Why is my series cart dropping Voltage at switch posts?

[Bnkedwards]

Hey all, long-time reader, first time poster here. So many of your posts here have taught me a lot about cart issues over the past few years.

Looking for some help trying to figure out what is the culprit of my cart (series ezgo marathon '91) dropping voltage. Batteries are good, soleniod is good, and I've replaced the switch already, but it didn't fix the issue. Here's what it's doing:

Volt meter hooked to neg side of battery pack and ANY of the switch posts shows 36+V. Hit the accelerator, hear the solenoid engage and it starts to move but voltage is erratic and drops down as low as 28V (both in F and R). Motor engages, but shakes a few times as the voltage drops down, then comes back up and tries to move again before dropping V again, then eventually the controller throws a fault light and turns itself off (alltrax).

Voltage across the battery pack stays above 36V the whole time, and i've also tested each battery under load and they're all good. Voltage across the solenoid stays above 36V on both sides during all of this also. I think it might be something wrong with the motor components but I'm not an expert. Anyone here have an idea or troubleshooting steps to diagnose that they can think of?

Here's a link to a video I took of what it's doing with the volt meter hooked up to it. https://www.youtube.com/watch?v=L8ZiZ-7D_BY

Thank you in advance for your help!

-Blake

[JohnnieB]

Your battery pack is a mismatched set of Super Start and Trojan batteries and are only about 50% charged.

Charge the batteries.

[Bnkedwards]

Originally Posted by JohnnieB

Your battery pack is a mismatched set of Super Start and Trojan batteries and are only about 50% charged.

Charge the batteries.

Thanks for the info Johnnie, the voltage you’re seeing in the video isn’t the full voltage of the battery pack, because the positive is connected to the A2 switch post (which is being fed from the controller). The full battery pack charge is 37.43V currently. Under load all batteries stay above 6v the whole time. You think I have a battery issue?

[JohnnieB]

Troubleshooting is best done with fully charged batteries and 37.4 is still only about 79% charged, so they need to be charged.

Which Alltrax controller (AXE, NPX, SMP or SR ???) and what error code is it giving.

Load tester for car batteries don't test deep cycle batteries very accurately, your cart itself is a better load tester for them.

What does the pack voltage drop to and what do the individual batteries drop to when you try to drive cart?

When battery manufacturers do non-destructive testing on their products they take then down to 1.75VPC (Volts per Cell) which is 31.5V for a 36V pack, so when you take your mismatched 36V pack down to 28V, some of the batteries may be dropping below the 1.5VPC threshold where irreversible destruction of the plates starts occurring.

[Bnkedwards]

Originally Posted by JohnnieB

Troubleshooting is best done with fully charged batteries and 37.4 is still only about 79% charged, so they need to be charged.

Which Alltrax controller (AXE, NPX, SMP or SR ???) and what error code is it giving.

Load tester for car batteries don't test deep cycle batteries very accurately, your cart itself is a better load tester for them.

What does the pack voltage drop to and what do the individual batteries drop to when you try to drive cart?

When battery manufacturers do non-destructive testing on their products they take then down to 1.75VPC (Volts per Cell) which is 31.5V for a 36V pack, so when you take your mismatched 36V pack down to 28V, some of the batteries may be dropping below the 1.5VPC threshold where irreversible destruction of the plates starts occurring.

Alltrax controller is an SPB4200. I’ve never hooked anything up to it to see what the error is, it just flashes a red light after it the voltage drops a few times and shuts itself off until I turn the key off and back on.

The battery pack drops to 36.4V overall when pressing the accelerator. When I mentioned that I tested them underload, I meant using my cart (I don’t have a load tester). All of the Trojans stay above 6.0, the one off-brand in there drops to 5.9.

Thanks again for helping me work through this troubleshooting!

[JohnnieB]

I suspect you mean SPB48200.

Here is a copy of the SPM/SPB operators manual. Blink codes are on page 28.

Connect a laptop and see what the Under Voltage slider is set to.

[Bnkedwards]

Originally Posted by JohnnieB

I suspect you mean SPB48200.

Here is a copy of the SPM/SPB operators manual. Blink codes are on page 28.

Connect a laptop and see what the Under Voltage slider is set to.

Yes, SPB48200, my bad. I charged it back up to full charge overnight, was reading 38.5V across the pack just now. I put it in reverse and let it struggle again until the controller faulted again. It’s giving me one green and one red blinks (short circuit). I haven’t yet plugged a laptop up to it, but will do that later.

[Bnkedwards]

@JonnieB, finally got some time to work on this again. Under voltage bar is set to 20v. Picture attached. I’ve got the cart jacked up and wires disconnected from the motor now. About to test the motor with a 12 v battery And check continuity now. Will post the results soon.

[Bnkedwards]

Jumped A1 to S1. Then connected 12 volts positive to A2 and 12 volts negative to S2. Motor turned the wheels just fine.

Also ran this continuity test and everything beeps as it should:

Get out your DVM and set it to the Continuity setting and:
- Test between A1 and A2 (Beep)
- Test between S1 and S2 (Beep)
- Test between A1 and S1 (No Beep)
- Test between A1 to Motor Case (No Beep)
- Test between S1 to Motor Case (No Beep)

[JohnnieB]

If you are getting the Short Circuit error code, disconnect the cable from the controller's M- terminal and try it.

If you still get the error code, the controller might be bad, so contact Alltrax.

If the error code disappears, there is a short in the motor or cables somewhere.