Hello all,

I am new to the truck world. We just got property and I needed a truck to haul lumber for the house and other odds and ends. So I found the 78 F100, for a price I could afford and got it. Tuns good, but seems to have some electrical issues. I have done a search and can't find anything that is similar to what I am having happen, so I will explain.

The trun signals inside the cab doesn't indicate they are on or not. I will check the bulbs first, if that is not it?????

The left rear brake light, doesn't work when hitting the brakes.
The left rear turn signal doesn't work.
The left rear running light does work, but goes out when hitting the brakes or trying to use turn signal. Back up light does work.
The dash lights don't work in the cluster, the heater back lights do work and the headlight and wiper dash lights do work.

The speedometer is 14+/- off to the lower, say's 41-42 when doing 55-60?

I am new to Fords or truck in general. This is my first vehicle that I have had to work on. I will be ordering a manual, Haynes or Chilton, which is better?

Thank you for your time and reply.

ArmyHooah

 

Welcome'to FTE.

Hacked wiring seems to be typical on old trucks.

First do a bow to stern visual inspection. Look for taps, crimp connectors and wires that change color on opposing sides of so called repairs.

Clean bulb sockets after looking for corrosion. Make sure bulbs are inserted correctly. Bulbs have two indexing nubs. It is possible to force a bulb into a socket incorrectly - I've done it. Verify that the correct type of bulb is in the correct sockets.

Check and verify grounds using an Volt-Ohm meter.

Apply power and working from back to front of a circuit, test various points along the circuit until you find power. Now, ask yourself if that power is supposed to be present or not. The simplest circuits to troubleshoot are the marker and the brake lights because they are either on or off. that's where I would start. Then move on to the turn signals which use a different filament in the bulbs.

In the end, it would not surprise me if your truck has both a bad headlight switch and turn signal switch but there is troubleshooting to do before coming to that conclusion. Take it one function at a time and the common components should become evident.

Haynes or Chilton? Either or both. I have a Chiltons and a vintage Clymer. What isn't shown or described can be found here or from fordification.net.

On the speedo, the previous owner couldda swapped the trans or rear gear. It could have the wrong driven gear at the end ot the cable which is inserted into the trans... Is the speed error consistent in all gears?

I recommend decoding your warranty plate to understand how it was equipped and compare that configuration to how it is equipped now.

 

HIO Silver,

Thanks for the suggestions, I am going to do some homework this morning and start looking into the problems.

The speedometer is different in all gears.

Thanks

 

As far as the instrument cluster lights go, I would be willing to bet that the backing plate for the instrument cluster has rotted out.
That's a really common issue on these old things, did you see any white plastic bits on the floor perhaps?
They reproduce these but I prefer to go to a salvage yard and pull one out of a late 80's to early 90's econoline. They used the same cluster in the vans for much longer, and somewhere along the way they began to use a much better plastic...

 

Great Idea, I will looking into this. I will need to learn what is interchangeable with this truck to help fix the little stuff. That really all that is wrong with it, the little stuff.

Thanks

 

Thank you both for your post, I am slowly figuring this out. I will explain where I am and maybe ya'll could point me in the right direction from here.

Took the radio out, it was a hack job and wiring was all messed up. Funny thing is, the blinkers work now. Except the right front turn signal stopped working, it has a new socket on it and a new bulb, it worked before I took the radio out. So now I'm really confused. I changed the flasher thingy ma'do. Nothing changed.

Now the left side; brake light works and the turn signals work only if the headlights are OFF. When I turn the headlights on the running light works, but brake light doesn't and neither front or rear turn signals light works, only on left side.

Also, while taking the radio out. The little screws that are holding the metal bracket on, what size are they? 6mm is to small and 7mm is to big, tried 1/4 to big and next size down is to small. Neighbor said it is a Bas%%rd screw?

The fan adjustment on the dash, for the heater/air conditioning just fell apart while taking the radio out. What are my options? LMC doesn't have them in the catalog, so am I junk yard bound? I kind of need the heat, air not so much. Windows 2/70 works fine for me.

Thanks for the patients with this newbie to working on vehicles. I can build a building from scratch and wire a power plant but have trouble with 12v automobiles always have....

 

sounds like a ground problem to me.
regular RA lifer

 

1975/79 F100/350 - 1975/80 Econoline - 1978/79 Bronco instrument cluster backs were made of a composition material that can TURN TO DUST before your very eyes!

1981: Ford came out with hard plastic cluster backs for Econolines, these were also the "service part replacements" for the original POS cluster backs.

Some of these POS cluster backs were replaced under warranty, some were replaced "customer pay." But some were not replaced.

E1PZ-10848-A .. Cluster Back-Use with Oil/Amp GAUGES / Obsolete

1975/91 Econoline - 1975/79 F100/350 - 1978/79 Bronco.

E1PZ-10848-B .. Cluster Back-Use with Oil/Amp WARNING LAMPS / Obsolete

1975/86 Econoline - 1975/79 F100/350 - 1978/79 Bronco.

AFAIK, no one repro's these cluster backs.

Dennis Carpenter in Concord NC (800-476-9653) has several 100 of the GAUGE cluster back and about 50 of the WARNING LAMP cluster back.

Carpenter bought all of FoMoCo's remaining inventory, so these are NOS, not repro's.

 

I'm still really struggling to figure out this wiring issue.

Now, Right front turn signal doesn't work. It has a new socket and good ground.

Left turn signal works front and rear, both have good grounds. Brake lights are working also.

When I turn the headlights on, all the crap starts. Left turn signals stop working, left only and the left brake light doesn't work. But the running lights work.

I just don't get it, I have checked the headlight grounds and the socket grounds for the turn signals. I'm lost.

 

I just ran into this on a '75, I would be willing to bet that the turn signal/brake light socket on the left side is wired wrong. One of the "hot" wires is being used as ground currently and vice versa. When you pull out the switch, the whole thing tries to short.

 

I'm pretty sure I got a repop out of LMC once, it was made from a somewhat translucent plastic that doesn't block out light very well. The whole panel flashes with the signals.....

 

I was thinking that when I started to tear into this, along with some of the other suggestions, checking the grounds. All the wiring except the radio looks to be factory, it evens have the black wrap on it from the factory. Not heat shrink or electrical tape, showing any signs of tampering with it.

After I get paid, I think i am going to replace the turn signal and headlight switch. Heck, I might replace the brake switch it's like $5 bucks.

We will see.

 

Could it be painted Black?

I was looking at the LMC book and was thinking of replacing the dash cluster. I pulled it out when I was fixing the radio wiring, a couple of the light sockets just crumbled on my, so I need to replace it anyways. Circuit board and all, If I'm going to keep the truck, fix it once and be done with it, that's my thinking.

Have also considered pulling everything out of the truck and doing a new harness and rewire everything. I hate electrical issues in a car/truck. Funny thing is I'm an electrician, give me a 5000 amp service at a factory anyway of the week, give me a car issue I hate it!