? That's about what it's worth.

Thanks for clarification. It's not about cooling my engine, but cooling my AC to prevent its cut off. As I stated in my first post, fans are dead and need replacement. The rest I found while in there. Pressure switch and pressure is ok. Unplugging coolant temp switch doesn't trigger voltage to fans (high speed) so probably K9 relay needs replacement, whatever.

I will replace that switch to blue sensor then, they both seem to be on M14x1.5 thread.

Repeating and building on what ah-kay said, the glow relay temp switch is usually on the side of the head by the IP. The bigger device on the side of the head is the gauge sender. The 3-prong temp switch is for the fans at high speed (lower temp trip) and AC compressor cutoff (higher temp trip). The temp sensor beside the 3-prong’er is for EDS.

The fans won’t trigger by disconnecting the 3-prong switch. 2 posts have to be bridged. I don’t remember which ones.

There’s a refrigerant pressure switch (no pigtails) that interrupts the signal to compressor clutch if system pressure is insufficient. There’s a refrigerant temp switch (with pigtails) that triggers the fans (mentioned above). Separate circuits makes Diesel300’s point that engaging the compressor doesn’t automatically trigger the fans. Conversely, it’s been reported that in really hot conditions, the fans come on at low speed even with the compressor off.

From your comment that the glow relay isn’t working as expected and that it’s wired to the 3-prong switch, I suspect the harness doesn’t properly connect engine and body. When restarting a hot engine with afterglow, the glow light will flash for less than a second, even if it might keep the glow plugs energized longer. Was it doing this with the 602?

Sixto
98 E320s sedan and wagon
02 C320 wagon

xtPicture here: https://imgur.com/5zTtwN8
Didn't take picture of the gauge temp sensor;
Gauge temp wire goes under intake manifold middle of head.
3 prong has 2 wire connector to AC (bu/gy & br/yl) and one wire connector on the third prong to glowplug relay (white).
Next to 3 prong switch there's another sensor, with one wire, to EDS (gn/rd).
Green wire is not connected to anything on both ends so disregard.
Water pump has a hole on the side but no sensor (thermo-time switch).

By the way, according to ETM the 1987 glowplug relay didn't use any external temperature sensor. Relay is from 350SD I think, since it is for 6 cylinders. Leaving its temp sensor cable disconnected doesn't change anything. I don't know yet where I will connect it. Just ordered 008-542-45-17.

My memory is fuzzy but I think the paired pins in the 3-prong are for the fans. Bridge the connector sockets and the fans should go full tilt. The solo connector is for AC compressor cutoff, not glow relay. That’s a switch, not a sensor.

A 603.960/1 won’t have a glow relay temp sensor. A 603.970/1 will have it on the side of the head one space aft of the gauge sender. Then two blanks, then the cabin heat supply port.

The afterglow relay will work without a temp signal but I think you’re getting full afterglow with every start which could shorten the life of the glow plugs. Doesn’t seem to bother folks who do the purple wire mod.

That’s right, the 50*C switch for the transmission amplifier goes at the water pump.

Sixto
98 E320s sedan and wagon
02 C320 wagon

I bridge them and compressor stops. Not sure about fans, will check.
So since the relay is not for this engine per se, where should I wire its temp wire? To one of the thermistor coolant sensor wires? I don't wanna chase a 1987 relay, because afterglow is an advantage.

On a side note, I picked up a W126 1st gen single aux fan today. Can it be mounted? All 124's are gone locally. It's hot and I need the AC badly.

You have to add a dedicated temp sensor for the glow relay. The 603.960/1 didn’t come with it. Fitting one to the side of the head looked difficult so I used the threaded port on the water pump housing ahead of the thermostat. I could do this because I removed the transmission vacuum amplifier.

Temp sensor 008 542 56 17 has a pin each for gauge sender and glow relay. I can’t find the part number of the glow relay only sensor. The 603.960/1 gauge sender has only 1 pin.

If you bridge the pigtails and the fans don’t blow, it could be a bad low speed resistor behind the left headlight.

Most folks here upgrade the early 124 single fan to later dual fan. I don’t recall that anyone retrofitted a 126 single fan. I think there are multiple sizes of 126 single fan. I found it dificult to remove the 124 dual fans. Once it’s out, it can’t be that difficult to fit a single fan.

Why not wire the fan directly to the battery (fused!) with a switch for the driver? At least until you sort the sensors and wiring.

Sixto
98 E320s sedan and wagon
02 C320 wagon

Took the fans out, finally...these damn rivets.
After cleaning out of the debris they work, but one fan works only in one direction and makes a noise.

My water pump comes from 350SD AFAIK, so it doesn't have any threaded holes, or at least I haven't spotted them yet.

Filp,

If bridging the temp sensor pins causes the compressor to stop, it sounds to me like the wiring is wrong. Given the engine transplant and the different pre-glow relay that is not properly connected (no head temperature sensor), there may be other things done wrong or not done at all by the previous "mechanics".

My advice would be to get the wiring diagrams for the '87 sedan, the '92 sedan, decide on what is possible and what is the best design to get the best AC performance, and then make the car match your design.

We haven't even discussed the KLIMA relay yet, hopefully that circuit is either 100% functional or someone did a proper job in removing the KLIMA and substituting relay and jumper to restore AC compressor function and transmission kickdown.

My '87 wagon had totally butchered AC wiring when I got it a decade ago. Took me a few weeks to tear out all the jury-rigged nonsense, replace the failed components, correct the wiring, re-seal the compressor, and bypass the KLIMA. Using the wiring diagram and studying the design to understand the function of each component and the signals that each wire carried was key.