How to test sending unit

dlampe · #1 (**permalink**) 09-30-2007, 03:18 PM

I am trying to test a low pressure sending unit that has a range from 240 (0 psi)ohms to 33(15psi). At 0 pressure it should read 240 but with the unit out of the car and on the table my ohm meter is not getting a consistant reading. I put one terminal on the stud and one on the base were the 1/8 pipe threads are. What do I need to do to test the sending unit? does it need some sort of 12v hook up?

Tom Wells · #2 (**permalink**) 09-30-2007, 04:27 PM

dlampe,

If the sender is supposed to measure 240 ohms, it should measure 240 ohms unless your voltmeter is bad

The voltmeter applies its own voltage (that's why it has its own battery). If in doubt, you could put a fresh battery into the meter...

Your method of measuring the sender removed from the car is correct!

Tom

RICK LAKE · #3 (**permalink**) 10-02-2007, 04:46 AM

Dean there is a 10% fudge factor built into senders. Grounds are VERY important for correct readings. Some senders can be adjusted. They have a small screw or bolt that is painted to hold spec. If you have tape(white) rapped around a pipe that needs a good ground, this may cause a problem. Did you ohm out your meter before reading it?

If you hook up the sendor and hook the ohm meter to the sendor you will get a reading. As pressure goes up, resistance or Ohm's reading goes down. Some gauges can also be adjusted. See you next week, I hope.

Motor is apart still.

dlampe · #4 (**permalink**) 10-02-2007, 05:27 AM

Thanks Rick, I changed the battery and it seems to be working. The gauge is another matter.

wtm442 · #5 (**permalink**) 10-02-2007, 06:54 AM

Go get a couple of inexpensive resistors (33 and 240 ohms, or as close as possible within that range) at Radio Shack and attach each resistor to the gauge terminals. This will at least tell you if the gauge is working properly.

If your test meter is working properly, disconnect the sender from the gauge and measure the sender resistance with the test meter. Good grounds are necessary. Check the accuracy and stability of the test meter by measuring the resistance of the two resistors you got at Radio Shack.

trularin · #6 (**permalink**) 10-02-2007, 07:23 AM

Dean, if you can locate it, try using Copslip as the thread sealer.

It has little copper balls that crush as the threads mesh when the sending unit is installed. It helps with the electrical connection to the block.

Additionally, you can connect a 240 Ohm resistor ( 2W ) to see if the guage moves to the correct location. I recommend you do it first at the block and then at the meter. This will deal with the harness as a factor, if it works at the block, the harness is working.

If it goes beyond this, PM me and I will try to help.