M35a3 - no headlights, no power to starter

[Mquirin]

I have an M35a3. It has run perfect for years. I haven't driven it for about 6 months, so went out to start it and batteries were dead. I charged and figured out 1 was dead, so I purchased 2 new ones. I installed the new ones, but I still can't start it. I put a voltmeter on them. I get 24v out the 2nd battery. When I have someone try to start it, I don't see any sort of dip to the 24v reading. I tested the headlights, but they don't come on (with or without the aux power switch flipped to on position - I don't think its required to be on, but can't remember). When I flip the aux power switch, it seems like some of the gauges fully read, but the fuel and battery gauges just move slightly but don't seem to register. When I attempt to start, I just get a click from the relay at the firewall. Batteries are new. I'm not getting any headlights or power to the starter. All connections cleaned and tight. Could it be a bad aux power switch, or something different? I've read about PCBs, but I'm not sure there is one in an A3. If so, I can't seem to locate it in mine. Any help is appreciated.

 

[glcaines]

If your batteries are new and fully charged, the problem has to be a poor connection. The starter not operating at the same time you have no lights means the problem is in common with both. You said the connections were all clean and tight. I'm assuming you are talking about the connections to the batteries. You also need to check the other connections, and especially the ground connections to the chassis and block. You can take one jumper cable and run it from the negative side of the first battery to a good ground on the engine block and see if you now have power. You can also do the same thing by bypassing other cables. When removing other connections for cleaning, make sure you remove the ground cable from the second battery first. Accidentally shorting a positive cable to ground on the 24 VDC results in major unpleasantness.

 

[Mquirin]

If your batteries are new and fully charged, the problem has to be a poor connection. The starter not operating at the same time you have no lights means the problem is in common with both. You said the connections were all clean and tight. I'm assuming you are talking about the connections to the batteries. You also need to check the other connections, and especially the ground connections to the chassis and block. You can take one jumper cable and run it from the negative side of the first battery to a good ground on the engine block and see if you now have power. You can also do the same thing by bypassing other cables. When removing other connections for cleaning, make sure you remove the ground cable from the second battery first. Accidentally shorting a positive cable to ground on the 24 VDC results in major unpleasantness.

Thank you Gary! I will give that a try today or tomorrow.

 

[dmetalmiki]

And check the heavy leads going to the starter motor. On the starter motor/and solonoid.
They may be loose or making a bad contact.
SOME deuces have a Second power solenoid fixed to the side of the engine block, and they can go faulty. Or, Have Loose connections.
If your has the twin solenoids be careful and mark where the wires go,
Some deuces have REALLY hard to fathom wiring in the starter circuits.

 

[Mquirin]

And check the heavy leads going to the starter motor. On the starter motor/and solonoid.
They may be loose or making a bad contact.
SOME deuces have a Second power solenoid fixed to the side of the engine block, and they can go faulty. Or, Have Loose connections.
If your has the twin solenoids be careful and mark where the wires go,
Some deuces have REALLY hard to fathom wiring in the starter circuits.

Thank you both so much. I went back to the battery box and unhooked everything. I hooked it all back up, but this time one power wire at a time. There were 4 in total (1 small one for solar charger, 1 small one to ?, and 2 thick ones). I hooked them all up, except for one of the thick ones. You can see it in the picture because it's hanging loose in the back (do you know what it powers?). I tried it again, and everything powered up and it started right up. I went back and hooked up the 2nd big one and it started back up.

Thank you both again for the quick response and suggestions!
Mike

 

[Floridianson]

Extra big hot wire was for the NATO slave cable.

 

[Mquirin]

Extra big hot wire was for the NATO slave cable.

Thank you.

 

[ldmack3]

I'm guessing there is a problem with your master switch. Per the schem power comes from the mag start relay wire #10 to the switch, pin A.

Does your low air buzzer work when you turn on the master switch? If so power is getting through the master switch. If not looks like the master switch or power path is bad. If not check for power on pin "A" of the switch. This comes the mag start relay via the start solenoid from the battery. Any of those connections loose or broken will cause the issue. Power on A but not B (output) indicates a bad switch.
This is all if the vehicle is wired per the diagram.

BTW there are diagrams with errors in them. In the one the LH battery is drawn incorrectly internally as far as the + and - goes. Another version has the d\starter solenoid drawn drawn wrong.

Ph and this for an A2 I believe. Not sure if A3s are the same wiring.

 

[glcaines]

I'm guessing there is a problem with your master switch. Per the schem power comes from the mag start relay wire #10 to the switch, pin A.

The OP stated that not only would the starter not engage, but the lights wouldn't work either. The master does not have to be on for the lights to work. The fact that when he disconnected the cables and reinstalled them everything now works means it was most likely a poor electrical connection. Also, the A3 has a different schematic than the A2.