87 Marathon - slowly comes to a stop when trying to accelerate

[oldschoolnewb]

Totally new to working on these - some electronics background.

I recently picked up an 87 Marathon. I drove it before I purchased, but it was a very flat street that I drove it on. I got it home, and I realize that if I am going "uphill", that it will lose speed and would come to a stop. My driveway is an incline, but you wouldn't notice if I hadn't really told you it was (which is to say it's almost level/flat). I could drive up it about 20' or so before it would come to a stop.

I started by testing the voltage of the batteries, and realized that the voltage was low and that a battery had died. I tried to resuscitate it, but decided to just buy replacement batteries. They aren't great - just the basic ones from Costco. I charged up the cart, and I was able to make it to the end of the driveway (about 200') last night - although it was going about 2 MPH...... When I come "downhill" it FLIES! From the end of the driveway to the house, I probably get up to about 20MPH!

I'm hoping that someone wouldn't mind pointing me in the direction of what to check next. To be honest, I'm thinking about picking up the carts unlimited conversion kit, but, I don't have a "working" cart now - so I don't want to spend a bunch of money, only to find out that it won't fix my issue..

The purpose of this cart is to drive recycling bins to the road, maybe toss fence pieces on it, potting plants delivery, etc on a 5 acre parcel that is "mostly flat".

Thanks in advance.

[scottyb]

Monitor the live battery voltage at the pack using a digital battery meter. IF the pack voltage is dropping more than 20% then suspect another battery problem. If no significant voltage drop when the cart is under performing then suspect a bad contact in the F&R switch.
Other possibilities are a broken resistor coil, Speed wiper contact compromised or not going to full throttle.

[oldschoolnewb]

Thank you Scotty - I'll start going in that direction and report back. Ultimately, I want to chat with you about your conversion kits - but, like I said, I want to make sure it works first! ha!

[oldschoolnewb]

The voltage dropped by about 1v when I was going up the driveway. The lowest it went was about 35.5.

Can you help me understand a little better how to troubleshoot the F/R switch? I'm not having an issue with either forward or reverse - would a faulty switch cause the cart to not go quickly uphill?

[scottyb]

If the contact is compromised then amperage flow would be reduced which could result in low speeds. NOTE all electric carts slow on hills.
So if you are new to carts you may simply be experiencing normal operation.

[JohnnieB]

Originally Posted by oldschoolnewb

The voltage dropped by about 1v when I was going up the driveway. The lowest it went was about 35.5.

Can you help me understand a little better how to troubleshoot the F/R switch? I'm not having an issue with either forward or reverse - would a faulty switch cause the cart to not go quickly uphill?

Since you have some electronics background:

The motor's ability to draw amps decreases as RPM increases due to back-EMF.
The amp flow through the motor determines the amount of torque produced by the motor and the motor will accelerate until the amp flow determined by the voltage applied to the motor produces an amount of torque equal to the mechanical drag (or load) of the cart.
When transitioning from level ground to an incline, the mechanical load of the cart increases (due to lifting the cart's weight against gravity), so the motor slows until it can once again pass enough amps to again produce the torque equal to the new load.

If there is excessive resistance in any cable, contact or connection in the high amp loop, (series circuit) volts are dropped across that resistance, so the motor has fewer volts applied and cannot produce much torque, so the cart goes really slow or stops on inclines.


Connect your DVM between the motor's A1 and F2 terminals and see what is applied to the motor on your driveway when pedal is on the floor. (Direct battery connection - bypassing the resistors)

If the lowest the pack went was 35.5V, than it shouldn't go much lower than that unless there is excessive resistance in the series loop.

[oldschoolnewb]

Originally Posted by JohnnieB

Connect your DVM between the motor's A1 and F2 terminals and see what is applied to the motor on your driveway when pedal is on the floor. (Direct battery connection - bypassing the resistors)

If the lowest the pack went was 35.5V, than it shouldn't go much lower than that unless there is excessive resistance in the series loop.

Something is creating resistance in the system. Putting the positive lead on A1 and the negative on S2 resulted in some interesting numbers. At the start (as I barely pushed on the pedal), I saw 36V - then it dropped to about 15V. As I drove up the driveway (again - a very slight uphill), the cart slowed to a crawl and at full gas, the motor was reporting as low as 8V, but as it sped up, got to about 15 on the uphill. On the downhill, with the pedal floored, the DVM was reporting about 22V.

I try to watch a video or 2 on how to test that switch.

[JohnnieB]

Amps through resistance produce heat and the same amps flow through all components in a series loop, so if anything is getting hotter than the other stuff (other than the resistors in the box) it needs to be fixed or replaced.

This is one of the times an IR thermometer comes in handy.

Also a voltage drop across any cable, connection or contact indicates the presence of resistance.

Don't know how many feet of high current cable there is in a resistor drive, but the intrinsic resistance of 25' of 6Ga only drops about 0.99V per 100A and 1' drops about 40mV.