Intermittently blowing interior fuse in my 97?

MadMikeyL

MadMikeyL

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I need some help with an issue I’m having on my 97. The 10A fuse for the interior lights keeps blowing every few days. Upping to a 15A made no difference. Prior to today, every time it blew was either getting into or out of the car, but today it just blew as I was driving down the highway, and when I got home, I replaced the fuse, and it didn’t blow. The tell-tale when it blows is the speedometer stops working, which may or may not be a symptom, since I’m not sure if that fuse is supposed to power the speedo. Anyway, does anyone have an EVTM for a 97 to post up the layout of the wiring for the interior lights? Or could anyone with a 97 4.6 car do me a favor and pull the interior lights fuse and see if their speedometer stops working?
 
An intermittent short like that is likely a wire across a metal edge of a panel. a dislodged grommit thru the firewall caused me similar probs.
Think of anything you had apart before it started.
The door bundle would bear checking.
Put an led or lamp where the fuse goes, and start wiggling wires, slamming doors, etc.
There are known issues with the bundle to the hood light bundle, trunk light bundle, and the one by the drivers'seat belt anchor.
 
Yeah, I’m guessing it’s a rubbed through wire, but I don’t know where. The only thing I had apart was end of last summer I redid the headliner. So I immediately went to the dome light. There was nothing obvious, but I taped up the wiring up there and the metal bracket that the light screws into. After that it was good for a few days, then blew again. I had also previously had a water leak into the trunk at the seam sealer by the back glass, so I thought it might have been from water getting into the keyless module right there. I ordered a good used module on eBay, and it was good for a few days, then blew again. Next I found a blown bulb, and I thought maybe it was the filament shorting out, so replaced that, and it was good for a few days, then blew again. I’ve inspected all the wiring I can get to without taking things apart, and tried wiggling wires under the dash, in the door jambs, in the trunk, above the fuse box, and haven’t been able to reliably reproduce it. Plus this last time it happened while I was going 80 down the highway, with no lights on at all, and after having driven an hour down to drop my son off at his mom’s, and then about a half-hour back, so probably not something in the harness going into the door, since obviously those were closed the whole time.
 
Mike, just taking a brief look at the manual, the speedometer indeed gets a feed from the INT LPS 10A fuse:

20260531_231934.jpg

Which is to say that the speedo not working after the fuse blows is, in my opinion, a mere side effect, not really of any significance for diagnostics.

Apart from that, here is the power distribution from that fuse:

20260531_231552.jpg
 
Interesting, I actually retrofitted a 97 cluster in my 94 at one point(SHO gauges and 97 bezels were a much easier to find at the time!) and remember having to add another ignition wire to get the speedometer to work, never would have thought it was off the interior lamps circuit, I never looked at a full 97 EVTM when I did it beyond the connector pinouts. 96 and prior strictly power the speedometer off the cluster fuse
 
XR7-4.6 said:
. 96 and prior strictly power the speedometer off the cluster fuse
Click to expand...

And I'm not sure why 97 uses the lamp fuse circuit. On the surface, the only main distinction is that the lamps circuit is hot all the time, not just with ignition on. Could there be any reason to power the speedometer with ignition  off?
 
1997ThunderbirdLXV6 said:
And I'm not sure why 97 uses the lamp fuse circuit. On the surface, the only main distinction is that the lamps circuit is hot all the time, not just with ignition on. Could there be any reason to power the speedometer with ignition  off?
Click to expand...


All I can say is from the retrofit the speedometer would not work at all (odo/trip included) until I added that extra wire. I didn’t get it then and seeing that being the source I even more don’t get it now

1780290121880.gif
 
If you have a multimeter here's a couple things that may help.
- you can pull the bulbs and with a good fuse measure both power and ground looking for less than 12v. Less than a full 12v on the power side would direct you where to look.
- with fuse pulled, measure resistance between power & ground at each bulb socket, bulbs removed
- This should be infinite, any resistance is bad, a very high resistance may not blow the fuse immediately and would point you to which wires to begin looking at.
- you can also measure the resistance from the fuse terminal to the hot side of the bulb terminal, with both bulb and fuse removed. resistance should be low, ideally 0.
- similarly measure resistance from the negative side of the bulb socket to a chassis ground on the car. again very low, ideally 0. I would not suspect this to reveal anything, but high resistance would be a bad thing, just not blow a fuse.
 
It's an intermittent problem, and the easiest way I've found to find these is to replace the fuse with a lamp, so you can see when it shorts. A short will fully light it, but an intermittent will flash as long as it's shorted. I've never found one with a meter until it shorts permanently
Two .187 faston crimps on an led lead will plug right into the fuse location.
 
So the mystery here is that it happened a few times when opening a door, but once while driving.

If it only happened when opening a door, you'd suspect damage in the courtesy light circuit  after the door push button switches and before the bulbs. But since it happened at least once while driving, you'd suspect the always hot portion of the circuit, so  before the door push button switches.

How does that fit together with it happening when opening the door?

