DFI MDL fuse keeps blowing

Tiny
SHELLY KALANI KAPELA
  • MEMBER
  • 2004 CHEVROLET MONTE CARLO
  • 3.8L
  • V6
  • 123,000 MILES
I was driving to work and went to pass a vehicle and my car shut off in the middle of acceleration. Thankfully I was able to get to the side of the road with no power. I ended up taking her to a shop and they told me the DFI MDL fuse had blown and they replaced it but couldn't find what caused it to blow. She ran for a couple weeks with no problems and now once again she has shut off. I replaced the fuse in my car but she keeps blowing the 15 DFI MDL fuse every time I turn the ignition on. When I crank her she only turns over. Any ideas on what could cause this fuse to be blown every time?
Saturday, November 30th, 2019 AT 3:09 PM

3 Replies

Tiny
CARADIODOC
  • MECHANIC
  • 33,723 POSTS
There's a whole bunch of things on that circuit, and to make it really confusing, the engineers used pink wires in every part, but they split it up, then fused each part separately. This fuse protects circuit 539, as shown with my pink arrows. The only thing on that line is the ignition control module. The wire going to it could have a bare spot and be touching the engine or a metal bracket, but the ignition module itself is the best suspect. If you unplug that module, then a new fuse doesn't blow, replace the module. If the fuse still blows with the module unplugged, I'll describe how to use a trick to avoid having to keep replacing fuses while we diagnose this.
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Saturday, November 30th, 2019 AT 6:34 PM
Tiny
SHELLY KALANI KAPELA
  • MEMBER
  • 2 POSTS
Okay, we removed the clip from the ignition control module put a new fuse in and turned the ignition on; the fuse blew again. So does that mean the problem is not with the ignition control module?
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Saturday, November 30th, 2019 AT 7:26 PM
Tiny
CARADIODOC
  • MECHANIC
  • 33,723 POSTS
Dandy. That eliminates the expensive part.

Sorry for the long delay in responding. Just got plowed out today after a big winter storm.

A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a 12 volt light bulb. A common 3057 brake light bulb works well because it has the type of terminals that are easy to clip to. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it's not laying on the carpet or against anything plastic. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out. In this case, move the wire harness around for the ignition module.

These drawings are two I produced that show part of the description for this trick. The bulb will limit current to a safe one-amp so the wiring will still be protected.
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Monday, December 2nd, 2019 AT 7:11 PM

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