1990 M-B 190e 2.6. Auxillary fans are not working. Fuses OK. All relays appear to check out. Suggestions? Does anyone know where the ballast resistors are located and how to test them? One auxillary fan spins freely, but the other auxillary fan may need to be replaced. If one fan is having problems, could that make both fans inoperable? Additionally, assuming the temperature sensor switch is a normally open switch, does anyone know how to check that part? Thanks!

I believe the sensor for the fan is on rear of the engine. Disconnect the ground and it should send voltage to the fans. You could have a bad relay but I have never had that issue. The ballast should be on the driver's side down by the AC drier. On the 2.6, it has an exposed coil resistor wire. If that is damaged or broken, then that is certainly part of your problem. The fans could also be bad, I have dealt with that on my 2.6. Probably a thing of the brushes are worn out, easiest way to test them is to send 12V to them from the battery directly (or fused is probably a better way to do it but I have never had one die from direct power from the battery). I had some that would run with the fans sitting on the ground but when put back in the upright position they would not run.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.

How do you know they aren't working. Mercedes in their wisdom created a system where the aux fans almost never come on. The strategy varies a little by model but not much:

The low speed will function when A/C pressure is above 300psi. This will only occur on very hot, humid days, and you'd probably need to be stuck in traffic. The switch is located on the receiver dryer (there are two switches there, it's the one with two leads). The resistor is located right beside the receiver dryer, on the inner fender near the engine.

I believe the 190E has a three terminal switch located on the water outlet for the high speed function. This will only come on when the water is nearly boiling...100C, 105C, 108C, depending on model. There is no resistor involved for high speed.

There are two relays for the fan. The low speed relay is in your electrical center, it's one of the silver ice cubes. It's likely to be a two pole 15A relay, on cars where the main fan has an electromagnetic clutch, it works both the clutch and the aux fan.

The high speed relay is usually located in a separate box forward of the main electrical center. The tricky thing is that high speed has no external fuse...the fuse is built into the relay. If you have an older black plastic relay for this function, it's been superceded by an orange relay that has a replaceable fuse built into the top.

You can revive a sticky fan by removing the blade and putting a drop of oil on either end of the shaft. The only trick is that the bolt that holds the blade is left hand threaded, so turn it backwards.

Wow. Thank you! Lots to work on. w123fanman, for clarification, when you say disconnect the ground, do you mean the ground on the fan(s)?

Does anyone know how to test the ballast resistor to determine if it is good or bad? I know this car well. The engine obviously heats up in stop n' go traffic, but not this much--that's what made me realize the aux. fans were not turning on. Thanks again for all the information.

Not sure on the resistance value of the ballast resistor, but typically they are easy to spot if they have failed because the wire will have melted or corroded away, at least that is what it was in my case on two cars.

You need to find the temp switch for the aux fan then remove the ground from that sensor. You may just be able to pull the plug off and have the fans come on, I'm not 100% on how that circuit works.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.

If we're talking about the switch on the dryer, just short the two leads together. The fans should come on immediately. The resistor is only a couple of ohms. If it has continuity, it's good.

Thank you. No progress to report, yet, but will let you know what we find out.

There are connectors from the fan pigtail, I recall under the headlights that you can use to apply 12V directly to the fan motors, which should allow you to determine whether the problem is in the control system or the motors.

Look at the following thread to see how one of my motors failed and how I repaired it.

Auxiliary fan failure analysis and used part request

In this case one fan worked, but the other didn't.

Duke

We have been able to get the driver's side auxillary fan working. The resistor seems to be OK. We're back to the possibility that the temperature switch needs to be replaced. It tests open, but if we can get it to close, that will tell whether the other fan needs to be replaced or whether this is just a switch replacement. Thanks again for all the information. We are narrowing it down.

Current conclusions: Bad auxillary fan temperature switch and bad passenger side fan. Thanks again for all the help!

You can test the switch by removing the connector and shorting it. That's how I tested my fans before reinstalling them. The switch actually has two circuits - one for the fans, and I can't recall what the other is for. I do recall that you short either diagonal pair on the connector and one or the other should activate the fans at high speed, and for that reason the connector "clocking" is not important and can be installed in any of the four possible orientations.

The switch itself will not close until coolant temperature reaches 105C, but by shorting a diagonal pair on the connector you can test fan function. It the fans work, then let the engine heat up to 105, and if the fans don't engage the switch is probably bad.

There was no problem with my switch since one fan worked, but the above test will verify that the fans themselves work, which is why I did the test before I installed the frame with the two fans back in the car. After I finished the installation I blocked the radiator with carboard and let it idle until it hit about 105C and both fans engaged at high speed, so I knew the swtich was okay and I did the installation properly, other than polarity on the rebuilt junkyard fan that I found and corrected during the preinstallation fan test.

Duke