What would cause the ohm's to fall after tightening up the screws?

I have an instrument cluster for an '84 Toyota truck that has a bad fuel gauge receiving unit. I ordered another cluster and the fuel receiving unit also tested below what the manual calls for with the screws in it. However with the screws loosened or removed the ohm's read right where the manual states they should be.

Pick of area I am dealing with.


The 3 screws(upper right section) are Fuel, engine, ignition
Manual states approximate readings should be:
Fuel-to-Engine 156
Fuel-to-Ignition 83
Ignition-to-Engine 239

Readings when screws are tightened:
Fuel-to-Engine 90
Fuel-to-Ignition 67
Ignition-to-Engine 79

Is the unit bad, or is the circuit panel causing the problem?

Would electric diode grease help or hurt?

You might try some emery cloth to clean up the copper where all the connections are
Russ <img src="/forums/images/graemlins/kewl.gif" alt="" /> <img src="/forums/images/graemlins/cheers.gif" alt="" /> <img src="/forums/images/graemlins/patriot.gif" alt="" />

"Ohms to fall" i.e. lower.. well as you tighten the screws on any connection the resistance will tend to drop as the connection becomes better..

So far as what the gauge is reading, I would strongly suspect its resistance reading will be non-linear .. i.e. the resistance reading will vary with how much current your ohm meter is attempting to put through it.

Try measuring it with the ohm meter on higher and lower resistance scales and see if the reading looks consistent. If not, the resistance is non-linear and the correct resistance will be read only when, a specified amount of current is applied through the meter.

A tale tale sign in the FSM would be if it calls for the use of a specific Toyota test meter to measure the resistance.

The ohm meters(both digital and analog) I have read correctly for the other items on the cluster, so it isn't that. The readings were also the same when using the higher settings.

After messing around with it yesterday, I determined that it is an issue with the ground circuit on the cluster itself. With the Fuel and Ignition screws in and tight, I would get the correct readings between all three as long as I left the Engine(-) screw out. I decided to isolate the ground and ground it to the vehicle.

This appears to have worked. When I plugged the cluster in, the fuel gauge was above the "Empty" line, and fell down to empty with the key on. I put 4 gals in and the gauge rose to 1/2, which is about right, there was about 3 gallons in it and it is a 13.5gal tank. Just to make sure it was the ground, I removed the ground wire and returned it to oem, and the gauge remained at empty. I will fill it up later this week and see if the gauge moves to full.