QuestionI HAVE A 1938 PLYMOUTH P6 AND AM HAVING TROUBLE WITH THE FUEL SENDING UNIT.I HAVE BEEN TOLD THE UNIT IS 78 ohm AND NOW HAVE 2 DIFF UNITS THAT ARE NOT WORKING.THE STOCK UNIT WORKED FOR A WHILE BUT STOPPED AND WENT TO EMPTY.I BOUGHT A UNIT THAT WAS A REPLACEMENT 78 ohm,WHEN I HOOKED IT UP,I GOT 1/4 TANK READING ON DASH GUAGE(AT EMPTY WHEN MOVING FLOAT)AND 3/4 TANK (AT FULL WITH MOVING FLOAT).OTHER IS A 30 ohm AND IS WORSE.WHEN GROUNDING OUT FUEL GUAGE GOES TO FULL.AND WILL DROP TO EMPTY WHEN REMOVING GROUND.HELP!!! THANKS, JEFF
AnswerIf I understand you right, you are testing the senders by hooking them up to the dash gauge and grounding the sender body to the chassis, then moving the float arm from one extreme to the other.
That is the correct way to test them.
If you have an ohmmeter, check the senders to see what the resistance actually is for the float arm all the way down and all the way up. It is possible that your "78" ohm unit isn't going all the way to 78 ohms, or all the way up to the minimum value required to make the dash unit read full, which is probably somewhere around 6 to 10 ohms.
The resistance should be maximum when the float arm is all the way down. In the case of the 78 ohm max unit, I'm surprised that it did not make the gauge read empty, or below empty. Either it didn't go to a high enough resistance, or it is the wrong resistance for your gauge, or there is something wrong with your dash gauge.
I would go to the local Radio Shack store and buy a selection of resistors so you can calibrate your dash gauge. If you can find a wire wound rheostat which covers a wide enough range of resistances (say 100 ohms max, 0 ohms minimum), you can calibrate your dash gauge that way. But, if even 78 ohms isn't high enough, you'll have to get a higher resistance sender.
I don't know the specification for your car, and I don't have any way to look it up either, but 78 ohms seems high enough that it should work - more typically the senders are around 70 ohms at empty, and around 6 ohms at full.
Did you check the resistance with an ohmmeter when the float arm was all the way down?
If you have another dash unit to double check with, you could try it with the 78 ohm unit to see if that makes it read right. If it does, then I suspect there is a problem with the first dash unit.
You didn't say what the 30 ohm unit did when you tried it. I would have thought it would have shown "full" when it was all the way up, and maybe around 1/2 tank when it was all the way down, if the resistance was too low for your car. If it did that, then your dash unit is probably OK.
I hope this is some help - I think you are going at it the right way, I just don't know what the proper resistance range is for your car. Perhaps you will have to get a factory shop manual for the car to find that out. It's a good idea to have one anyway. You can probably get one from one of the auto literature dealers. I use Ed Faxon at www.faxonautolit.com but there are many others who offer the same service.
Good luck to you - those are really great cars!
Dick
