I need some help with dash gauges. I have a 1970 F250, 360 with dash gauges. When I first got the truck the gauges didn't seem to register their true values. The temp and oil gauge read on the low side and the fuel gauge didn't work. After troubleshooting the fuel gauge system I found that the sender was bad (float full of fuel) and the IVR wasn't within specifications. I replaced the fuel sender and IVR. The fuel gauge reads 3/4 full when full. I cleaned all ground connections I could find. The gauge tests good. The oil and temp readings stayed the same at the low side of the gauge. I checked the gauges and both oil and temp gauge check good. I replaced the sending units and checked the wiring which turned out to be ok also. Last weekend I was traveling at night and used the hi-beams and after several minutes the headlights started to flicker off and on. I pulled over, set the light switch off hi-beam and let the switch cool (it was very warm). When I returned home I replaced the headlight switch and dimmer switch. Both where corroded inside.
Now here is the strange part: When I start my truck in the mornings (it's still dark) I turn the headlights on and warm up the engine. The fuel gauge reads full when full and the oil and temp gauges read about in the middle of the gauge. Which is where I would expect them to be. As I drive to work the with the headlights on all the gauges are working normal. (as I would expect them to do) When I come home from work with the headlights turned off the gauges go back to the old values.
Does anyone have an idea of what may be the cause or better yet a solution to this problem? I believe it has to be in the wiring harness or I'm thinking that my voltage regulator may need to be adjusted ot replaced. My thinking is that there is higher draw from the altenator when the lights are on and less when the lights are off and this is in fact what is causing the different readings on the gauges. As always any and all info is appreciated. Thanks
Jeff

 

Have you read a test procedure for the gauges ?
First off remove the sending unit and clean it then replace.
Use conductive sealer on threads...with 5 volts being sent ...it is the Amount of ground that determines the reading on the gauge.The more ground out the the higher the reading.

1st) Try testing with analog Multi-meter...then with a test light.
Measure the voltage at the S/U plug ...should be 5 volts Blink...blink...blink.

Do the same for oil gauge system.

 

"My thinking is that there is higher draw from the altenator when the lights are on and less when the lights are off and this is in fact what is causing the different readings on the gauges. "

The IVR(instrument voltage regulator) is supposed to keep this from happening.

Have you done the grounding test on all the sending units? Especially the gas guage. Take the sending unit wire off the tank, and with the key on, ground the wire. The fuel guage should swing all the way full scale one direction. Then take the wire off, and the guage should swing the other direction.

If you are suspicous of grounds, then try grounding the sending wire to the frame, then the tank, and then the sending unit itself to see if there is any difference in the readings. You could also run a wire directly to the battery negative to see what kind of reading you get there.

About the headlamp/guage problem, I could see a possible problem if the instrument cluster was not grounded properly. When you turn the headlights on, the dash lights come on in the cluster, and if they did not have a good return ground, the current may find another path through the guage circuit, making it read differently.

 

Yes, I've done the grounding test on all the gauges. Each gauge swings full right when tested. I tested each gauge per the factory manual. I did not use any conductive sealer on the oil or coolant sending units. I installed them dry. How is the instrument cluster grounded? I'll test the voltage to each gauge today. Thanks for the info
Jeff

 

"How is the instrument cluster grounded?"

Sometimes only through contact of the back of the cluster to the metal edge of the opening in the dash.

Years ago I took my 65 mustang dash apart so I could re-paint the dash. Put it all back together and noticed the dash lights were doing all kinds of crazy things. I would put the turn signals on, and the dash lighting lights would flash! I finally figured out the paint I sprayed had covered up some bare metal spots where those speedy nuts are clipped around the opening to hold the cluster in place. I scraped the paint off and it has worked fine ever since.

 

It grounds through the connector then to the dash brace.

Have you checked for a ground wire strap from engine to firewall ?

 

Just finished checking the sending units. All checked fine with 5 volts pulsating to each sender. I do have an engine ground strap at the back of the engine to firewall. I did not remove it nor clean where they bolt. Looks like the original. It looks like it needs to be replaced. Some insulation is missing, it's cracked and looks corroded. Is there another strap located elsewhere? Thanks for your imputs.
Jeff

 

That strap being corroded is a possibility. Also don't be afraid to add grounds. If you suspect the dash cluster may not be grounded properly, and you can see where it should be grounded(I don't know if your year has the circuit board or not), then by all means run a short piece of wire from a piece of metal under the dash to the cluster. Then the ground strap you spoke about should ground the firewall/dash sheetmetal, and the engine block is obviously already grounded by the big negative wire from the battery.

You may want to run a ground strap from the negative battery or engine block, to the frame too. The frontend sheetmetal. body, and engine/tranny are all mounted in rubber mounts, so an aux ground won't hurt going to the frame.

I had an older truck that suddenly everytime I hit the brake pedal, the dash lights would light up. That turned out to be a grounding problem on the bed. The whole bed of the truck had somehow become un-grounded from the frame. A small jumper wire bolted from the bed to the frame fixed that one.