XJlimitedx99

Howdy all,

I've been trying to diagnose my intermittent power locks over the past few days. I performed the test sequence called out by the FSM and everything checked out, short of one circuit.

The door ajar/key in circuit running into the passenger side door switch measured 40 ohms when it should be continuity. I traced the circuit back to a connector in the driver's kick panel that came from the driver's door ajar switch. The connector was corroded pretty bad and would intermittently trigger the door ajar functions as I handled the wiring. The ground from the connector measured continuity.

I bypassed the connector and checked the resistance over at the passenger door switch; 2.5 ohms. "Great, fixed," I thought. I then reconnected the battery and checked resistance again; back around 40 ohms.

I went back and forth a few times disconnecting and reconnecting the battery with the meter in the switch and the resistance was jumping between 2.5 and 40 ohms by applying voltage.

Is this normal? Mr. Ohm told me that voltage is proportional to the product of resistance and current ( V=iR ) but shouldn't resistance be unaffected by the applied voltage because of the flowing current?

I don't have a ton of experience diagnosing electrical issues like this but I found that to be quite peculiar. One thought I had was there could be another ground that has higher resistance which is diverting more voltage into the circuit. I'm going to keep playing with it today and see what I come up with.

Figured I'd poke you guys with this while I'm at work because I thought this would be a fun one to talk about. We never have good talks anymore. Its always "How can I fit big tarz with a lowering kit?" or "How can I turn my rusty tractor into an amphibious rocket-ship?"

BlueRidgeMark

Can't do that. If you have battery applied you'll screw up your reading. The reason is that your meter is applying a known voltage and measuring the resultant current to calculate resistance. Throw in an unknown voltage and things get wonky.

Resistance readings must be taken on a cold circuit.

awg

I fixed diagnosed and repaired these issues in both my '96

the drivers door harness gets intermittent, caused by ****ty quality wires

the EASY way (I eventually) sorted this was to piggyback wires

they would read ok with a continuity tester, but not work when current was applied

fb97xj1

What he said.

Measuring voltage = battery connected
Measuring resistance = battery disconnected

JeepingDan

Isolate the circuit to measure resistance also.

BlueRidgeMark

This reality of life trips up a lot of people. They reason that if they can measure continuity, or measure voltage (with a meter), then the wire or connection is good. They they go crazy trying to figure out why the window (or headlight or whatever) won't work when they KNOW the wiring is good.

Not true. If there is high resistance in the circuit, a meter may measure the correct voltage, because the circuit doesn't have a load on it. Once you put a load on it, the voltage drops dramatically.

It's analogous to a pipe running to a faucet. It's supplied by a tank at 50 psi. Put a gauge on a tee just before the faucet and you'll read 50 psi, same as what's at the tank.

Now put a hose on the faucet with a high-pressure nozzle (very small opening) at the end. Open the faucet and the pressure will drop some, let's say, to 40 psi.

Now put a restriction in the pipe between the tank and the gauge. Not a complete stopper, but a significant restriction. Say, a valve that's 95% closed.

Close the faucet at the end and you'll see 50 psi, same as before.

Open the faucet, though, and you won't see 50, you won't see 40, you'll see maybe 5 psi.

Take off that high-pressure nozzle and you'll see pretty close to zero.

(I'm making these numbers up as examples.)

If your wire is broken and has only one or two strands left intact, it will measure very good on an ohmmeter, and a voltage measurement will look fine. This is because the meter doesn't draw any appreciable amount of current. It's like having the faucet closed.

But put a load on it like a window motor (or maybe even a test light), and you'll be wondering why nothing works.

That's why I like to use a meter AND a test light. The light will often show a problem like that where the meter will not.

XJlimitedx99

Ahh, I see! That makes sense. Thanks for the explanation as well as that analogy in your other reply. I never really considered exactly what the meter is doing when measuring. I was in the "it measures good so it must be good" group.

I am going to start using the meter and test light combo like you suggested.

My locks have worked since I made this repair. TBH I can't remember if the circuit was live when I made the first measurement at 40 ohms that made me go hunt for problems. Time will tell if I actually fixed them or if they've just been cooperating.

Thanks for pulling this thread back up from a few days ago, Mark. I thought it was gone with the wind.

BlueRidgeMark