Backwards Batteries

[Falangee]

Hello all new member here with a real good one for you all.

I am currently trying to solve a major issue on an M817 circa 1982 5 ton. Upon my inspection of this vehicle I found that the batteries had been connected backwards. Positive on negative terminal and negative on positive terminal. Prior to my findings there was no low air buzzer warning, starter engagement or lights so it was suggested that the Protective Control Box could have gone bad. One was bought and installed (still with batteries hooked up backwards) and that's when the first signs of trouble reared their heads.

Upon connection of new box and turning the battery switch on there was smoke detected coming from the engine compartment. Location of smoke is unknown due to the fact that only one person was doing the work and couldn't see it's origin. So it was assumed that another box had been fried. Another was ordered and the person who worked on it was sent on his merry way. This is were I come in and find the connection issue.

After disconnecting the batteries, downloading all the TM's to make sure of proper hook up, the new box was installed. Upon reconnecting the batteries there was no low air buzzer, lights or starter functions. After getting a second set of hands involved I found that when the Battery Switch is turned on the relays in the Protective Control Box are engaging but nothing else is working. At this point I noticed that the ground to the Cold Start Relay was seriously burnt but the wire was still intact.

After following the wiring diagram I've narrowed my sights to a couple of possibles. The Cold Start Relay, the Instrument Panel Circuit Breaker or possibly the Alternator as the break in the circuit. Any ideas or suggestions would be appreciated at this point.

Thanks all for your time Falangee.

 

[m16ty]

Let me see if I understand you, are you saying the batteries were hooked up as positive ground?

 

[Falangee]

Yes they were hooked back up as a positive ground or backwards. The batteries had been taken out to charge and the person that put them back in hooked it up wrong. The truck had worked properly until this occurred.

 

[goldneagle]

Yes they were hooked back up as a positive ground or backwards. The batteries had been taken out to charge and the person that put them back in hooked it up wrong. The truck had worked properly until this occurred.

The center hole in the battery clamps is a different size POS and NEG. The negative should not even fit on the POS post. Did they remove the cables from the clamps?

 

[Falangee]

The batteries had long since been replaced with regular commercial truck batteries. Yes the positive clamp had been changed and the negative clamp is well worn. Both terminals are on the same end of the battery close to the frame.

 

[dmetalmiki]

Electronics HATE incorrect (reversed) connection, and are usually rendered useless by the practice. Once a diode is blown, it stays blown. Transistors, the same.
In the old days reversal was simply rectified by connecting the battery or batteries the correct way (polarity) and briefly closing the the charging regulator points correctly polarized the generator (dynamo) and everything else in the sparks department, like coil etc was good to go. Those were the days.

 

[The HUlk]

Upon reconnecting the batteries there was no low air buzzer, lights or starter functions. After getting a second set of hands involved I found that when the Battery Switch is turned on the relays in the Protective Control Box are engaging but nothing else is working. At this point I noticed that the ground to the Cold Start Relay was seriously burnt but the wire was still intact.

After following the wiring diagram I've narrowed my sights to a couple of possibles. The Cold Start Relay, the Instrument Panel Circuit Breaker or possibly the Alternator as the break in the circuit. Any ideas or suggestions would be appreciated at this point.

Thanks all for your time Falangee.

-Have you checked resistance across the CB you mentioned?
-If a ground to a relay was overheated there is a chance the relay was damaged.
-I'm not familiar with the wiring on 800s but would guess the alternator gives an "engine is running" signal to prevent engaging the starter while running. I know the 939s have this. If something in the alternator fried/shorted it is possible that would be a cause for no start if a similar no start signal is used.
-Was the buzzer module known to be working prior to the reverse polarity?

 

[Falangee]

Yes I will have to agree on all counts.

 

[rchalmers3]

I suggest you isolate all possible components that may contain a short, then hook up the batteries (correctly!) and add one component at a time, meanwhile monitoring for magic smoke loss.

Rick

 

[Falangee]

Yes I will have to agree on all counts dmetalmiki. The HUlk I haven't checked the CB resistance yet, found it on the wiring diagram quicker than I found it on the truck. Am going to track down a relay believe that is a suspect also. The alternator I'll have to pull and take to a local shop. The air buzzer was working prior to the incident.

 

[o1951]

Always check the easy stuff first. Make sure both batteries are still good and fully charged. A low battery could cause you to waste a lot of time and $$$$.

Alternator may have survived. The nose knows.

Not familiar with yours, but most have a fusible link to protect in event of diode failure. If so, that would probably melt before alternator is cooked.

 

[tractors0130]

Does it have 2 batteries wired in series or 4 batteries wired in series / parallel configuration? Like mentioned before, check each battery individually and always connect the vehicle negative cable last and disconnect it first to prevent chance of arcing positive to ground when installing. Also, make sure all connections are tight before bumping the starter or else you can melt a battery post or clamp. On the batteries wired in series, there will be positive to negative connections to make 24 volts. Smoke coming from any electrical device is never a good sign. Post some pics., good luck!

 

[o1951]

Does it have 2 batteries wired in series or 4 batteries wired in series / parallel configuration? Like mentioned before, check each battery individually and always connect the vehicle negative cable last and disconnect it first to prevent chance of arcing positive to ground when installing!

That is a good question. When you said "The batteries had long since been replaced with regular commercial truck batteries." my mind went to tractor trailer configuration - 2 commercial truck batteries for 24 volts.

 

[The HUlk]

The alternator I'll have to pull and take to a local shop. The air buzzer was working prior to the incident.

-I'd hold off on pulling the alternator unless you know it's bad. Disconnecting the "engine is running" wire from the alternator should bypass the function to troubleshoot whether or not the alternator has shorted the signal preventing the starter from engaging. When my alternator went bad on my M939, I disconnected all of the alternator wires and tied them back so they wouldn't short out. The truck could still be strarted and operated using only battery power without an alternator connected. I would assume the 800s would do the same. WARNING: Make sure to remove the battery ground before disconnecting alternator wires to prevent accidental shorting.

-Regarding the buzzer, I would check for power and ground to the buzzer module. If it is receiving power & ground then I would assume the reverse polarity took out the buzzer module. If it is not receiving power or ground then work back from there using the wiring diagram to figure out why.

 

[Falangee]

Many thanks all. Due to the many other hat's I wear I haven't gotten back to actual work on the truck. However I have read and digested the pointers and suggestions that all have made. As soon as I can do the testing and repairs that have been offered here or found other issues (fingers crossed and tongue twisted), I will give an update to overall progress. Once again many thanks.

I was asked about a picture and the one I have handy is how I found the batteries connected. As I had stated in an earlier post the Batteries had been swapped out for commercial truck batteries long ago. Prior to being removed for charging they had been connected properly and all systems functioned. Both batteries are the same and as you look at the picture the positive is on the left side and the negative is on the right side of each battery. Needless to say that was my starting point for sticking my nose into the mess.

Once again many thanks to this point will update as I know.