w126 Blowing Fuse #5 - PeachParts Mercedes-Benz Forum

JamesDean · #1 03-04-2010, 08:11 PM

Hello,

I've been tracking down an electrical problem on my friends 300SE and figured I would open it up and see if you guys had any input:

For whatever reason, the car keeps popping Fuse #5 (8-Amp)

I've been told this might be due to the aux water pump.
I've unplugged the pump (two pin connector next to battery, in metal holder) and the fuse still blew.

Sometimes the fuse will last a day or so and then blow..

Heres the conditions when it blows:
1) just started the car
2) blower on high, recirc pump on, pbu set to economy or middle button
3) set to heat
4) shifts into reverse, backs out of parking spot
5) applies brake, shifts into drive, waits for trans to engage
6) applies gas..
7) fuse pops.

Everytime the fuse is replaced the pbu comes back to live and everthing works well.

Other things on the Fuse #5 Circuit:
1) Electronic Engine Controls / Air Injection Management
2) Transmission Kick Down
3) Stoplamps/Backup Lamps
4) Automatic Climate Control

Thus far I dont believe its:
#3 because all the lights work and are proper bulbs
#4 fuse still blew when aux water pump was unplugged. [Might need further investigating]

I'm not sure about
#1 -- The smog pump was removed after it seized up and its connected were sealed off. However diagrams indicate the MAS is also hooked into this fuse.

#2 -- Not sure how to diagnose this or test.

Other things to note:

We recently replaced the starter on the car and my friend believes he mistakenly grabbed on wire under the intake manifold and might have broken its end connector. I'm not sure what this wire might be or what it might go to..it could be completely unrelated.

Any insight would be much appreciated.

Thanks

-Kris
#2

JamesDean · #2 03-05-2010, 01:03 AM

It would appear that when the fuse pops its only after the car has been shifted from reverse to drive and the accelerator has been pressed down...

mbdoc · #3 03-05-2010, 08:02 AM

Try disconnecting the air injection pump clutch & IF that doesn't help,
try disconnection the electrical connector on the kickdown switch under the gas pedal.

JamesDean · #4 03-05-2010, 11:02 PM

it would appear this only occurs when the engine is cold. thats the only time i've been able to get it to repeat.

Its not the climate control stuff. I had that all off and it popped.

So I'm now investigating the transmission stuff..

I suspect it to something involved with the upshift delay solenoid. Does anyone know where the wiring runs for that? My buddy pulled a wire by accident when we were installing the start and might have broken something.

I tested the kick down and it worked while driving, kicked down and didnt have any problem, fuse didnt blow.

Renntag · #5 05-01-2010, 10:17 AM

If there are multiple circuits involved, perhaps there is a short from one circuit to another, exposed wires? Maybe the jarring of shifting/accelerating causes a wire(s) to move slightly and ground out.

Crazy thought:
Try running a jumper wire to the driver compartment to bypass the fuse. Attach inline a circuit breaker to reset while driving/testing. This may give more indicative conditions.
Worth a try anyway.

Best of luck. Whacky electrical issues can be frustrating.

JamesDean · #6 05-01-2010, 11:30 AM

We ended up pulling the trans upshift delay relay and the fuse hasn't blow since...