Inaccurate/Faulty gauges

[jgf100]

HI, the gauges in my 70 f100 either are inaccurate or don't work at all. The fuel gauge only goes to 3/4 full and the temp gauge has no reading. I've replaced the wiring harness(was needed anyways), gauge voltage regulator, new braided grounding wire from engine to cab, printed circuit board, fuel sender and coolant temp sender. Also removed external voltage regulator and use a high amp alternator. The oil pressure seems to work pretty good. I've grounded the wire from sender to gauge and the gauges do move to the full/hot position.
What else can I look at?
Thanks for the help.

[Fraser]

Constant voltage transformer. It attaches to the printed circuit and supplies the correct voltage to the gauges.

[jgf100]

Constant voltage transformer. It attaches to the printed circuit and supplies the correct voltage to the gauges.

Hi, thanks for reply. I did change the voltage transformer that is attached to printed circuit. Still see the issue.

[1972hiboy]

Time to break out the multimeter. some if not most of those instrument panel voltage regulators are adjustable. You will need to find a spot on the back of the gauge and probe voltage and verify that you are seeing a pulse of 5vdc ( or at least determine the best you can that you are as close as possible) once thats verified time to attack the gauges one at a time. I know the fuel level gauge has needle adjustments for full empty and full full via tiny screws behind the gauge. Just gotta massage the limits on the gauge out. then go to the next gauge. but its important to make sure you start the dash out on a level playing field by making sure you are getting pulsed 5vdc because the voltage will change the sweep of all the gauges.

[Jbarnes79]

In addition to 1972hiboy's recommendation... I believe the voltage regulator is case grounded. Give the housing an additional clean ground and see if that helps.

[zgerbic]

With electrical problems, making sure good grounds are essential. You did not say if the gauges ever read correctly in the past. Working gauges that now don't work are troubleshoot differently than those that have never worked.

If you want to know if your gauges are accurate, check this post: http://fordification.com/forum/viewtopi ... er#p773508

If you can build one of these or at least get a couple of resistors to check for low and high readings on the gauge, you can determine if the gauge is ok and reasonably accurate. Substitute the tester/resistors for the sender. If the gauges read ok, then the instrument voltage regulator is also good. This only leaves the senders as problems.

If gauges are tested as working and reasonably accurate, troubleshoot the senders. Fuel gauge reading low is often from a bad float on the sender, bent wire that the float is attached to or a weak/poor ground. Remove the wire from the gas sender and measure the resistance when the tank is full. It should read between 13 ohms and 8 ohms, spec should be 10 ohms. Keep in mind to subtract the resistance of the test leads themselves as they may be a couple of ohms themselves. It is likely the resistance across the fuel sender will be 20 ohms or so which makes the fuel gauge read low. Next is to pull the fuel sender and measure the resistance while moving the float wire from one end to the other. It should move from about 73 ohms +/-5 ohms (empty) to 10 ohms +/- 3 ohms (full). If not, the gauge is defective. If it reads right, the float arm might be out of adjustment. Hold the sender as it would be in the tank and gently raise the float till it stops and see if it moves too high and would hit the top of the tank. Also check if the sender has absorbed gas (foam float) or some gas has leaked into the float (plastic/brass hollow tank). This is like checking the float in your carburetor.

Temperature gauge reading wrong is often from a bad ground between the threads of the sender to engine, bad sender or incorrect sender resistance range (not the right type of sender for the gauge type you have). Let the engine cool down, remove the wire from the sender. Measure the resistance from the sender terminal and ground. It should be around 73 ohms cold. If not the sender is bad or the wrong one. If it is a higher resistance, like 200 ohms, it could be a sender meant for another type of car, perhaps a GM or later model Ford. If you heat up the engine to operating temperature and take the resistance measurement again, it should be around 25 ohms (which would make the gauge read about the middle). Buying a new temperature sender today can be a problem since many are inaccurate when new or some generic resistance range that sort of works. If you determine your sender is bad, check the new replacement before installing. Check the cold resistance for about 70 ohms to 80 ohms, and put the sender in boiling water and it should be about 15 to 7 ohms. If not then return it for another new one.

The measurements I provided are shown with a bit of +/- range because the senders and gauges are not real accurate to start with and the factory does not provide any calibrated markings on the gauges either.

Another link to look at is: http://fordification.com/forum/viewtopi ... 87#p774187

Hopefully this will provide some additional info to reach a fix for your gauges.