91 Marathon 36v slow go

boatcrazy67

Apologize up front for the long post, but I've posted a few questions on a general issue I am having and have now been able to do a series of tests and have provided all my results below. Hopefully this will make more sense and will help solve the issue I think I am having.

I'm new to the cart world, picked up a 91 marathon with a rear seat/bench set up. Would like to be able to move 4 people around simple gravel roads and grass. No golf course, trails, etc. All original set up on the cart with original FNR switch, controller and I think solenoid. When I bought the cart the first issue I noticed was a battery with melted area around a terminal. It was just one battery, assumed caused by loose cable arching, the cable was still good. I replaced the melted battery with a Duracell GC2 which matches the other 5 batteries.

Basically the cart seems to work fine on level surface and downhill, but once on mild slope it slows, and on higher inclines it crawls and sometimes stops altogether. If you are stopped on an incline and go full pedal to get up the incline, sometimes won’t move at all.
I've found some very helpful testing steps that I've tried to follow and have posted my findings below. Any help at all I would appreciate.

Raised rear of cart on Jack Stands for testing.

1) Reading at battery pack main – terminal and + terminal after full charge reads is 38.0v.

2) With key switch on, forward gear, and + DVM probe on "battery side" of solenoid large post (assume this is the solenoid post that has the wire running to the + side of battery where charging cable connects to). Reading is 38.0v full BPV.

3) With – probe on main – terminal and + probe on "controller" side of large solenoid post (this is the other solenoid post). Voltage was 31.9. If more than 3 volts difference from BPV resister is faulty? This solenoid has both a diode from small post to small post and a resistor from large post to large post. I already replaced the diode. The resister ohms test is .331 with Ohms on the lowest setting on my DVM. Replace resistor?

4) Inductive Throttle sensor test. At pin 1 White wire on controller, with pedal just pushed until solenoid clicks should give low end reading (for series carts & DCS carts .4-.6 volts (low end) to 1.5-1.7 volts (high end). The low end reading I get is 3.3v when the solenoid first clicks. With pedal pushed to the floor it should give high end reading, the reading I get at full throttle is 1.6v. What does the high 3.3v reading at first solenoid click indicate?

5) Checking controller input. Connect - probe from DVM to B- terminal and + probe to B+ terminal on controller. The reading should indicate approx. 0 volts with resister removed and the pedal not pushed in the past 15-30 minutes. I removed the resistor but the diode remained in place. The reading I got was .03V. Slowly depress pedal to engage solenoid, check the meter. The reading should show BPV which it does, depress pedal and watch the reading go down near 0 volts, the reading I got did NOT go down. What does that indicate?

6) Voltmeter testing between M- and B+( with FNR in F). The voltage should go from 0 when solenoid clicks to full pack voltage when pedal is on the floor, the reading I got from was 0v to 36.1 volts (lower than the original main BPV of 38v. The voltage between M- and B+ with pedal down should match the voltage between B- and B+ terminals on the controller with the pedal down. The reading I got was 36.1 volts.

Not sure where to go at this point. Other things I have done include:

1) Upgraded all cables to 2awg. The cable running from main + on battery pack to A2 on controller was too short, so I joined to shorter 2awg cables together with nut and bolt to make it long enough. I will replace that but just did it for testing.

2) Tested the potentiometer. Originally was going from 0 to a little over 5k ohms but adjusted the linkage rod so now goes from 0 – 5k ohms.

3) Cleaned all cable connections to motor. Removed motor to inspect brushes, etc. brushes looked new. Blew out dust, verified all wires are connected to brushes and installed back to cart.

4) Used stainless steel nuts and lock washers to replace any that appeared rusty. Replaced all the nuts on the cable connections to the motor with stainless steel.

5) Replaced the diode on the solenoid because I thought it was bad. Don't think it was. This solenoid has a diode and a resister connected.

6) FNR switch looks ok from outside. All the prongs are bronzed colored and not burnt. I've never had the voltage at the FNR switch be close to BPV. Not sure if it needs to be.

7) At full throttle, while on jack stands, checked each individual battery with the DVM. Readings on all batteries were around 6.0 or 5.9 v with pedal to the floor.

Totally at a loss at what to look at or try next. I know this is not a powerful cart or a speed demon, but really don't think the struggling/stopping on incline is normal. Any suggestions at all I would really appreciate. Not adverse to upgrading things I need to, just don't want to start replacing parts without having some clue. Thanks a bunch.

