My 76 F100 has no power to the instruments. The only things that activate when the key is turned is the idiot lights for the brake and fasten seatbelt. None of the gauges will operate. I've replaced the fuses, and even swapped instrument clusters to check and see if it was localized to just the one I had, which it isn't.

I'm also having issues with getting the fan to work for my a/c and heat. I hear only what I can describe as a "relay" under the dash clicking, but the fan motor will not operate. Can someone please provide a diagram for removing the heater box on a truck with factory a/c? I'm not quite sure where to begin here.

I've got a few more electrical issues to cover, but for now these are the most pressing ones.

Thanks in advance y'all.

 

For your dash lights, check to make sure that all your bulbs are tit and that you have SOLID connections between the bulbs and the circuit board

 

Power to the gauges is not protected by a fuse. The gauges themselves are powered by a 5-volt regulator on the back of the instrument cluster. The regulator is powered through an 8.5-ohm resistor wire which is part of the dash harness.

Either you do not have power to the cluster through the resistor wire, the sending unit to each gauge is coincidentally bad (which can and does happen), or the ground out at the sending units is bad (which can really only be an issue with the fuel gauge). The ammeter (if equipped) is not part of the instrumentation rail.

I realize I'm not answering your question, but do any of the speeds work at all?

 

FMC400, thanks for the info bud. I'm guessing the fuse in the fuse block for the instruments is just for illumination then? Here I was hoping it was going to be an easy fix.

I also wouldn't think that I got two clusters with bad IVR's, and gauges? Is there any way to test them? Also where in the harness is the resistor located? If I have to replace it then I'd at least like to know where it is.

And with the heater, no I'm not getting any fan speeds at all, when it is turned on all i hear is one solid "click" under the passenger side of the dash, near the heater core/box assembly. I've found the relay, and that is what is clicking, but I get no other response than that.

 

That's correct; the small fuse in the fuse panel labelled "INST" or something similar is for the dash illumination.

It's highly unlikely you got two bad ICVR's in a row. You can test the ICVR by hooking up a test light; it should flash about once per second. Here is some more info on the ICVR: https://www.ford-trucks.com/forums/1...-the-icvr.html

The resistor is a wire bundled in the dash harness; you won't see it unless you completely peel away the harness wrap. These hardly ever go bad; ICVRs seldom go bad either. In most cases the problem is downstream at the sending unit. Turn the key to RUN or ACCY and ground out the sending unit lead; if the respective gauge pegs all the way to the RIGHT; then everything upstream of the sending unit is fine.

Do you have access to the blower motor connector? If so, do you get 12 volts there with the speed all the way up and the relay clicked ON?

 

fmc, the odd thing is...when I got the truck all of the instruments worked, and my wipers worked. Suddenly they stopped working, and everything I have tried has not restored them. I'll try to ground out the wiring going to the sending units for the instruments and see what that tells me.

The heater has never worked, so I'm kind of saving it for the "last push" to get fixed. I'm not even sure where the power connector would be on a truck with factory A/C. I'm more familiar with the 80-86's and can pretty much identify everything blindfolded by feel on those.

I know i'm at least getting 12v from the fan switch to that relay under the dash, but after that I'm clueless what I need to do next. I really need to get the shop manual for my truck, but I was hoping someone would have the electrical diagrams for me to sort this out easily. I do have the manual for a 74, but I doubt much is the same.

 

If the instrumentation was working and then all of the sudden stopped, I would be focused on the instrument cluster connector as was originally suggested. They have a habit of slipping out.

Factory wiring diagrams for 1976 are available here: 1973-1979 Ford Truck Wiring Diagrams & Schematics - FORDification.net

 

Thing is, I already swapped clusters... and the connector was/is tight on both. Not sure what else to make of it, but I'm going to keep digging. Thank you for the info fmc!

 

Got some more information today, still haven't grounded the wiring to the sending units yet but I did think to check something else.

I decided for giggles to check the fuse box and see what fuses had power. Every single one of them had power but the instrument fuse/illumination fuse in the box.

i'm not sure what to make of this aside from maybe a broken wire or bad contact in the fuse box. This single fuse had no power at the contacts, across the fuse, etc. Fuse did not look like it was blown, but I may be wrong. I'm going to go back to the shop in awhile and check it over real good.

FMC, you got any more ideas?

 

As previously mentioned, the instrumentation (including the ICVR and all gauges) are not protected by the fuse panel in any form or fashion. You could completely remove all fuses, and the gauges would know no difference. Circuit protection for the instrumentation is accomplished with an 8.5-ohm resistor wire upstream of the ICVR. Among other things, one of its purposes is to provide current limiting (and therefore circuit protection) if a gauge shorts out. Again, fuses have nothing to do with the gauges themselves.

The short INST fuse is solely for the dash illumination (not only the instrument cluster, but also the headlight/wiper panel and heater controls). Power to this fuse comes from the headlight switch as a BLUE with RED stripe wire. The path for this circuit is: power into headlight switch, dimmer rheostat inside switch, BLUE with RED stripe wire out of headlight switch into fuse panel, fuse, BLUE with RED stripe wire from fuse panel, to the rest of the bulbs.

If you have no power at this fuse, then the headlight switch is bad, or the dimmer is turned all the way down. Again, it has nothing to do with the gauges.

 

Sometimes the instrument cluster connector seems tight in the cluster, but one or more of the contacts are not making a good connection between the connector and printed circuit. I briefly had some of the same issues after I took my cluster out and put it back in.

 

FMC, thanks again for the info. I know the fuse did not have anything to do with the gauges themselves, i just found it odd that there was no power on that circuit when there should be. I just kind of checked it for giggles sake ya know?

The headlight switch does work, but could probably be turned down. I'll swap it with another known good switch that I have and retest that circuit if I still get nothing after adjusting the dimmer for the dash illumination.
I'll search tomorrow for the resistor in the feed to the instrument cluster and see if it is burned out or not. I'll also recheck the connector for the instrument cluster and make sure it is inserted all the way.

I do appreciate the help guys!

 

I had the same issue with my instrument lights not working when I first got my truck. It ended up being a bad headlight switch. Maybe someone already mentioned that (havent read every response).

 

Ok I now have some real and relevant data that may help here.

I removed the instrument cluster and checked to make sure the plug was seated all the way. I even removed the plug and reinstalled it. Still no difference.

Then I removed the instrument cluster, and took my multimeter and probed the contacts at the instrument cluster plug. I don't know precisely which ones are supposed to have power, but I know that there is power available at the plug, with two connections at least with 12v. A few others pulsate between 0.3v and 1v.

I also pulled the ignition switch connector and tested it for power just on a whim and it also has power where it is supposed to. (I knew this already, but i wanted some hard data on voltages there).

I also looked at the instrument cluster wiring harness, and attempted to find the resistor that is supposed to be inline with the wiring but did not find it. I'll take another look at it tomorrow, I just wanted to pass on the info I did find and see if anyone else has any ideas.

 

I finally peeled back enough of the instrument cluster wiring pigtail to see the resistor that is inline with the green/black wire coming from the plug. It ties back to the main harness at two wires that are green/black and green/red.

I measured voltage at this wire and it is 0.06 volts, and I cannot get it to give a resistance reading. It has no markings on it, does not say how many ohms it is, or any other information.

If anybody knows what the particulars are on this resistor, or what to replace it with please let me know. I want this little saga over with asap.