So my 78 ford f100 custom has a problem with the instrument panel. I'd love to get the original guages working so any thoughts would be great. Here's what I know.

Speedometer works ... no surprise.
Turn signal lights and hazard lights work.
The dome light behind the seat works.
No blown fuses or bulbs ... at least that I could find.

The cigarette lighter light, dash lights, instrument panel lights and guages themselves all do not work.

Can anyone give me a starting point?

I've looked and there really doesn't seem to be any corrosion issues.

 

The dome light and cigarette lighter pull off the same circuit, so if the dome light works, you're going to have to start behind the dash and see how the wiring looks. The cigarette lighter gets its power from a 3-terminal junction to the left of the glove box. A jumper wire runs from the block to the back of the lighter, and the lighter grounds by the housing threading around the opening.

Dash lights source off wiring that is blue with a red stripe. It's possible the headlight switch rheostat is corroded or worn. If the tab loses contact with the spring, the dash lights go out.

For the gauges, make sure the connector on the back of the cluster panel is properly seated.

 

I just noticed while playing around, that every once in a blue moon, i'll get the seat belt light and ringing noice when turning the key to ignition.

Also, the headlights seem very tempramental about when to turn on, and when not to, sometimes they'll just turn off randomly without having touched the ****.

 

I took out the headlight switch rheostat and cleaned up the connections, there was quite a bit of corrosion. Didn't bring the lights back on though.

It strangly seemed to help the belt light and the ringing noise but that maybe my imagination.

And the iffy lights, was more from the brights push button on the floor than the ****.

 

question about this red and blue wire, is it supposed to be in the harness that's attached to the instrument panel? because I can't find one on the harness. I'm looking through the my little haynes book now for the wiring diagram.

 

One possibility for the gauges, there is an instrument voltage regulator which is mounted to the cluster back plate and plugs into the printed circuit board (plug looks like 9V battery plug). If the IVR is bad or going bad, this could cause erratic or no gauge operation.

As far as lights, it could very well be time for a new headlight switch. In my case, however, the panel lights were inoperable due to a brittle/crumbling cluster back plate. Over time, the OEM back plates get brittle and once they degrade far enough, it will cause inoperable panel lights. In my 77, the panel lights worked, then worked imtermittently, then ceased working. When I pulled the cluster out, it disintegrated in my lap. I have since replaced it with a new back plate from Bronco Graveyard and all the lights work.

 

Ok, a little update on the guages. It looks like someone had installed an aftermarket guage set at one point. The wires to the oil pressure and engine temp sensors seem to be disconnected. In their place are wires that lead into the dash and then just end. After cleaning out the interior, I found an guage like the ones you would buy at an o'reileys. I hooked the wires up and the little thing does work, at least 2 of the 3 guages. I, however, would prefer to use the original guages.

The sensors look like they have been modified in some way and the original wires don't look like they will ever fit again, could it be that the sensors themselves have been removed to use the aftermarket mechanical guages?

 

The gauges that came in your truck originally are electric gauges. The "sensors" you described are commonly referred to as sending units and originally they are basically variable resistors. Now, the original factory gauges are inaccurate to begin with and after 30 years they are worthless. Even if you replace the sending units, the needle isn't gauranteed to be right.

So, a lot of people (including myself) install aftermarket gauges. Within those are two types - electrical, again, and mechanical. The electrical aftermarket gauges operate on the same principal as the original factory gauges, but the sending units and gauges are not gauranteed to be compatible between the two (different range of resistances). The mechanical gauges work by actually running the signal up to the dash. The oil pressure gauge actually runs an oil line to the gauge, and the temperature gauge runs a long metal line from the sending unit bulb to the gauge. Mechanical gauges are far superior because they give you a true up-to-date reading. Electrical gauges are innacurate because even when working properly, there is a middle man that adds innacuracy. They convert the signal to a resistance, and read the resistance.

Hopefully somewhere in that explanation is the answer to your question.

 

Thanks fmc400, yeah I think you've answered it, I really don't like the look of in operable guages so I guess I'll have to find some way of making the instrument cluster look nice without the guages being present.

Looks like I'll be going with aftermarket guages