I normally understand electricity... but I am not sure if I have a problem or a problem of understanding. I am working on my WUR, the heating element is supposed to have 12V on the connection on top for the heater, but I am getting 10V. After following the circuit and studying the wiring diagram I saw that the WUR is supplied by output from the fuel pump relay, Actually I am running the fuel pump with a switched jumper instead of the relay, but it seems to me with the switch on it should give the same results as the relay. There is 12V to the input of the relay connector but with the fuel pump running the output side is drawn down to 10V, which is what I am getting at the WUR. If I disconnect one of the fuel pump leads I get 12V at the WUR. Is this normal or is there a problem with the pump or the wiring to the pump. I have cleaned up the central electical panel and most of the grounds.
Thanks in advance.

 

Hi Grant --

The fuses in the early panels are notorious for corroding in place. A good place to start might be cleaning the fuse connections and maybe replacing the fuel pump fuse.

You can use your voltmeter to test the voltage drop through wiring sections and fuses with the circuit active. Set the meter on DC volts and connect the probes to the ends of the connection you want to inspect. The meter will read the loss/voltage drop directly. Use this to test for voltage drop across your switched jumper too; not all switches are created equal, and many are not up to the load presented by the two fuel pumps plus the WUR heater in your '78.

Got one of those IR heat gun thermometers yet? Heat is a byproduct of resistance in your wiring and connections. Not quite as handy as a Flir IR camera, the handheld thermometers nonetheless are a decent runner up. Know that the sensed area "blooms" with distance; get close to localize the source of the heat.

 

i suppose you measured that without engine running ?
Could be a "weak" batterie that get's down to 10V due the fuel pump(s) running ?
Did you check batterie voltage at the same time ?

 

Thanks guys. I charged the batterie and reset the switch connections and got another volt or so. I am not losing anything through the switch itself, the problem seems to be the fuse. On my panel some of the original fuses have been replaced by soldered in adaptors for modern fuses, including the fuel pump fuse. The soldering looks terrible and when I put my (brand new) temperature gun on the soldered joints I get higher than ambient temperature... so I think that may be the problem. I will put resoldering on my list of things to do.

 

Grant --

The fuse itself is a resistor intentionally, such that current passing through it heats the metal. Too much current equals high enough heat to melt the metal "link" in the fuse. So, expect a certain amount of heat when current flows. Once you identify the source of the heat, use your DMM as I described, testing for the voltage drop through the fuse and holder contacts. Your original post asked for help finding the component or resistance that's causing the voltage to the pump to be reduced. The DMM voltmeter is definitely the weapon of choice, with it's ability to directly measure the voltage lost through any component.