Here's a guess: damage in the wire going to the mirror switch. Per the diagram above, the mirror switch gets always hot power from this circuit. The short could be triggered by casual vibration while driving as well as by the vibration from opening/closing the door.

Questions to substantiate/disprove my guess:
  • On the door incidents, did it happen immediately on opening? Or was there maybe a delay? Did it even happen on closing?
  • Did it ever happen when opening the passenger door?
 
The door wires in the 'tube' going to the door flex every time you open or close the door; there was barely enough room to run a 12awg speaker wire thru there. Using a lamp lets you wiggle stuff, and look for a flash, instead of trying to read a meter. An led and a 180 ohm resistor draws very little current, and flashes faster. And leds are easy to see. I've diagnosed a medical scanner with that trick; there was a half a mile of wiring in there.
 
I’m not sure exactly when it happened prior. It always happened during daylight, so my first indication has always been the speedo not working, and until yesterday, I always noticed that immediately after pulling onto the road, so no idea if it happened when I got out of the car, or got into it, or while it was just sitting there. I did try wiggling the wires in the door jamb while watching the dome light, but never found anything. I’ll try the LED and 180ohm resistor trick. Although now that I’m thinking about it, won’t that load in place of the fuse interfere with full voltage getting to the speedo?
 
Yes, it wont run stuff; it's just for looking for shorts. you could attach it across the fuse, and run it up to where you can see it, and it will light when it blows.
 
I made a related observation today, but I remember seeing this once before several weeks ago:

The speedometer sort of sticks to zero. I drive off, no speed indicated. I reach 20-25 mph, and suddenly the needle jumps up and reads normally.

Any ideas?
 
1997ThunderbirdLXV6 said:
I made a related observation today, but I remember seeing this once before several weeks ago:

The speedometer sort of sticks to zero. I drive off, no speed indicated. I reach 20-25 mph, and suddenly the needle jumps up and reads normally.

Any ideas?
Click to expand...

I doubt it’s anything electrical related, most likely the 30 year old lube is just a little sticky before the speedo motor overcomes the friction as the speed rises.

My SC speedometer does it randomly too, though it’s a different design from 97s. Sometimes I’ll notice it reading 0 when I’m going like 35 and smack the dash and it jumps and settles where it’s supposed to and work normal for the rest of the drive, days, weeks etc. before it does it again.

And no I don’t know what oil type/viscosity to use, or I’d have done it already 😆
 
XR7-4.6 said:
I doubt it’s anything electrical related, most likely the 30 year old lube is just a little sticky before the speedo motor overcomes the friction as the speed rises.
Click to expand...

Well, first thing, I checked my interior lights, knowing what we've learned from this thread; and they worked normally.

30 years old, that's always everyone's excuse for everything. Damn.

XR7-4.6 said:
My SC speedometer does it randomly too, though it’s a different design from 97s. Sometimes I’ll notice it reading 0 when I’m going like 35 and smack the dash and it jumps and settles where it’s supposed to and work normal for the rest of the drive, days, weeks etc. before it does it again.
Click to expand...

Your car doing it,  too, always makes me feel better.

XR7-4.6 said:
And no I don’t know what oil type/viscosity to use, or I’d have done it already 😆
Click to expand...

Wait, there must be some small mechanism lubricant. Maybe something you buy where they sell remote control cars, etc.?

One could also clean the mechanism with some electronics cleaner spray.
 
1997ThunderbirdLXV6 said:
Well, first thing, I checked my interior lights, knowing what we've learned from this thread; and they worked normally.

30 years old, that's always everyone's excuse for everything. Damn.



Your car doing it,  too, always makes me feel better.



Wait, there must be some small mechanism lubricant. Maybe something you buy where they sell remote control cars, etc.?

One could also clean the mechanism with some electronics cleaner spray.
Click to expand...
A low viscosity lube like gun oil or 3 in 1 oil is not good, I'd use thin grease, like phonolube. Or gun slide grease.
Electronic spray=no. maybe in '80,but not now. Solvents changed.
 
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Well minor update, I haven’t gotten to the bottom of this yet, but at least now it is blowing immediately all the time, so hopefully that will make it easier to troubleshoot when I get the time. I took the car out to Carlisle, and it was fine for about 2/3 of the drive out there, then blew again while cruising on the highway. When I got to the hotel, I tried to replace it, and it blew right away. I tried another fuse before heading home, which blew right away, and another this morning before dropping my son off at school, and that also blew right away, so whatever was intermittently shorting is now fully shorted.
 
@MadMikeyL
Can you replace the fuse with the door closed? Or holding in the door courtesy switch? Would be interesting to know if it only blows with door open.

Also, I'm guessing the answer is yes, but when blown, do power mirrors not work?
 
That makes it a hell of a lot easier to find, hopefully.
Start disconnecting connectors, to localize where it is; front/rear, dash/ door, etc. Putting a lamp in it now will help a bunch.
 
What I was working on was in two rooms, lol, and no help.
Necessity breeds creativity. You'll move something seemingly random, and it will change. Mine had a metal shard off a gear stuffed thru a wire bundle, shorting two wires.
 
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