150A fuse blows while cranking

Tiny
SAVAGE11
  • MEMBER
  • 2002 MG ZS
  • 60,000 MILES
I have an MG ZS that blows the 150A fuse everytime I try to start it. It originally blew while I was driving down the autobahn, and the car hasn't started since. The day it blew I had done some body work on the fenders, so both were taken off and then reattached after the work. I also put a new driveshaft in the front left hand side. We also had to weld a new exhaust hanger on to the car as the previous one had broken off and the exhaust was dangling. After all of the work was done the car started and drove just fine, I drove home, cleaned the inside of the car, drove to a vaccuum to clean the inside, left the car running, again, no problems. After leaving from the vaccuum and driving towards the autobahn, the check engine light came on, and I started hearing an awkward sound (it's been awhile but if I remember correctly I think it was a ticking sound) I pulled over and popped the hood, but it was dark and I couldn't locate the sound, after getting on to the autobahn to drive home, the car just died. I was driving down at maybe 100Kph for maybe 3k and the car just shut off. My headlights/dashlights work, and the car cranks, but the power windows, ac/heater, stereo, horn, dome light, and power door locks are all dead. After checking all the fuses, it is only the 150a that is blown. I have since checked the wiring diagrams in the Haynes manual and each system that is dead in my car runs through the 150a fuse that is blown. So after replacing the fuse and trying to start the car again, the symptoms remain the same, and the new fuse blows as if nothing has changed. Please help.
Friday, June 1st, 2012 AT 5:17 PM

1 Reply

Tiny
CARADIODOC
  • MECHANIC
  • 34,330 POSTS
First of all, all of the circuits you mentioned will have their own sub fuses. Anything shorted in those areas couldn't possibly draw enough current through their little wires to blow a 150 amp fuse because the wires would melt first. That means the short has to be before those circuits branch off, and since they're using such a large fuse, it would be typical to be going to the output terminal on the generator or the feed cable to the starter but starters aren't normally fused and they could easily draw over 150 amps anyway.

Since I'm not familiar with your car, start by measuring for voltage on the generator's output terminal. If voltage is missing when the fuse is blown, see if there are multiple smaller wires bolted to it. If so, remove them for now.

It's also possible there is a smaller positive battery cable bolted to the under-hood fuse box. If there is, and you DO have voltage at the generator, unbolt the fuse box from the car and look underneath for anything that is touching metal.

The next thing to try is to use a pair of small jumper wires to connect a 12 volt light bulb in place of the fuse. When the short is present and the circuit is powered up, the bulb will be full brightness and hot so don't let it lay against plastic parts or on carpeting. The goal is to keep the short occurring so you can systematically disconnect things until something makes it go away. When it does, the bulb will go out or get dim. If there were multiple wires connected to the generator, reconnect them one at a time to see if one makes the bulb get bright, then follow that one and try to split it into multiple circuits.

You could even have something coming apart inside the generator and touching that output terminal but that would be very uncommon.
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Friday, June 1st, 2012 AT 6:50 PM

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