Bought a '79 F350 a couple days ago and I'm tracking down the issues that need to be fixed. Right now the priority is getting the instrument cluster working. The speedometer doesnt work (apparently it did on and off then stopped) previous owner thinks its electrical not the speedo gear. Also the cluster lights dont come on at night. Tried playing with the dimmer, no luck. What I need is some help, advice with the wiring back there, and maybe a breakdown of the fuses so I can start trouble shooting this. I'm hoping I can find a short back there to patch and miraculously the dash lights and speedo will work again. All the other gauges seem to be working fine, but the gas gauge I've noticed takes a long time to adjust. Maybe related, maybe not. Any helpful advice will be great. Going to try and find a chiltons manual as soon as I can. Thanks for the help in the meantime.

 

the speedo issue has nothing to do with electrical. the whole system is a gear in the transmission, a spinning cable from there to the gage, and a mechanical gage. if the odometer is working then the gear and cable are ok, so the gage itself is the problem. if the odometer isn't working, pull the cluster out and spin the little nub that the speedo cable goes into, and it should register a response on the gage. spinning it with your fingers is usually fast enough to get a response, but if not you can use a cordless drill at low speed. from there, you'll know for sure if your problem is in the cluster or not.

for the lighting, i know that in 80s trucks, power flowed through the marker lights fuse to the headlight switch, where it goes to the marker lights and the dimmer for the dash lights, and from there to another fuse before it gets to the cluster. i suspect its the same on your 79, but i don't know what fuse is which, except that it should be a 5a fuse going to the cluster lights.
its also fairly common for a previous owner to have neglected it long enough that all the light bulbs in there are burned out, and it has no wiring problem at all.

the gas gage is always slow to adjust. thats why fords have a steady needle whereas chevy has a bouncing needle on the gas gage.

 

That is very helpful info! Thanks man, that'll give me something to start with. The last few part stores haven't had a manual for this year so I may be ordering one online. Thanks again!

 

Would anyone by chance know what size speedo cable I'd be needing? Tooth count, length? I see there are 95'' and 118'' lengths, not sure on tooth count either...
Thanks again

 

for speedo cable, the gear is sized to match your combination of axle ratio and tire size, and is a seperate piece from the cable.
i suspect the cable would be the shorter one for 2wd models, or the longer one for 4wd models.