Blowing #5 Fuse - PeachParts Mercedes-Benz Forum

statefan29 · #1 06-28-2008, 01:05 PM

I have an 87 300SDL and the blower was not working properly. I tracked it down to the 30amp fuse (just outside the fusebox) and replaced it and I had great AC for 2 days.

Yesterday - the AC stopped and all I get is HOT air out of the defrost. I noticed that the #5 fuse was blown and I replaced it with another white one. The blower and AC worked when the key was in ignition slot II but when I start the car - it blows the #5 fuse. I tried another new fuse with the same result.

Any help would be greatly appreciated. We finally have some warm weather - and I am getting roasted driving with the HOT air coming out (it doesn't matter what setting I have on the CCU).

The Gears · #2 06-28-2008, 01:37 PM

What else is on the #5 fuze

brandlj · #3 06-28-2008, 02:35 PM

number 5 fuse controls your ccu, aspirator blower motor,aux coolant pump,switchover valve for the climate control,A/C compressor control unit,starter lockout and backup light switch,aux fan pre resistor relay.

If I was a betting man, I would pick the aux coolant pump first. i bet it is seized.Unplug it, put in a new fuse and try it again.

John

The reason that the air is coming out the defrost only when the fuse blow is because that is the default setting for the CCU

compress ignite · #4 06-28-2008, 03:43 PM

2nd choice would be Auxiliary Fan Resistor (Not the Relay) [could be the same
cause,the resistor will abuse the relay if non-fused or if the fuse can't act
quickly enough]they are known to melt.

Brian Carlton · #5 06-28-2008, 03:49 PM

Put the CCU on the low speed fan setting, replace the fuse, start the engine and report back if it blows the fuse.

statefan29 · #6 06-29-2008, 11:04 AM

Yes - with the CCU on low - the #5 fuse still blows right after starting the car. I haven't tried unplugging the pump yet - that will be my next step.

Thanks for the suggestions!

statefan29 · #7 06-29-2008, 02:33 PM

OK - I'd like to unplug the aux water pump - but I can't find where to unplug it! Where on an 87 300SDL is it located? Sorry for the novice questions but I've been looking for a bit under the hood and an stumped... Is it somewhere to the right of the battery?

brandlj · #8 06-29-2008, 03:06 PM

The aux pump is located on the firewall next to the battery. I have found it easier to remove the battery and then you will see the mono valve assembly which also has the aux pump attached to it. Not a lot of room to work on ir in there. When I changed out my pump and monovalve this past winter I also removed part of the 2nd firewall. It was then very easy to get to everything.

statefan29 · #9 06-29-2008, 03:25 PM

Thanks for the quick reply. I pulled the battery and unplugged the pump but the new fuse still blew as soon as I started the car (with the blower on low).

Any further suggestions?

Brian Carlton · #10 06-29-2008, 05:02 PM

Quote:

Originally Posted by statefan29

Thanks for the quick reply. I pulled the battery and unplugged the pump but the new fuse still blew as soon as I started the car (with the blower on low).

Any further suggestions?

Pull the Klima and try a new fuse.

statefan29 · #11 06-29-2008, 06:09 PM

Thanks for the reply. I did pull the Klima and the fuse did not blow!

Now for a stupid question - does this mean a bad Klima (so replace it) or was this a troubleshooting step that indicates a problem elsewhere...

Thanks

Brian Carlton · #12 06-29-2008, 10:09 PM

Quote:

Originally Posted by statefan29

Thanks for the reply. I did pull the Klima and the fuse did not blow!

Now for a stupid question - does this mean a bad Klima (so replace it) or was this a troubleshooting step that indicates a problem elsewhere...

Thanks

Well, now it gets tougher.

Disconnect the positive lead at the compressor clutch, install the Klima, and try it again.

statefan29 · #13 07-08-2008, 07:49 AM

I removed the top wire (3 pronged) to the clutch and replaced the Klima and the fuse did not blow. Plugged the wire back in and the fuse did blow.

So - I think I'm closer to diagnosing this as a bad clutch. Can anyone confirm this for me?

By the way - assuming the clutch is the problem - how difficult is it to replace? Thanks for all your help.

statefan29 · #14 07-08-2008, 09:45 PM

I found an old thread where someone recommended replacing the #5 white fuse with a blue fuse. Is that really a good idea?

brandlj · #15 07-08-2008, 09:54 PM

If a blue fuse is a larger amp fuse, this is a BAD idea. It means now it will take longer to blow and whatever is blowing it will still be getting voltage to it which means it will be getting HOT and you know what can happen to things that get HOT....They start to SMOKE... and where there is smoke there is eventually FIRE!!!!
JOhn