TRoubleshooting electric fans - PeachParts Mercedes-Benz Forum

reddogracing · #1 08-21-2015, 07:23 PM

I need a troubleshooting guide for my 1993 2.5 300D electric fans.

I have two of these cars I can not get the electric fans on either to turn on, but not sure how to go about it.

I jumped the switch on the A/C on both cars, nothing. I think I have the right one, the top plug stops the compressor, the other two wires have no effect on the A/C or the fans.

The gray car gets wamr in traffic due to weak manual fan clutch, but seems like the electric should kick in and cool it off. Running the defroster will drop it 30 degrees.

So if you know of a link to a troubleshooting guide for the 2.5 NOT the 3.0 please point me to it.

Mxfrank · #2 08-21-2015, 08:04 PM

I have the diagram for a 300E, most cars of this generation used similar systems.

reddogracing · #3 08-21-2015, 09:48 PM

Thank you

That diagragm shows no temp sensor switch?? odd

reddogracing · #4 08-21-2015, 10:16 PM

so i figured out a little more. I found a two wire sensor/switch near the upper radiator hose. If I ump across there the fans blow hard, on both cars. So new temp switches possably lower temps switch ???

pmckechnie · #5 08-22-2015, 06:42 AM

On my 91 300D 2.5d and most other Mercedes there is an axillary fan resistor and auxiliary fan preresister relay that runs the aux fans at low speed when the A/C pressure is over 12 bar. I have seen these resistors go bad. On my 91 it is located just behind the left headlight under the hood.

PaulM

jay_bob · #6 08-22-2015, 07:22 AM

The temp switch on the head for 88 and up 124s/2nd gen 126s is really a variable resistor sensor. It is read by the climate controller to tell the aux fans to come on and if the temperature keeps climbing to inhibit the A/C. The low speed relay is driven by the dryer switch and the high speed relay is driven by an output of the climate control box in the dash.

The diagram mxfrank posted is accurate for your car according to the factory manual.

Yes the fans are driven straight from the battery with no overcurrent protection. That is just nuts. I really wonder sometimes what they were thinking. Lots of things like that on the 124.

Note for anyone coming along that has an 87 see this post instead:http://www.peachparts.com/shopforum/diesel-discussion/370870-w124-c-new-pressure-switch-auxillary-fan-worked-twice-now-nothing.html#post3506733

Mxfrank · #7 08-22-2015, 08:45 AM

When you see a direct battery connection like this, it usually means the relay is internally fused. Later relays had a plug in fuse which could be changed. But with the early ones, if the fuse went, the relay was toast.

Since the main fan comes on when you short the sensor, your climate control head unit is ok, which is good news. This probably means the entire high speed circuit is fine. But high speed will only come on when the engine is very hot...probably around 105C. The low speed is triggered by the pressure switch on the receiver/dryer. Most likely problem is a fried resistor, as has pmckechnie pointed out...test it for continuity. If the resistor is ok, replace the relay and you should be fine.

I'm adding some photos...the first is an internally fused relay, as found in my '87 190DT. Notice the schematic on the top of the relay shows a fuse. These are really awful...you can't tell if the fuse is good by looking, and can't replace the fuse. When the fuse blows, it actually starts the plastic burning, but there isn't enough air in the case to sustain a fire, unless the case is compromised. The other photo is the new design, with a removable fuse in the top. The new one is plug compatible with the old one, and should be substituted before you have a problem.

reddogracing · #8 08-22-2015, 06:14 PM

thank you all i will contnue to plug away at it.

If i jump the two wirees at teh dryer and get nothing, if i leave that jumoed and jump the resistor i should get fans correct?

I know that the grey car needs a fan clutch, my concern is why didnt the electric fans kick in. It was almost pegged tempo gauge, and I could tell the difference between the two cars.

We actually were driving both cars home, a three hour trip. I drove black, daughter drove grey. ran into a bad bad traffic jam and grey was getting too warm.

i popped both hoods, and could well the differnece in air movement from the fan, and that the grey was indeed warm.

i could rig this up, but i like things to work as designed

Mxfrank · #9 08-22-2015, 09:59 PM

If the temperature was over 105C, then there is also a problem with the high speed sensor.

Yes, jumping the receiver switch AND the resistor should cause the fans to run on high speed, if the problem is the resistor. If doing this doesn't work, then replace the relay for sure.