Help with B5 Fuse Blowing?

[youngsyr]

Hi All,

I picked up a 2001 996 Carrera Tiptronic Cabriolet (C2, pre facelift, RHD Euro version) a couple of weeks ago and the past week it has started blowing fuse B5 (7.5 amps), which means the folding roof mechanism, electic seats and electric mirrors no longer work.

When I replace the fuse, everything works perfectly for a short while (as little as 5 minutes), then the fuse blows again and back to square one.

I couldn't pin down what is causing the fuse to blow, but I'm pretty sure it blows whenever the car is started.

I also took a look under both seats and as far as I can tell, all of the wiring is ok, i.e. no visible worn wires or shorts.

Can anyone help me pin down what may be causing the issue and how to fix it?

Many thanks. :)

[xmac]

I would try to force the failure to help narrow it down. Buy a box of 7.5A fuses and try to force it to blow by replacing it, then one-by-one immediately adjust mirrors, seats, then the top. All of them have motors that could have a problem.

[youngsyr]

Ok thanks, will do. I was hoping someone might have a place to check for a short, but I guess I may have to take it to have a professional look at it.

[JFP in PA]

Ok thanks, will do. I was hoping someone might have a place to check for a short, but I guess I may have to take it to have a professional look at it.

Problem is that there are multiple items on that circuit, and one of them is overloading the fuse. To resolve that, you need to test each device on the fuse circuit individually to isolate the problem(s).

[youngsyr]

OK guys, I've had a play today with some new fuses and it seems that no particular system is causing the issue.

I started off by starting the car with no fuse in the B5 slot, then put in a 10 amp fuse with the engine running (slot is meant to have a 7.5 amp fuse).

I could operate both electric seats (manual plus memory adjustments), roof and driver side wing mirror, but after about 10 seconds of adjusting the passenger side wing mirror the fuse went. I could still adjust the seats at this point, but the roof wouldn't open.

I then did the same again, put a 10a fuse in with the engine running, and I could play with all of the roof, wing mirrors and seats for several minutes (until I got bored!) without the fuse blowing.

So, at that point, I left the 10 amp fuse in, turned the engine off and then turned it on again, the second I tried to start the engine, the fuse blew.

Tried again with 2x 7.5 amp fuses and this time just turning to pre-ignition (the step before starting engine on the key turn) the fuses blew.

So, it looks like a surge on pre-ignition and start up plus possibly an issue somewhere else which cause the first 10 amp fuse to blow when adjusting mirrors.

Any suggestions on a trouble shooting or even better a fix? :)

Thanks in advance for any help! :)

Edited May 10, 2014 by youngsyr

[JFP in PA]

OK guys, I've had a play today with some new fuses and it seems that no particular system is causing the issue.

I started off by starting the car with no fuse in the B5 slot, then put in a 10 amp fuse with the engine running (slot is meant to have a 7.5 amp fuse).

I could operate both electric seats (manual plus memory adjustments), roof and driver side wing mirror, but after about 10 seconds of adjusting the passenger side wing mirror the fuse went. I could still adjust the seats at this point, but the roof wouldn't open.

I then did the same again, put a 10a fuse in with the engine running, and I could play with all of the roof, wing mirrors and seats for several minutes (until I got bored!) without the fuse blowing.

So, at that point, I left the 10 amp fuse in, turned the engine off and then turned it on again, the second I tried to start the engine, the fuse blew.

Tried again with 2x 7.5 amp fuses and this time just turning to pre-ignition (the step before starting engine on the key turn) the fuses blew.

So, it looks like a surge on pre-ignition and start up plus possibly an issue somewhere else which cause the first 10 amp fuse to blow when adjusting mirrors.

Any suggestions on a trouble shooting or even better a fix? :)

Thanks in advance for any help! :)

I would start with both a voltage drop and continuity (checking for shorts to ground) on each of the circuits (backup lights, top control module on cabriolets, seat and mirror memory control module, and outside mirrors) on that fuse. I would also consider individually testing each circuit for current draw. I would also immediately stop using any fuse rated higher than 7.5 amps until the problem is resolved; running higher rated fuses increases the chance of damaging the system further.

[youngsyr]

Thanks, but that is a bit beyond me - I wouldn't even know how to do a continuity test on each circuit. :(

I do have an amp meter somewhere though! :)

Perhaps it's time to get an auto-electrician involved?

Edited May 10, 2014 by youngsyr

[JFP in PA]

Thanks, but that is a bit beyond me - I wouldn't even know how to do a continuity test on each circuit. :(

I do have an amp meter somewhere though! :)

Perhaps it's time to get an auto-electrician involved?

If you are not able to do it, that would be your next step.

[youngsyr]

Update - auto technician initially though the problem could be "the reverse [back up] light switch falling out of position and shorting on the exhaust", but after inspection it was a cable running through the body somewhere that was chafed and was shorting.

All fixed now, I'll speak to the electrician and try to get more information as to the location and circuit the short was on.