Instrument Cluster Problems

[Jzbochat]

Gauges all worked fine yesterday. Went to start up my truck, Temp gauge is reading way hotter than the motor actually is. usually it reads on the E in TEMP, today it was reading on the M on TEMP close to the P. I hooked up another gauge and it was reading 190, which is what my 390 always runs at, so I know it wasn't overheating. The temp gauge will go down when I'm driving then go back up, which sounds normal, but it's a big change in temp on the needle. Fuel gauge goes to full then goes back to it's original spot. Turning the rheostat on the headlight switch makes the fuel gauge go all the way to FULL?!?! Rheostat also turns on both on my turn signal indicators. They don't flash, just stay illuminated. It also cuts off the high beam indicator. Left and right turn signal indicators work, but the opposite side will faintly light up as well. If I turn left, the right indicator will faintly flash as well, and vise versa. Hazard indicators are very dim as well. I've grounded all the gauges and they go to maximum, so I know it can't be the gauges. I tested the ICVR and it has a constant and a pulse, so that's good there. I cleaned the cluster ground. I've replaced the harness for the oil pressure, and temp gauge too. Along with new sensors for both oil and temp as well. Oil gauge is perfectly fine though. It's like my gauges are getting a drop/spike in voltage. Any ideas on how to fix this? Could it be the circuit tape on the back of the connector? It's starting to frustrate me.

[Thipdar]

Gauges all worked fine yesterday. Went to start up my truck, Temp gauge is reading way hotter than the motor actually is. usually it reads on the E in TEMP, today it was reading on the M on TEMP close to the P. I hooked up another gauge and it was reading 190, which is what my 390 always runs at, so I know it wasn't overheating. The temp gauge will go down when I'm driving then go back up, which sounds normal, but it's a big change in temp on the needle. Fuel gauge goes to full then goes back to it's original spot. Turning the rheostat on the headlight switch makes the fuel gauge go all the way to FULL?!?! Rheostat also turns on both on my turn signal indicators. They don't flash, just stay illuminated. It also cuts off the high beam indicator. Left and right turn signal indicators work, but the opposite side will faintly light up as well. If I turn left, the right indicator will faintly flash as well, and vise versa. Hazard indicators are very dim as well. I've grounded all the gauges and they go to maximum, so I know it can't be the gauges. I tested the ICVR and it has a constant and a pulse, so that's good there. I cleaned the cluster ground. I've replaced the harness for the oil pressure, and temp gauge too. Along with new sensors for both oil and temp as well. Oil gauge is perfectly fine though. It's like my gauges are getting a drop/spike in voltage. Any ideas on how to fix this? Could it be the circuit tape on the back of the connector? It's starting to frustrate me.

I think you may have more than one problem here.

The coolant temp gauge will go "hard ON" if the signal wire gets grounded to the engine. Considering how old some of these wires are, and how brittle the engine heat can make the insulation, I'd suspect some of the insulation has fallen off of the wire from the Coolant Temperature Sender. If that's the case, replace the old wire. Consider putting the new wire inside of some split loom tubing for a little additional protection. If you are certain that the wiring coming from the sender is good, your sender could be going out. For a V-8 engine, drain some of the coolant before you pull the sender, or it could make a little fountain on the intake manifold and you'll get coolant in places you don't want. Once the new sender is installed, replace the coolant. I don't know if the same warning applies to an I-6.

Fuel Gauge and light rheostat aren't supposed to interact with each other, but the fuel level signal and the Courtesy Lamp wires do run together to get from the back of the cab to the dash. It could be that they are both being pinched by the metal clip that's above the driver's door - carefully pull your headliner out and look at the wiring that goes along the top of the Driver's door jamb. If it isn't at that clip, it's most likely somewhere along that run. It may help your diagnostics if you disconnect the wire from the Fuel Tank Sender (or the Courtesy Lamp - or both), so that you have some slack to allow for a visual inspection on the wires.

Before going into detail on the Instrument Cluster, I want to know what year truck you're talking about... a '69 ain't the same beast as a '70... it would also help to know which engine you've got.

[Jzbochat]

I have a 1972 f100 with a 390. I was poking around today, all the wires are in great shape. Noticed a tab was missing on the back of the circuit board where the plug goes. Turns out its the constant 12 volt tab that broke off. I will be ordering a new printed circuit board and hope that fixes the problems.

[Thipdar]

I have a 1972 f100 with a 390. I was poking around today, all the wires are in great shape. Noticed a tab was missing on the back of the circuit board where the plug goes. Turns out its the constant 12 volt tab that broke off. I will be ordering a new printed circuit board and hope that fixes the problems.

I sure hope that's it.
Seems odd to me that the circuit board contact would fail like that... since it's not a moving part
Let us know how it goes, and don't hesitate to ask questions if you need to.