Hoping someone can help me or point me in the right direction. Like many of these old trucks, the electrical system in mine has been "modified" many times over. I'm trying to return some kind of functionality to the stock temp gauge.

It's a 77 F250 w/ a 460.

Temp sender in block seems good but this is where it gets weird -- the wire from that sender may go to the dash (but grounding it out doesn't peg the gauge to full hot, so I'm guessing it doesn't) -- but I do see continuity between that wire and a terminal on the starter solenoid -- WTF?

That same solenoid post is then connected to an electric fan relay.

It's clear to me that someone tried to make the fan come on when the engine gets too hot -- but -- that doesn't seem to work because the fan comes on when the engine is off/cold and the key is in the "on" position.

I can easily tackle and set up the fan/relay the correct way, so let's just take that out of the equation.

What I can't do is find the original wire that goes to the dash temp-gauge... Does anyone have a wiring diagram or method so that I can figure out what color wire this needs to connect with or at least short it out so I can see if the gauge reacts at all?

 

Accessory power is supplied to a small electromechanical regulator on the back of the cluster through an 8.5-ohm resistor wire. The supply side of the temperature gauge is connected to the output of this regulator. The sense side of the gauge leaves the printed circuit on the back of the cluster and runs out to the sending unit as a RED with WHITE stripe wire.

 

Thank you! The red/white wire is the one that is at the sensor but I will check with continuity between that and the gauge.

Shorting the sender side of the gauge to ground should result in the temp gauge going all the way to "HOT" correct?

 

That's correct.

 

I've finally found time to take the dash apart and try to figure this out. I assume the Red/White stripe wire is in position #8 on the 20-pin plug on the back of the panel?

I'm getting something very weird when I put my multimeter on that -- it "sweeps" from 7v - 1v back and forth. I have a digital multimeter so it's hard to tell exactly what it's doing.... Any thoughts?

 

I don't have access to the factory diagrams for '77 (just '76 and '79) so I can't confirm the position in the plug, but you can trace out where it should go by starting at the "sense" input of the temperature gauge, tracing where it goes along the printed circuit and out to the connector, and matching it up. In other words, let your truck tell you.

The meter readings you're seeing are from the instrument cluster voltage regulator "upstream" of the temperature gauge. The regulator is an old-school electromechanical regulator. It switches back and forth between 12 volts and zero at a rate of less than 1 Hz, to give an RMS voltage of around 5 volts. The sweeping effect seen on your meter is just the meter's autoranging function trying to keep up. The numbers don't really mean anything. The only way to get a really good measurement of the regulator output is to use an oscilloscope that can perform an RMS measurement of a slow-moving waveform.

 

Thanks, FMC -- The correct wire is indeed the R/W in position #8. When I ground this wire, it pegs the gauge. Still getting that weird voltage sweep but understand what you are saying about the regulator (I've never run into this before.)

I can't find any cuts or splices in the R/W wire but a large portion of it is inside of the wire-loom and I don't feel like making a mess of that. I ran a new wire to the temp sensor and I'm at least getting a reading at the instrument panel now. I am going to replace the temp gauge (and try to borrow a real temp gauge) to make sure I am covered. The cooling system on this truck is interesting (to say the least) and I want to be sure I don't risk overheating it when I get it back on the road.

 

Why are you replacing the gauge? You said you shorted the wire and the guage did peg, so it's probably working.

And you are correct, the whole point is moot, the factory gauge doesn't tell you anything anyway, a aftermarket gauge with real numbers on it will tell you what you need to know. You should get a oil pressure gauge also.

You are still getting that "weird" voltage because of what was posted before, that's how it works, that is normal for the voltage supply to all the gauges, even the fuel guage.

 

Sorry, typed that wrong. I'm going to replace the temp sensor. Even though the temp gauge doesn't tell me anything... with the right sensor, it should tell me if I'm overheating or getting too hot.

 

Make sure you do not use thread tape on the sensor. If you use any type of sealer, use it sparingly and just on a few of the threads. This sensor needs a good ground to the engine block to work correctly.

 

Thanks! $8 later and I've got a perfectly working temp gauge again..... After idling for 20 minutes it just stopped dead in the middle of NORMAL.

 

Yea. whatever temp that is

In the winter when it's really cold outside(don't know where you live, but about 20 F) you can guess at one of the temperatures. If you keep an eye on it, you will see the needle go up on the guage, and then suddenly drop back. That's the thermostat opening and then closing. It will do this a few times till it settles down. You can figure that part of "normal" is the temp of your thermostat. My old truck would bounce back and forth at the "o" of "normal", so I knew the "o" on my truck was about 190(that was the rating of my thermostat I had installed).

 

Great tip. I'm in Minnesota.... so... it gets DAMN cold.

I've got a 195 thermostat in there and I could see it bounce a little bit. I more or less just wanted to be able to give the girlfriend a heads up that if she is ever driving and it goes towards the top end of the gauge... PULL OVER!