Multimeter readings on the wires to the fuel pump... problem?
05-16-2011, 03:16 PM
#1
Instructor
Thread Starter
Multimeter readings on the wires to the fuel pump... problem?
First, let you know that I have another thread with the different tasks I'm working on to make my Euro '80 928 run again (previous owner has it stopped for 3 years).
But I would like to ask your opinions about the check readings I did yesterday on the fuel pump cables. I don't know if that resistance values in those cables are normal, but I have doubts:
To give additional information I should say that the old pump from previous owner on the car was removed because it didn't worked, we can not hear anything from it, maybe a low 'clok' sound when connected but never move (we thought it was jammed). We bought a new pump, and start working good, run smooth for 30 min when bridged on the ce-panel before fist cranks. But after 4 tries with the car almost running alone it stopped working (same 'clok' sound, not moving). We thought it was jammed again with dirt from the tank.
I'm working now in the process of cleaning fuel system, removing tank for flushing it, etc, but woudl like to ask in case there is an electrical problem that is frying the pumps. I'm completely new about mechanics so have that in mind if answering. Thanks a lot.
ps.- Here is a picture of an electric schema I have from a usa 1980 if that helps. Mine is euro, don't know if it is exaclty the same...
But I would like to ask your opinions about the check readings I did yesterday on the fuel pump cables. I don't know if that resistance values in those cables are normal, but I have doubts:
To give additional information I should say that the old pump from previous owner on the car was removed because it didn't worked, we can not hear anything from it, maybe a low 'clok' sound when connected but never move (we thought it was jammed). We bought a new pump, and start working good, run smooth for 30 min when bridged on the ce-panel before fist cranks. But after 4 tries with the car almost running alone it stopped working (same 'clok' sound, not moving). We thought it was jammed again with dirt from the tank.
I'm working now in the process of cleaning fuel system, removing tank for flushing it, etc, but woudl like to ask in case there is an electrical problem that is frying the pumps. I'm completely new about mechanics so have that in mind if answering. Thanks a lot.
ps.- Here is a picture of an electric schema I have from a usa 1980 if that helps. Mine is euro, don't know if it is exaclty the same...
Last edited by rockatansky; 05-16-2011 at 03:35 PM.
05-16-2011, 04:59 PM
#2
Drifting
Apparently your reading the coil resistance of the 'supplementary air valve' off the thick red wire. The other measurement seems reasonable, but the schematic shown is not complete enough to know where the wires go from there.
05-16-2011, 05:20 PM
#3
Rennlist Member
Rennlist Site Sponsor
Don't worry about the resistance reading, as it is meaningless to you. As Borland says, you are reading at least the resistance of the air valve that is on the same circuit.
One comment about your testing - voltage is not power. Checking for full voltage on the fuel pump cables is a good thing to do, but it is not the definitive test.
Imagine that you have a break in the power supply wire or the ground wire to the pump, so that there is one tiny strand of copper making contact across the break. A voltage check would show full voltage, so that you might assume that the circuit is OK. But - the single strand would not be able to flow enough power (Amps, not Volts) to operate the pump.
If the broken wires sometimes make contact due to movement, temperature, etc., the pump would work. When the major part of the conductors lose contact (again due to movement, temperature, etc.), the pump would not work - but you would still have full voltage.
A test load, such as a test light, is a better check when you have a puzzling electrical problem. You might even want to consider making a test light from a headlamp bulb, which would require a meaningful amount of power to bring to full brightness. If you do make such a test light, be VERY careful to avoid melting/burning plastic, carpet, upholstery, etc.
One comment about your testing - voltage is not power. Checking for full voltage on the fuel pump cables is a good thing to do, but it is not the definitive test.
Imagine that you have a break in the power supply wire or the ground wire to the pump, so that there is one tiny strand of copper making contact across the break. A voltage check would show full voltage, so that you might assume that the circuit is OK. But - the single strand would not be able to flow enough power (Amps, not Volts) to operate the pump.
If the broken wires sometimes make contact due to movement, temperature, etc., the pump would work. When the major part of the conductors lose contact (again due to movement, temperature, etc.), the pump would not work - but you would still have full voltage.
A test load, such as a test light, is a better check when you have a puzzling electrical problem. You might even want to consider making a test light from a headlamp bulb, which would require a meaningful amount of power to bring to full brightness. If you do make such a test light, be VERY careful to avoid melting/burning plastic, carpet, upholstery, etc.
