Electrical 92 GT Convertible - Fuse 13 Continuity Testing Question

Blown88GT said:

1. I got my tester yesterday. While you're waiting for yours, you need to make a fuse box adapter. Take a blown fuse (you have many), solder 2 wires to the test points on top of the fuse. This is what the "tester fuse" alligator clips will connect to.
2. I don't know.
3. Looks like it is.
4. I didn't open the "tester fuse" up, it's a thermal automatic resettable circuit breaker. The "tester meter" works by magnetic induction. You don't have to open any splices or bundles, just yet. You connect the "tester fuse" to the adapter you made which plugs into #13. You will work with the meter back from the fuse box (#13). If there is no short, you won't see anything. Make sure the dimmer is set for "dome light on". You want to force the "tester fuse" to trip, if the short exists. You don't need the test light, the dome light will serve as a test light.

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WillWho? said:

Hope y'all don't mind me jumping in here. There are two other components in common with the dimmer that also energize some fuses and some fuses energize those components.

When a light is dimmer, generally it means voltage is dropping either at or before that light. That means the intermittent short is likely a close arcing short. It is not likely a direct short which is why it takes a bit to heat the fuse (breaker) and blow it.

The Multifunction Switch (MFS) and Ignition Switch Module (ISM) are both common to the dimmer and head light switch. Both oh those have trouble shooting guides in the EVTM. Both are known common points of failure.

Remove the lower steering column. The MFS is by the driver's left knee and the ISM by the right knee. The ISM is the most likely point of failure. The plastics get brittle with age. There is a full blown recall on those, but it is not worth the time taking it to Ford. The part is under $20 at local parts stores. It is about a 20 minute job. I believe you need a security torx driver to remove it. (It has been a while.) Be careful when you first touch the ISM, they like to crumble into a gazillion pieces with some getting your eyes. Sometimes just the retaining clips break and you can push the connector tighter. Zip tieing the connector to keep it from coming loose is OK for a day or two. However, you are risking yoyr life, or even worse your car burning burning to the ground. The rest of that brittle plastic is just waiting to crumble.

The MFS has a very good troubleshooting guide that will let you know if it has an internal failure. Having one fail is not as common as the ISM.

For the main/fog switch, you should probably make a modification that is quite simple to emilinate the poor design of that entire circuit. I will resist aiming a dig at automotive electrical engineers.

Here is a diagram on how to modify the fog circuit by adding a relay. This is the easiest fix. BTW, rather that letting the fogs energize from the low beam circuit, you can energize from the parking lamps or an accessory fuse panel. That way you can have fogs with just parking light, or fogs with just fogs. Just run a 12g wire from the relay to the fogs, and a 12g wire from the 12V source to the relay. The factory wire on the relay trigger is sufficient since the amp load no longer passes through that wire. You could add a relay for the headlights too, and completely remove the amp load from the switch. The headlight/fog switches were TSB's for 87-93. The factory never fixed the problem. Even when my 89 caught fire on a trip home from New Orleans. Fortunately, I do not panic and removed the hottest fuse as I moved into the emergency lane. I made a field expedient modification (aka MacGuyver) to have headlights the rest of the trip. Ford had not issued the TSB and I made the repairs at my expense. Which was fine, I added relays and had fogs available with parking lights.

I am not an electrical engineer. I am a retired aircraft instrument mechanic (analog/electronic), former AC/DC theory instructor, advanced circuitry instructor, micro-miniature solder instructor, multilayer circuitboard repair instructor, aircraft instrument calibration specialist (analog/digital), & forensic aircraft instrument analyst. This was for aircraft that travel at speeds from slow to Mach 2.5 and faster.



Another problem is the courtesy light switches in the doors and the hatch. The internals get oxidation, wire insulation gets brittle, and those lights tie into the dimmer. So if the ISM and MFS check out OK, take a look at those.

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Blown88GT said:

What does the current (old) switch & plug look like? You can tell by looking at it if it was overheating. The switch is not only a switch but a thermal circuit breaker.

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Blown88GT said:

It's possible that relay could be the source of the intermittent short.
Does the radio illumination dim when the headlight switch is turned on?
That's all it's for.

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Blown88GT said:

Makes me wonder if that's where my intermittent short might be. The under hood light thing is probably just a coincidence.
Probably best to pull the top off the center console & have a look. Never had the top off it, doesn't look too hard to get to. 20 years ago, I had a lot of heat coming out the console for 1 day. It went away & never came back. There was a recall on the cats, Ford Replaced them. Sold them as like-new when replaced with hi-flow cats.

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