BGW

__________________
This advertising will not be shown in this way to registered members.
Register your free account today and become a member on Buggies Gone Wild Golf Cart Forum

JohnnieB

Apparently you have a 91 Marathon cart with an electronic speed controller.

Attached is a schematic for its drive system.
Also attached is a picture of the controller I pretty sure is installed in your cart.
Plus a voltage vs State of Charge (SoC) chart.

Since you've posted two numbered sets of statements/questions with both starting at "1", I'm breaking my reply into two posts rather than renumbering.

1. At 38.0V, your battery pack is only about 90% charged.

2. Your assumption is correct and the voltage is the same as the pack voltage.

3. It depends on what the range of lowest Ohms setting on your DVM is. If is 1KΩ, the resistance is 331Ω. That is a bit high, but will not effect the cart's operation in any way, shape or form. The cart does not need that resistor to operate. However, the correct value should be about 250Ω.

Also, I suspect your are using a troubleshooting guide for a later model controller, so the 3V drop does not apply to your controller.

4. You do not have an ITS throttle. Instead, you have a 0-5K pot box throttle.

To check it, pull the black and white wires off the controller and measure between the wires with an Ohmmeter.
With the pedal up, you should measure about Zero Ohms.
With the pedal on floor, you should measure about 5000Ω.
With direction selector in reverse, you might read 2500Ω with pedal on floor (Half speed reverse), but maybe not. It depends on how it is wired.

5. The statement highlighted in RED is what you should see when measuring between B- and the M- terminal on the controller. At the controller's B+ terminal, the voltage will go to pack voltage and stay at pack voltage.

6. I suspect the voltage between M- and B+ is acting normally and the pack voltage is dropping to 36.1V when amps are being drawn from the battery pack. Verify by checking pack voltage when pedal is on floor.

The other possibility is that there is excessive resistance in the high current cables, connections and contacts.

----------
I've got some Honey-Do stuff to take care of, but I'll be back to comment about the second set of numbered statements.

Attached Images

	Pot Box cart schematic.jpg (110.7 KB, 0 views)
	Curtis 1204 profile.JPG (23.3 KB, 0 views)
	JohnnieB's SoC Chart.JPG (63.9 KB, 0 views)

JohnnieB

Now for the second group.

1. The cable on the controller's A2 terminal does not connect directly to the battery.

That may be why the cart isn't performing very well. Connect the high current cables (2Ga) as shown on the schematic I posted above.

2.

3.

4. If the motor studs are brass, the nuts and washers ought to be brass.

5.

6. When the pedal is pushed just far enough for the solenoid to click (motor not turning), all the voltage on all the F/R's high currents studs should be close to being the same and equal to the voltage between the controller's M- and B+ terminals.

When wired correctly, and the pedal is pushed far enough for the motor to be turning, the voltage of two of the F/R studs will be equal to the voltage between M- and B+ on the controller and the other two will be equal to it, lees the voltage drop across the stator windings of the motor.

7. That adds up to a pack voltage somewhere between 35.4V and 36.0V, which is fairly consistent with what you get between M-and B+ on the controller with the pedal on the floor and the rear wheels off the ground.

---------------

Backing up a ways to your statement that one of the batteries had a melted post and was replaced.

A common mistake is over-tightening the nuts on the battery studs. The torque range is 95-105 Inch-Pounds, which translates to only 7.9 to 8.75 ft/lb.

There reason for such a low torque spec is that the stud is actually a SS hex head bolts with the head embedded in the lead battery post. Lead is a soft, malleable metal and if you torque the nut too much, the head turns and the electrical contact area changes from the hex head's six flats to its six points and the resistance of the SS to Lead electrical interface skyrockets.

Any resistance anywhere ion the amp delivery system does two things:

a. It generates heat when amps flows through the resistance and a lot of amps are flowing when the motor is running.

b. Electrical energy being converted to heat in the high current cables, connections and contacts, is electrical energy that should have been converted to motor by the motor, but is being lost.

boatcrazy67

Johnnie B, thanks so much for reading all that information and spending the time to respond. I really appreciate it. The first thing I will check is the wiring and what is connected to A2, etc. It might be correct and I just stated it wrong in my message. I couldn't find a writing diagram with that level of detail so it will be very helpful. I'll check tonight and report back. Thanks.

JohnnieB

At first, it is akin to tracing a single noodle through a plate of spaghetti.

After a while, you'll be able to draw a schematic for your cart while sleeping.

