'89 marathon loses power after a few seconds.

[c.p.]

[JohnnieB]

Originally Posted by c.p.

The voltage actually "bogged down" more when it was running right. There was very little drop when it was acting up, so either the motor wasn't asking for the current, or the controller wasn't willing to give it up!

Thanks for the info. What did we do before the internet?

Great looking Gasser.
I'm envious!!!!!

Back to the Marathon:

In addition to the motor not asking for Amps (brushes???) and controller not passing it, every cable, connector and contact is also suspect.
It is a series circuit and total Amps passes through all of them equally.

The fact that fewer Amps flow (less battery voltage drop) when the cart is acting up tells me the resistance in the series circuit increases.
The fact that it occurs suddenly makes me lean toward it the problem being mechanical (Loose or frayed) rather than thermal (dirty or corroded).

Through the "Magic" of the Internet, here's a schematic of the High Current circuit in a generic series cart.
When all the safety interlocks and control circuits are eliminated, what's left is fairly simple and straightforward.
From what you've posted, all I can eliminate so far is the Battery (There is a charging issue, but not causing the sudden power loss), the fuse (if it exists), the A2 cable (if it exists) and the precharge resistor.

It may very well the motor or controller, but if it was my cart, I'd eliminate the easy stuff first.

[c.p.]

AWESOME schematic. Answers many of my questions.

I'm not near the cart right now, but I can't say I noticed a resistor in there. What's it's purpose?

[JohnnieB]

Originally Posted by c.p.

.........., but I can't say I noticed a resistor in there. What's it's purpose?

There's a dozen or more MOSFETs in parallel between B- and M- that generate a bunch of electrical noise as they Pulse Width Modulate the DC Current to the motor, so there's also a whole slew of filter capacitors between B- and B+. The resistor across the solenoid's contacts allows them to precharge so the instantaneous peak current isn't as high when the contacts close. Prolongs contact life.

In looking at numerous schematics, wiring and block diagrams of various carts and controllers, I believe the resistor also allows the controller to be powered up and waiting before it gets hit with the total battery current it is supposed to control.

Not all carts have the resistor, but it is recommended.

There is also a diode across the Solenoid coil that may or may not exist.