HVAC blows fuse! - PeachParts Mercedes-Benz Forum

dieseldaddy · #1 08-01-2002, 09:52 AM

Has anyone else had the 'spontaneous' fuse blowing while driving down the road?

My wife was driving yesterday (90 degrees) and she said that all of a sudden the A/C blower went off!

I got home and drove it for a few minutes and it was not just the blower but A/C compressor as well.

Checked the fuses and sure enuff...blown fuse.
I also noticed that the temp guage when I was driving it was at about 115. Now, this wagon's temp guage (1981 300TDT) has never gone above 90-100 (degrees) and it has been warmer outside than this lately! She was only about 5 minutes into her 30 mile drive...and she was not happy!

So I changed the fuse and turned on the A/C...*SNAP* goes the fuse after about 2 seconds.

What could be causing this all of a sudden??

Any quick responses would be appreciated as we are planning on taking the wagon on a long roadtrip/vacation next week.

Thanks in advance.
-Brian

Gilly · #2 08-01-2002, 08:49 PM

What fuse number is it? Possibly a bad aux fan motor is blowing the fuse since it seems linked to a high engine temp problem.
Gilly

sainttammany · #3 08-01-2002, 10:38 PM

My 82 300d blows a fuse like that, but only when the fan speed is set on the variable speed. I just run it on low or high and no problem.

RG5384 · #4 08-02-2002, 01:55 AM

mine will blow that fuse spontaneously as well, but only like every couple of months or so. I dont know what causes it either, but it sure is frusterating, especially with people in the car!
Ryan

dieseldaddy · #5 08-02-2002, 07:42 AM

It is the #8 Fuse.
I have blown 2 more...with the last one lasting a whopping ten seconds!

It does not seem to matter if the fan is set to variable or just low or high.

This last fuse (third time is a charm?) seemed to work... I am just a little nervous to drive it for a long distance (300+/- miles) in the heat of 90+ degrees.

I checked last night and I could not see anything blatantly obvious. I will have my wife drive it more today and let the AC run.

Thanks everyone for your great and SPEEDY input!

-Brian

RG5384 · #6 08-02-2002, 10:19 AM

i wonder if buying a glass fuse for that one would solve the problem? Glad it's working, now it just sounds fluckish like the rest of ours

just remember to carry a ton of extra fuses with you!!
Ryan

Gilly · #7 08-02-2002, 08:12 PM

I looked at the electrical diagram, the fuse feeds 3 basic items; The blower speed control unit and ultimately the blower motor itself, the pushbutton panel, and the A/C compressor relay.
I would take it a step at a time. I think initially my suspicion would be the blower motor, I think I'd go directly to the blower motor first and unplug the electrical connection and see if the fuse blows.
Will the problem occur if the "O" button is pushed on the climate control panel?
You could try, if the disconnecting the blwer motor doesn't help, disconnecting the connector at the push button panel, and if that doesn't help maybe disconnect either the harness at the AC clutch or remove the AC clutch relay.
Gilly

dieseldaddy · #8 08-02-2002, 11:02 PM

Thanks Gilly!

No, the fuse only blows when the blower is ON (high or low...auto doesn't seem to kick the fan on anymore. hmmmm...)
The fuse will not blow (seemingly) until the blower is ON in either of the two A/C settings. My next test is to be *absolutely sure* that the "EC" setting with the blower on High or Low does not blow the fuse.

I thought that it was overheating related...but I fixed that problem and it still happens. Also, my aux fan did not come on at all when I was overheating. The problem was that the heater hose coming out of the drivers side of the engine block to the firewall had weakened and developed a crack, letting coolant fizz out. Once I replaced that hose and added distilled water and coolant, the temp backed right down to 80.

So now I will do as you say and eliminate the components on Fuse #8 one by one. Does it make sense that my aux fan stopped suddenly? I cannot see any problems in the wiring i.e. frays or bad connections...

Not sure what to do now... we leave first thing Monday AM for Canada and I do not have much time this weekend at home to work!

Gilly · #9 08-02-2002, 11:11 PM

Without breaking out my 123 chassis disc again, it would seem feasable that because fuse 8 powers up the push button panel that it may affect operation of the aux fan.
At this time, it would seem like the problem would be with the AC compressor clutch, so I'd suggest going there first now. (EC mode does not engage the compressor, all the other modes can and will engage the compressor clutch, except "O" or "off" mode)
If memory serves, there is a squarish plastic box on the drivers side wheel well. I think the compressor relay is one of the 2 (?) in that box (under the lid), or you may try just directly disconnecting the wires from the compressor clutch itself.

Gilly

dieseldaddy · #10 08-02-2002, 11:29 PM

Once again, Thank you!

I will try that still tonight and let you know...
I only have 2 fuses left and the local parts store doesn't open until 8:00 tomorrow morning so I should probably take a break!

Your response is leading me to believe that my A/C compressor is on it's last leg. Is this a possibility??

I REALLY hope not as this is the only of the 2 MBs we have that the A/C work! (and it is my wifes...

)

dieseldaddy · #11 08-03-2002, 01:39 AM

OK, It is now 1:30AM and I *think* it is fixed...for now.
I have been doing a *TON* of searching tonight on this subject and I kept coming across several people who say MB recommends replacing the #8 and the "C" Aux. fan fuse with a 25Amp fuse instead of 16Amp.
So, after five blown 16Amp fuses in the last 2 days I say what the heck! and replaced with 25Amp.

I purchased this car in February. Replaced all the fuses almost right away. I did not really pay attention to what WAS in there...maybe 25's were already in there, but I went by the fuse card in the fuse box cover. Now that we are finally using the A/C, could we be noticing the fuse-blowing only now?

Anyhow, it appears to be working tonight (geesh, I guess it is MORNING now) as I drove until the temp got to 80 degrees celcius with the A/C on FULL BLAST. Ambient temp was actually only around 60*F at this time, but that shouldn't matter, right?
I will have to try it again tomorrow morning when it is much warmer and take it for a long drive. Bummer.

Gilly · #12 08-03-2002, 08:56 AM

Well, laziness is prevailing again, but I would swear that on the wiring diagram on my service CD that fuse 8 was listed as a 25 amp fuse. Maybe there was a change made, and a lower amp fuse is listed on your fuse card, which was changed later in production, and the wiring diagrams were also changed later on.
Gilly