Preglow Relay Shutting Off Blower Motor - PeachParts Mercedes-Benz Forum

Jollyroger054 · #1 07-08-2013, 01:28 PM

Hey Guys. I've read through just about every post regarding blower motor issues and have learned a lot but still cannot find one that addresses my particular problem.

I have a US 1987 300d with the turbo om603. My problem is that my HVAC blower motor works intermittently on pretty much all speeds. I've dug out the blower motor and the regulator. The motor tested fine and looks to be near brand new. I'm getting good voltages from the 3pin regulator connector (12v on constant, and 1-8v on control pin). So from what I've read so far, it sounds like the regulator is faulty. (I've also swapped the CCU with a spare to rule it out)

Now the weird part, while testing the climate system I turn the ignition on (without starting the engine) and set fan speed to low. The fan operates for roughly 30sec or so and then shuts off when the preglow time relay clicks. Once the relay clicks I cannot get the low speed to work at all. If I hit the high or auto button the motor blasts back on.

Diving a little further I found that if I disconnect the 4pin plug on the relay the low speed functionality returns.

Do you guys think I have a short-circuit rather than a faulty regulator?

Any help would be appreciated!

jay_bob · #2 07-08-2013, 01:32 PM

If you are operating the blower without running the engine that may be by design.

What happens if you start the engine?

When the blower dies, does the control voltage on the porcupine (regulator) go to zero?

Jollyroger054 · #3 07-08-2013, 04:59 PM

Thanks for the reply

I tried running the fan on low with the engine running. Still no change. Fan dies after the preglow switches off. I checked the control voltage and it was right around 1.6v if I remember correctly. Auto put me at 6v, and high at 7.6v. So its not zeroing out.

Jollyroger054 · #4 07-08-2013, 05:45 PM

Well I don't know why I didn't think of this earlier, probably just too consumed with the problem at hand. But I got desperate and was about to swap a regulator from my 88' 300ce when I thought: why don't I just plug the diesel regulator & motor into the ce since there's literally no dismantling required lol.

I got the exact same result on low! Obviously minus the preglow thing. The low speed didn't function what-so-ever and auto/high were at full-speed. If I'm not mistaken, pretty much isolates the problem to the regulator correct? (I tested another motor as well on low with same results).

Any suggestions on where to buy one cheapest? Partout vs new online?

jay_bob · #5 07-08-2013, 09:31 PM

First check the ground connections on the PBU, the glow plug relay control, and the porcupine. Note that the blower motor is grounded through the porcupine, since the drive circuit in the porcupine is not much different than a car stereo amplifier.

Then I would check the X4 connection block, this is where the hot side is in common between the porcupine and the power side of the glow plug relay.

According to the FSM, the permissible range of control voltage on the porcupine is 0.4 to 9 V dc. However the usual values are 1.0 V for low, 1.4 to 4.7 V for auto, and 6 V for high.

If you need a schematic go to w124-zone.com and register then go to the factory service manual section. It is also available on startek but for some reason the w124-zone site's copy is cleaner.

Jollyroger054 · #6 07-10-2013, 10:13 AM

I'll check the connections this afternoon. Just to clarify, is the glow plug relay control that you referred to the same thing as the preglow relay I've been talking about? And also I seem to be a bit behind on the acronyms lol. What does PBU stand for? Thanks for the tip on the fuzzy schematic. Startek's wiring diagram is pretty bad indeed.