Clark Electric: Where is this voltage coming from?

[JoshuaM]

We have a Clark electric forklift which has blown two fuses. The battery has been removed from the frame to diagnose where the short is but I am getting an unexpected voltage reading of 0.6 to 0.4v between the frame and the ground. It makes resistance testing described in the maintenance/repair manual useless.

The manual says I should not have any reading less than 2MegaOhm from circuit to body, but this voltage is making that impossible to measure accurately.

Where could this voltage be coming from or how do I isolate so I can do the correct troubleshooting procedures? Or are there any alternative procedures I should be using instead of the manual to find the cause of the blown fuses?

 

[Delmer]

Is your voltage between the wiring harness and ground (with no battery installed), or between the frame and ground? (ground cable from the battery?)

In either case it seems pretty straight forward to find the voltage, you hook up so you can see the voltage on the meter and start eliminating things.

If it's the ground cable to frame, then I bet you have enough corrosion on your connector to make a mini cell. If it's the harness to ground, start pulling fuses until the voltage goes away, might be the same corrosion on some component causing your voltage.

 

[BigWrench55]

Electric forklifts do not use a chassis ground. All ground comes from the battery. It is possible that you are reading voltage from a capacitor. If the battery is still connected but you are using the estop switch to kill it then the voltage is coming from leakage from the battery. I would check for continuity from the circuit that is blowing the fuse and the ground lead at the battery. Then you will find the short to ground and then it's a matter of chasing the harness around. Good luck to you.

 

[JoshuaM]

Update: Via Delmer's suggestion I stared at the ground and started removing things "down current" until I found the mystery charge was from the controller circuit (it must have a capacitor because the battery is fully removed from the machine and unplugged. So not a shorted circuit...?

Next was a battery test, it had me check the + and - terminals for voltage to frame faults. It instructed me to remove the battery from the chassis and plug the battery into the harness. Then attach voltmeter pos+ probe to pos+ post and the neg probe the frame. The result was 2v; the manual says 2-3v is unacceptable so find the fault.

I also checked the negative just to be sure and got 0.9v with probes on the frame and negative terminal. (weird where is the voltage going? as I understand it voltage is a flow so a reading of 2 volts has to be going somewhere right?)

I decided to start with the cable from the plug, I removed the positive wire leading from the plug to the circuit. The same layout of the test but now the positive lead doesn't even contact to the body, same results.

In the morning I will try cleaning the battery on the assumption that this slight charge must be going across the battery cover which is slightly dirty and has been overfilled at one point by my boss, other suggestions are welcome. (maybe I am missing something obvious)

 

[Delmer]

If you're missing something obvious it's sure not obvious to me

I guess I haven't paid much attention, so electric forklift chassis's aren't grounded to negative? or just that the main current is carried by cable not the chassis.

Voltage is NOT flow, it is potential (pressure). Amperage is flow. With these "phantom" voltages, if there was flow there would be much lower voltages. The slight leakage with everything supposed to be disconnected is the problem you're trying to find, I think...

I don't think the dirty battery cover will have an affect with the battery out of the forklift, you can check to the battery case though.

A capacitor would discharge in a few seconds if you connect the terminals, and then check the voltage again.