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Discussion Starter #1
Today, I noticed that I was missing a 10 amp fuse (Fuse 18, per my research) labeled "Back" on my '91 VR4. Since this appears to control a few things (Backup lights, Light Automatic Shutoff, SRS Diagnostic Unit), and since my reverse lights were not working, I decided to insert a new 10 amp fuse into the missing fuse socket. Upon doing this and turning my key to "ACC", I noticed an electrical smell. But before I could react, a loud pop came from the engine/hood area, and black smoke started to rise out of the passenger side of the hood (and into the cabin via the air vents). I turned off the car, and removed the 10 amp fuse. I've checked all the remaining fuses on the car, and all are good.

Fuse18-3000gt.jpeg

Without fuse 18, my car still starts, and everything generally seems to work (although I have not extensively run through all the electronics yet), I was just curious if anyone might know what happened. Something surely got fried, right?
 

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There's no way an under 10 amp current let the smoke out of anything... I'd suspect you fired up a system that allowed a relay to engage a much larger current draw that had a problem. I'd look over the power distribution diagrams with that in mind to narrow the suspects...
 

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....Without fuse 18, my car still starts, and everything generally seems to work (although I have not extensively run through all the electronics yet), I was just curious if anyone might know what happened. Something surely got fried, right?
Check to see if your headlight still work.
 

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Discussion Starter #4
Check to see if your headlight still work.
Yes, headlights still work fine. In fact, I just did some test drivin', and everything still seems completely functional. No new warning lights, or strange sounds, blinking, etc... I just wonder what component in the engine would "pop" and blow black smoke, when the car is in "ACC" mode. In fact, it might have done this with the car in "OFF" position too.... not sure.

I've researched the schematics on Fuse 18, and best I can tell...
  • Back-up lights still do not work
  • Turn-signal and hazard flasher unit work - both signals work (inside and outside) and the hazard button still works. Not sure how they could work without Fuse 18?
  • SRS diagnosis unit - No idea if this works, but since it's located under the center console, that is not where the smoke came from. But still, perhaps it connects to other things which are located in the engine? The smoke first appeared coming from under the hood... and then (since the fan was running), it was sucked into the cabin via air vents... best of my knowledge.
Then, the other mystery is...
  • Why was Fuse 18 missing in my car? Did the previous owner or previous mechanic remove it for some reason? Too bad this car came with zero documentation and mechanical history... but I guess all cars come with some secrets.
 

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I'd look for something aftermarket that could be causing you problems... Like a relay added in to run a since removed backup camera, or a partially removed remote start. If no joy there, the only thing that I would suspect of being a potential candidate is the tail light relay.
 

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I like (sorry) this mystery (as I have plenty of my own...check my posts...)
I agree that it's very strange that your turns and hazards work without fuse 18.
Almost makes me think down Larry's track, something aftermarket installed poorly and then removed, OR some short in the wiring "somewhere" in the fusebox wiring (I don't even want to think about THIS)...

I REALLY hope you find and solve this issue and THEN, post the results, PLEASE.

I will continue to watch this thread in case I have input....

Bob.
 

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So the 10A Fuse you did put in, didn't melt? The car hasn't been in reverse during that test?
The SRS Light does come on and extinguish or never works? (SRS Light Bulb removed from cluster?)

The reverse light has the switch in the gearbox housing facing to the radiator, either the switch or wires to it are shorting? But then again, most likely that would burn the 10A Fuse but worth investigating, you can disconnect the switch at the gearbox and measure continuity against ground on both pins. One should have it the other doesn't. (Black-White wire should not, that is the power supplied by the fuse18).

Just a guess, but take a Multimeter and measure continuity against chassis (ground) at the Fuse18 pins too. There should be none.
 

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Discussion Starter #8
So the 10A Fuse you did put in, didn't melt? The car hasn't been in reverse during that test?
The SRS Light does come on and extinguish or never works? (SRS Light Bulb removed from cluster?)
Nope, the 10A Fuse is still fully functional after I removed it.... no melting. I did put the car in reverse, right after putting in the 10A into Fuse 18. Still no reverse lights, and then the boom and smoke. It all happened very quickly (within 5 seconds or so). The SRS light has always been on the dash since I got the car, and continues to light up. Thanks for the suggestions. I'll give them a try tomorrow!
 

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.......Just a guess, but take a Multimeter and measure continuity against chassis (ground) at the Fuse18 pins too. There should be none.
Not completely accurate….. many meters will indicate continuity to ground, since there’s path thru bulb elements, internal components of like SRS Diagnostic unit or Light Automatic Shutoff unit.

It’s tricky when using continuity meter setting, since you’re basically reading resistance (ohms) and it depends at what ohms reading the meter manufacture decided to activate audible continuity sound. That’s why I only consider zero ohms as complete continuity to ground or good wire test from end to end.
 

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Discussion Starter #10
Just a guess, but take a Multimeter and measure continuity against chassis (ground) at the Fuse18 pins too. There should be none.
One of the Fuse 18 pins does indeed have some resistance/continuity to the ground. So, does this mean that something on this circuit is shorted? I guess the mystery then is, if a wire or device is shorting while connected to this pin, it should have melted Fuse 18 when I had inserted it, correct? Also, could the continuity with ground explain why my Headlights / Hazards and SRS are still working without Fuse 18? My head hurts, lol o_O
 

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Go back and re-read White's post . What he's saying is that a single 16w backup bulb is around 9 ohms. Two of them is half that, 4 1/2 ohms. That's just the bulbs. Who knows what, exactly, the ETACs or anything else is adding (OK, mathematically speaking, subtracting..) from the equation.
"Having some resistance/continuity to the ground" is EXACTLY what he was warning you about.
"Some resistance to ground" is meaningless. In 12v systems low resistance (to ground) is normal.
Like you said though, "he mystery then is, if a wire or device is shorting while connected to this pin, it should have melted Fuse 18 ". Which brings us back to what RealMcCoy mentioned.
Look at it this way. What if someone wired in one of those annoying as fuck back-up beepers. Let's say they powered the beeper through a relay from, oh I don't know, maybe the audio circuit. They then triggered the relay off of the back-up lights. In theory whenever the back-up lights were on the relay would fire and the audio circuit would drive the asshole beeper.
Now imagine that the beeper fried. The fuse 18 back-up light circuit would be fine.

The audio circuit would also be fine. It's only when the back-up light triggers the beeper that you'd notice a problem. Pull the back-light fuse and the problem goes away.. (except for the back-up lights not working....)
I agree with RealMcCoy. Look around for any aftermarket add-ons..
 
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