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)

If you were getting cold air, the compressor coil was activated. It is the device which "turns on" the air conditioning. It's an electric coil on the front of the compressor which is used to generate a magnetic field - it's basically a modest size electromagnet. When active, it literally pulls the clutch plate against the rotating pulley, and the compressor starts spinning.

Anyways, my point is the compressor coil was active all those 2-3 minutes. I can't think of what else could kick in on that circuit after a few minutes.

This is a 124 chassis, right? The electric water pump shouldn't be a problem. Sure, they go bad, but the PBU includes short circuit protection against a bad water pump. As well, the pump doesn't run until the PBU decides to mix in a little heat. This only happens after the car cools down, and begins to get too cold. So you won't see any symptoms from a bad aux water pump in the first few minutes after the car is started. (Well, unless you live somewhere not very warm.)

Radio Shack is a decent place to buy a DVM. Make sure you get a digital unit, with autoranging. They're common as dirt, so shouldn't be difficult to find. I haven't bought one for 10 years, but it was only $25 back then.

Thought of something. If the resistance on the compressor coil checks out OK, try installing a new fuse and running until (?) it fails. Then immediately recheck the resistance. It may only short out when it's hot, but be OK when cool. It's unusual, but I've encountered wierder things.

- JimY

The CCU does indeed have protection for the aux water pump circuit, but excessive current draw from it can cause CCU failure. In fact when you buy a rebuilt CCU, they won't warranty it unless you check replace the aux pump.

To check the aux pump, simply rig up some jumper wires with a DC Ammeter in series. The pump should draw no more than 1.3 Amps. If you change antifreeze annually you will in all likelihood never lose an aux pump.

Good luck,

I didn't have the time to purchase a DVM this weekend and troubleshoot, too much going on with wife and family. What I did do however, was reconnect the heater water valve and install the PBU from my 87 wagon into the 91 sedan. Took the 91 out for a ride and everything seemed back to normal. There was air to the windshield but it was cool not hot, same with the air from the side and center vents and this was with the car fully warmed up. Pulled over and checked the fuse box and #7 was still good. Ok, great I'm thinking so I go ahead and turn on the A/C and compressor engages, interior fan comes on blowing cooled air, then all of the sudden, bam, nothing (ran ~2 minutes again, maybe not quite that long). Go home and sure enough #7 fuse is blown again.

So I'm thinking it's the A/C clutch coil based JimY's commnets last week and some further reading, specifically the DIY on A/C control diagnostics on this site. Quote from Steve Brotherton's article...

"Clutch coil current should also be inductively checked off the jumper. The amperage should be less than 5a and is usually around 3a. If a controller is bad all the time, definitely check this current before replacement. Unfortunately, this is also intermittent and will burn the controller just like a fuse. If a new controller comes back, I would definitely suspect the clutch coil."

This quote speaks to exactly the behavior I'm seeing in my system since my 87 PBU brought the system back.

I'm hoping it's a simple R&R and with no R-12 recovery needed. Mine A/C unit has a 10mm nut holding the clutch to the compressor.

JimY and/or Larry, have either of you any experience with replacing the A/C clutch on a Nippendenso compressor ?

Thanks Again for listening...

- Mark

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

I'm currently experiencing the exact same problem in a 1987 300TD. The AC clutch plate has gone haywire and is clattering around on the pulley assembly. The #7 fuse will blow whenever it feels like it, so I don't think this is an issue with the aux water pump. However, it had a nasty tendency to blow heat constantly which was rectified by replacing the aux water pump with a good used one.

It's been a while but I wanted to report back. I ordered the A/C clutch and rebuilt PBU from Phil here at Fastlane. The A/C clutch really isn't all that bad a job. This is on a 1991 300D 2.5T though and there is a little more room up front than there is on my '87's. I would advise removing the fan clutch, fan shroud, belt, and PS pulley as it makes access that much easier in such a tight spot. The most difficult part of the job is R&R of the spring clips since you are bascially doing it blind with the compressor in situ.

The rebuilt PBU now no longer causes the system to blow the #7 fuse when the car is started and I don't have heat blowing all the time now. However, the replaced A/C clutch did not correct the issue I'm having. Price paid for not testing as I was advised to.

After replacing the A/C clutch I ran the system in the garage for about 5 minutes and all seemed to be running Ok. Backed up the driveway and as soon as I put the car in 'D' everything cut-out again (compressor and interior fan). So it would appear as though the A/C clutch and coil were good to begin with.

I now have other issues though and the A/C will have to wait. The transmission on this car puked on me as of Saturday afternoon, slips in reverse. A check of the transmission fluid found it to be dark almost black. And this juice has only been in the tranny case for about 10 no more than 15K miles.

So now she sits...

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

Does the fuse blow every time you go from reverse to drive? (I realize this may be difficult to test with a toasted tranny) The reverse lights are also protected by fuse #7, could it be something as simple as the reverse switch or wiring to it is flaky?

__________________
1987 300D Turbo
2000 VW Golf TDI
2008 E320 Bluetec

I had all ready well reached my frustration point Saturday when all this went down, literally...

Yes, the #7 fuse has blown consistantly 3 times shifting into drive after backing up with the A/C running. I had at one point thought about the back-up lamps on that circuit or may be the neutral safety switch but didn't think either of those would short or draw enough current to blow the fuse and really thought if that were the case it would happen when moving from P to R. So, may be its neutral safety since it occures from N to D. I'll look into this when I get the chance again.

I'm hoping to 'revive' the tranny with a complete fluid and filter change. I've read posts stating the tranny problem got worse as a result, but oh well, I'm giving it a shot anyway, may be I'll get lucky. It may be a while but I'll report back when I get that far.

Luckily for me, my '87 wagon never sold and the Beast is back on the road again...

Mark

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