You're doing okay.

boatcrazy67

ok was able to check a few things. Not sure what it means but here is what I found:

1) Between B- and M-. BPV is 38.0. Put probes on terminals, reading starts at 35.3 before key is on. When key is on slowly goes down to about 31.8 and seems to stop there. Hit pedal to the floor and goes down to 0.

2) At controller, probe on B+ terminal and negative main battery, key is off, reading is 35.3. When key is turned on goes down to 31.8 and bottoms out there.

3) Tested Pot Box. DVM set at 20k ohms setting. Pedal up, ohms is .2. Pedal down in forward, reading is 4.9. Pedal down reverse, reading is 2.4.

4) Between M- and B+, FNR in forward, no key on reading is 0. with key on and when solenoid clicks then a full pedal reading goes up to 37.7v

Also found ohms test for FNR switch. at 200 ohm setting on DVM, Forward readings were .7-.8 Reverse readings fluctuated some but settled around 2.9 to 3.2. Tested several times.

Hoping this helps. I'm really confused. Unless I'm doing the FNR test wrong, it seems like those readings are out of spec.

Thanks for the help.

Volt_Ampere

How good are those batteries. You may want to get them load tested. Sounds like they may be dropping voltage under load.

boatcrazy67

Volt - probably a good idea. I did do a test of the BPV while sitting still, and then when moving under load up slight incline. I found voltage at main terminals was 38v and barely drops to 37 when moving up slight incline.

JohnnieB, I was wrong about the wiring to A2. Everything is connected probably based on the diagram you provided.

JohnnieB

1. For the B- to M- test, there are two critical voltage measurement points
a. When pedal is pushed just far enough for solenoid to click. At that point, the voltage ought to be the same as the battery pack voltage.
b. When the pedal is on the floor. At that point, the voltage ought to be Zero, or very close to it.

Located between B- and M- is an electronic "Valve" that controls how much electrical energy the motor receives from the battery pack.
When the valve is closed (pedal up), the entire battery pack voltage is dropped across the valve.
When the valve is fully open (pedal on floor), none of the battery pack voltage is dropped across the valve.
(The reason the voltage does not go completely to Zero is because the electrical valve is not perfect and there will be about 0.01V measured for each Amp of electrical current flowing through it.)

Due to there being a resistor bridging the solenoid's contacts, a voltage can be measured before the solenoid contacts close, but for this specific test, the readings before the solenoid clicks (contacts close) are meaningless. (And cause confusion.)

2. If the F/R lever is in either F or R, voltage is applied to Pin-3 (blue wire) on the controller when the key switch contacts are closed, which powers up the controller and more amps are drawn through the pre-charge resistor, so there is more of a voltage drop across the resister and less voltage at the B+ terminal. This becomes a moot point when the pedal switch closed and the solenoid energizes, eliminating the pre-charge resistor from the circuit.

3. Looks like the the throttle resistance is 200Ω with pedal up and 4900Ω with pedal down. And that half speed reverse is connected.

Measure the Ohms when the solenoid clicks. It should be 50Ω or less, the closer to 0Ω the better.
Ideally, you want very close to 0Ω when the solenoid clicks and then a slight dead band of pedal movement before the Ohms start increasing.

The pedal down Ohms is within specs, but near the low end of the specs (4800Ω to 6000Ω), which affects top speed.
Ideally, the throttle should reach about 5500Ω slightly before pedal touches floor.
Stay well below 7000Ω because if the controller sees 7000Ω or above, it shuts down. (Safety feature)

4. As expected.

5. Your Ohmic readings for the F/R switch contact are high. All four measurements should be 0.4Ω or less.

Forward is A to D and B to C.
Reverse is A to B and C to D.

Excessive resistance in the F/R contacts will reduce top speed and the cart will slow more on hills.

boatcrazy67

On the PotBox setting I can adjust that so its closer to the readings you suggested.

Attached is the FNR test instructions I found I think from this site. By A, B, C, D I assume you mean the posts on the FNR switch? Based on this diagram I attached my forward ohm test was on #5 and #3 posts and reverse test was on #4 and #8. Is that correct?

Unless my test procedure was wrong it sure seems like the FNR switch may be the culprit. So my next question...what would be your next step? Can you clean the FNR switch at all, or should I just replace it? If I do replace it, should I go with a HD switch in the event I ever upgrade the controller or solenoid in the future? Or stick with the stock switch? For the little extra money I was thinking upgrade to HD since it cannot hurt my current setup and I'll be set if I do upgrade in the future.

Thanks!

Attached Images

FNR switch test.jpg (122.0 KB, 0 views)