In addition to always blowing hot air (out all vents) but mainly defrost, I keep blowing the #7 fuse. I started unplugging what's on that circuit and today I unplugged the aux h2o pump and water valve and the fuse was un-blown longer (would blow right at start up before unplugging these). As a result, I could start A/C with PBU, fan blowing inside car, compressor ran and both aux fans run. Air out of center vents was getting cool, sides were still blowing hot. Got to the top of the driveway and everything quit (inside fan stopped blowing) and sure enough #7 was blown again (ran for maybe 2-3 minutes).

When I first pulled the plug for the aux h2o pump, it was covered in dried windshield washer fluid (obviously something had been leaking a long time). Could this have shorted the aux pump which then in turn fried my PBU?

Any insights greatly appreciated

Mark

__________________
1987 300DT 127K
1987 300TDT 283K (Sold)
1999 528iTa 75K
1991 300D 2.5T 214K - Dead Tranny
1984 300DT 375K (Sold)

From what I understand, the aux coolant pump begins to draw more current as it ages. At some point, the current draw can fry the pushbutton unit.

I believe there is a modification that can be made to reduce the likelihood that the pump kills the PBU, and it involves using a slightly lower rated fuse than is normally in there. I've never done this, as I've never had this problem.

I'll tell you what. Rather than making US look up what circuit that the #7 fuse is in, why don't you give us a hint? Even if you don't have a service manual, look on the fuse box cover and tell us what it says.

I think I know which circuit it is, but if you will tell us, then we can advise further.

Good luck,

From what I can recall other devices on the circuit include:

Back up lamps
Heater Valve
Water Pump - Which I'm guessing is the Aux h2o pump
Heated washer nozzels
Climate Control

I think there may be one or two other devices I cannot remember here at my desk and wife has the car. Will update any missing COB today 8-Jun-05.

For any one reading who has been here, is there any way to STOP THE HEAT. I've unplugged the aux h2o pump and heater valve but it's still pumping it out heat. Will removeing the PBU controller stop it ?

Mark

__________________
1987 300DT 127K
1987 300TDT 283K (Sold)
1999 528iTa 75K
1991 300D 2.5T 214K - Dead Tranny
1984 300DT 375K (Sold)

The mono-valve in the climate control system is held closed by the application of 12V from the climate control ECU. Unplugging the mono-valve or removing the power from the climate control system will cause the mono-valve to open and provide full heat. I've heard this was an intentional design "feature" to ensure that the windshield will not fog/ice over if the climate control system fails. Of course if the system fails when it's 90F outside I would guess the driver would be less than pleased with this "safety feature".

Fuses blow due to excessive current draw from one or more consumers that are fed power via the fuse. You will have to monitor the current draw on the #7 fuse and disconnect consumers until you determine which circut is drawing excessive amounts of current.

I'd start by unpluging the climate control ECU.

Ok, there were a few other devices on fuse #7 (compressor, relay aux fan, transmission electrics.).

Per Tim's advice, I unplugged the climate PBU controller and the car stopped blowing #7 fuse. Looks like the climate PBU is the culprit. Does this sound right ?

__________________
1987 300DT 127K
1987 300TDT 283K (Sold)
1999 528iTa 75K
1991 300D 2.5T 214K - Dead Tranny
1984 300DT 375K (Sold)

It's tough to imagine the PBU drawing enough current to blow the fuse. That thing is just a printed circuit board inside - I think it'd melt before the fuse blows. BTW, how many amps is #7 - there are a very few things I haven't memorized about the 124...

I'd guess compressor clutch coil. Get a DVM and measure the resistance of that sucker. Use ohms law to calculate the current draw - should give you some idea whether the coil is likely to be blowing the fuse. It's easy to measure the clutch coil resistance by pulling the Klima relay. Just measure between pin #7 (or is it #5?) and ground - that's the lead to the compressor.

- JimY

Jim,

I'm not as comfortable troubleshooting electrical stuff as mechanicals and thus the reason this post is a bit vague, my apologies to all lending a hand here!!

The #7 fuse is 16A.

You mention the compressor coil... For a breif period of time while testing I had A/C, 2-3 minutes maybe, then everything shut off. This was with the PBU in place and the aux pump and heater valve unplugged. I was getting cool/cold air out the center vents.

Is it possible the PBU made a call to some other device (compressor clutch coil you mention) and thus blows the fuse?

Looks like I'm off to buy a DVM to do some testing.

Thanks again to everyone from an electrical neophyte...

__________________
1987 300DT 127K
1987 300TDT 283K (Sold)
1999 528iTa 75K
1991 300D 2.5T 214K - Dead Tranny
1984 300DT 375K (Sold)