Odd electrical issue??

[ivbeenrokd]

I'm working on an M35a2 with heater and electric wipers that I believe are from an a3. Neither the wiper motor or heater motor work. I tested voltage at the wiper motor and found 25.5 volts and determined the motor must be bad so I removed it. I then found 25.5 volts at the heater motor and thought it must be bad as well. I had another heater motor here so I connected it and it didn't work either. I then took the new heater motor to the battery and tested and it works fine. The big question is if I can get 25.5 volts at the heater location then connect my heater motor to the same positive and ground I test from and it not work what's the deal?

Any thoughts/help would be greatly appreciated.

Thank you,
John

 

[steelandcanvas]

Go directly to the battery with both motors, if they work there, and your getting 25.5 volts at the connector, it would be a bad ground.

 

[ivbeenrokd]

Do you know if the cab has a specific ground strap to the frame or if it's just via physical connection?

 

[steelandcanvas]

Was the heater motor mounted on the fender when it didn't work?

 

[welldigger]

A3 heaters are under the dash. Do all of your guages and guage backlights work?

 

[steelandcanvas]

A3 heaters are under the dash. Do all of your guages and guage backlights work?

He's working on an A2.

 

[ivbeenrokd]

All gauges and backlighting work. Heater was using a ground location on the fender when testing at that location. Same ground I used with my test meter for testing.

 

[welldigger]

He's working on an A2.

I know but he said he believed the wipers and heater were from an a3 in the op. However if it was mounted on the fender then yes it was an a2 heater.

A3 wipers ground to the windsheild frame. At least thats where mine are grounded. Hook everything back up and run a temporary ground back to the batteries. That way you can at least test if it is a grounding issue.

 

[steelandcanvas]

Do you know if the cab has a specific ground strap to the frame or if it's just via physical connection?

I may be wrong, but I don't believe there is a ground strap between cab and frame. I do know of the ones beween the transmission and frame, and between the radiator core support and frame, there may be others. Check all bolted connections between the fender and the frame and between the Negative cable and frame.

 

[ivbeenrokd]

Well, the plot thickens. I ran a ground wire from the battery to the wiper motor and to the heater motor and they still don't work. I measured with my meter and still get 25 volts using the factory heater + wire and the new long ground wire. I took the heater motor back to the battery still using the long ground wire and it works fine. So, I have a good known ground wire and 25 volts showing on the meter but it still doesn't work......

 

[steelandcanvas]

Do you have a Hi/Low heater fan switch?

 

[ivbeenrokd]

I do. I get 25 volts at the heater with the switch flipped in either direction or 0 volts with it off. I also tried the long ground wire to the wiper motor with no operation there either.

 

[ATPTac]

Sounds like a problem you'd run into with modern computer controlled cars... this is a long shot, but maybe you have an extremely intermittent short on your power supply. Is the 25v consistent or does it fluctuate at all? I've seen a ton of extremely weird electrical issues with cars in the past where we replaced the related harness and it fixed the problem even though all the test results came back normal. Try possibly tracing the supply wire and make sure its in good shape and doesn't have any weird pinches or obvious breaks or signs of overheating. Not trying to send you on a wild goose chase but it could be something odd like that.

 

[welldigger]

Voltage vs. amperage. Just because your getting enough voltage doesn't always mean your getting enough amperage to run the motor.

 

[doghead]

Measure the positive wire(full length) for resistance(ohms).

It may have a bad connection or internal corrosion or damage.

Run a jumper wire providing 24v+, to the input of the switch, and see what happens. Then to the output side of the switch.

 

[mbehne]

^^^^What they said.

I've seen it many times, it only takes one strand left inside a wire to provide the voltage but it will not carry any current. A quick and easy way to be sure is take a good headlight and hook it up. If it will not light you have a supply problem. Most new test lights and DVOM's are a very high impedance so they can't tell you wire condition at all.
The best way to determine wire condition in this instance, is with a voltage drop test. Hook your DVOM's positive wire to the battery positive terminal and the negative wire to the positive wire on either motor. Without the motor/load running you should have battery voltage. When you energize the circuit you should see the loaded voltage shown on your meter drop to less than 1volt indicating very little potential difference.

Mike

 

[steelandcanvas]

It may have a bad connection or internal corrosion or damage...

I'd start by tracing wire #10 coming off of the magnetic relay. This should go to the circuit breaker on the firewall. The circuit breaker for the heater is sometimes above the start switch, on the backside of the dash. Check resistance at these points.

 

[ivbeenrokd]

Thanks for all the great advice/pointers! I ran out of time this afternoon to continue but I plan to get back on it Saturday or Tuesday. I'll chime back in then with either a success story or more questions.

 

[ivbeenrokd]

I was able to spend some time today on this again. My first step was to take a power feed straight from the battery to the input of the heater switch as suggested. With this the heater worked on low and high and the wipers worked on both low and high. I then started jumping farther up stream and finally determined it must have simply been a corroded connection. After unplugging and reconnecting many of the power feed connections it worked fine.

Thank you all for the help!

 

[Jeepsinker]

You just think you fixed it. The problem will reoccur. For the problem to have been corrosion it would have had to be a VERY badly corroded connector. I'm betting that you have a wire that has broken inside the insulation and is making contact for the time being. Sometimes the problem goes away for a while and then comes back at the worst possible time. Been through this a million times, just not on a deuce.