Testing E420 Aux fans

[MB_FanAddict]

When I pull off the connector on the blue two prong temp switch or jump the fan switch on the drier the fans don't come on. When jumping the drier switch the low speed relay clicks. Both fan relay fuses are good.

Can someone tell me which sockets to jump to bypass the relays so I can see if the fans will come on.



Thanks,
Chris

[Stan Pittman]

Did you check the resistor for the aux. fans. Most of the problems are the resistor.

[MB_FanAddict]

The resistor could be bad, but the resistor has nothing to do with the high fan speed. Correct?

If anyone could send me the wiring diagram or troubleshooting info from the manual for the aux fans on my 1994 E420 I'd appreciate it.


Chris

[mbdoc]

Do the relays CLICK when you jump the switches?

IF so then you need to check the wiring at the ballast resistor. The low speed signal goes thru the resistor & the high speed wire connects to the resisror on the other side.

If you need wiring faxed leave me that number on a "PRIVATE message".

[y2kimmel]

If you short the coolant temp sensor, this will activate the cooling fans. The sensor's resistance is inversely proportional to the coolant temp[ (higher temp - lower resistance). Making an open circuit, as you described in your e-mail, will do nothing.

Try this (I used a screwdriver to short the sensor leads) and let us know if the fans click on.

Regards,
Troy
1995 E420 79k

[MrCjames]

If you are testing a 94 124 body with a 119 engine than the blue 2 pole switch at the front of the engine when disconnected (open) high fan speed should engage. This signal is sent to the A/C pushbuttons, if memeory serves me correctly, and in turn controls the high speed aux fan relay.

Before going too deep, check the voltage at the fan resistor. You should have twelve volts on the one side when the blue switch is disconnected.

[MB_FanAddict]

The fans came on today when I pulled off the temp switch connector. I read somewhere that this trick doesn't work all the time. (my problem?)

So anyway, I'm going to replace the resistor which should fix the fans not coming on at low speed.

The only other problem is that sometimes the aux fans will come on at high speed on and off in short bursts (1-2 sec) when the car is idling or just after starting it. Anyone have any ideas on this? I don't think it's the temp switch as I tested it and it has a steady resistance.

Chris

[wjm]

The only time I had a problem with my Aux Fans, it was due to the wiring going to the resistor being rusted and coming apart.

The resistor is hidden on the front driver side of the engine compartment below a cover near/below the alarm horn (if I remember correctly).

Willy

[MB_FanAddict]

After disassembling part of the car to get to the resister and then taking of it's cover, I found the top wire on the fan resister had corroded. I repaired it and now the fans work on low speed. I didn't need a new resister after all. The problem I am having with the high speed fans is being caused by the temp switch. Retesting it I found the resistance of the sensor erratic, it kept switching back and forth between normal and infinity when the car was at a steady 80c. Searching the board I found someone else that had this problem with there 560SEL.
See:http://www.peachparts.com/shopforum/showthread.php?threadid=7301

I should have the new switch installed by tomorrow.

Thanks,
Chris

[MB_FanAddict]

I replaced the temp switch and it fixed the problem I was having with the aux fans cutting on and off at high speed.
My aux fans have been working as they should since then, coming on low speed with the a/c. The car has not gotten hot enough for the fans to come on high speed yet though.

Thanks guys for all your help.

Chris

[400E]

Chris,
Are you saying that the fans come on at low speed whenever the AC is on? If so, that's NOT normal operation. They should only come on when the refrigerant pressure exceeds a certain limit, which characteristically only happens after the AC has been running a little while, and ambient temp is high.

If they are coming on whenever AC is running, suggests to me you have a bad pressure switch, or are running too high of refrigerant pressure for some reason.

[MB_FanAddict]

Steve,

No the fans don't come on immediately with the a/c but I have another problem, once they are on they don't shut off when the car is idling even if you turn the a/c off (and I've waited 15min). The fans only turn off when you drive the car. If you turn off the car while the fans are on and then restart it 1-2hrs later the fans will run for 5sec and then turn back off. I know it's not right. I'm sure it's the switch on the drier (while working on getting the fans to work I found someone had bridged the wires on the drier switch so the fans would run all the time but I removed it. Do you think it could have messed the switch up somehow?) but to change the switch the system would have to be evacuated and refilled. Right now the a/c works great and it's not like the fans are stuck on all the time so I'm just going to leave it for now. Unless you or anyone else have any other thoughts or ideas...

Thanks,
Chris

[400E]

Chris,
I think your plan is a solid one. I agree -- doesn't seem worth it to evacuate the system for what seems to be a malfunctioning pressure switch, esp. since the fans, if anything, are running more than they 'need' to. That's a much better 'problem' to have than not running at all. You are conscientous enough to know that your aux fans weren't working before you changed the temp switch. On the other hand, there are a LOT of MB owners with non-working aux fans who don't know it, and may be shortening the life of their AC compressor.

Many folks don't know that low-speed aux fan operation is really intended to preserve the compressor, not to keep the cabin cool (although that's certainly a nice side benefit).