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M915 Electrcal grimlins

wreckerman893

wreckerman893

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I got the fuel line issue with the new (to me) M915 resolved.

Reinstalled the driveshaft and took her out for a shakedown run.....runs and shifts out good.

The batteries were dead so I had to slave it off.....she now has four new batteries installed.

She cranks good but there is a clicking sound coming from the box attached to the back of the alternator that I think may be a short or relay sticking. As long as the batteries are connnected the clicking continues.....I am keeping them disconnected to keep from running them down.

There is a big yellow wire coming off the starter that is burnt to the point the insulation is melted off. This wire goes into a harness that goes around to the front of the engine and comes out and goes into the box attached to the rear of the alternator (I assume it is the same wire). There is another smaller yellow wire that is equally cooked that goes to the same terminal as the big yellow wire. No other wires seem to be affected.

The headlights, turn signals and flashers work but none of the electrical guages are working (oil press, voltage). The Jake Brakes are inop now too.

I opened all the panels on the dash and did not see any signs of short circuiting or cooked wires. Didn't hear any clicking coming from the breakers in the dash.

I don't have a TM or wiring diagram for the truck so I am shooting in the dark.

My big fear is that the alternator is cooked......I am sure they aren't cheap and I am on a low budget for repairs until my VA check kicks in.

Any ideas from the educated masses????

Here are some pics. First two are on the starter side...the rest are from the alt side.
 

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NDT

NDT

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Sounds like the transformer/rectifier box attached to the alt has smoked. I would isolate it by disconnecting all the wires going in and out of it and see if the 12 volt part of the alternator is functioning. If so, possibly only the transformer rectifier needs to be replaced. I believe the instruments not functioning is not related to the alternator. You can still use the truck without the transformer/rectifier charging the second bank of batteries, just give them a hot charge every week or so with a shop charger.
 
R

R Racing

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Call me I can clue you in. just went thru it myself Thats the 24V feed from the transformer if it exceeds 15 amps it will smoke the transformer and wire. 443-624-4197 just rewired my m920 with a 200 amp 24v alternator.

Jim
 
wreckerman893

wreckerman893

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Call me I can clue you in. just went thru it myself Thats the 24V feed from the transformer if it exceeds 15 amps it will smoke the transformer and wire. 443-624-4197 just rewired my m920 with a 200 amp 24v alternator.
Click to expand...
Jim,

Thanks.....I have missions for the next two days so I may give you a call Wed or so.......I would love to resolve this with little or no expense and a minimum of gnashing of teeth and wringing of hands.

Did you remove the inner fender panels and take off the tires? Looks like they come off easy...would prob be alot easier to get to the starter and alternator with them out of the way.
 
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R

R Racing

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I would like to help. Just a off the wall question . Do you have the 12v feed hooked up at the right spot? Its a brownish or redish wire in the battery box about a 6 gauge wire. If its not the 24v side will try and charge all 4 batteries and your gauges wont work ( thus causing melted wires). I may have a spare alt for ya if need be. I'll get it checked out tomorrow to make sure its good or not ? Its transformer was questionable at 1 time but might be ok.
 
R

R Racing

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Actually was able to swap out the alternator without taking the tire off in less than a hour. It just makes you look like a carnaval act doing it LOL :???: The starter I did from both top and bottom. But I don't see a reason for you to remove it in your case.
 
nk14zp

nk14zp

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NDT said:
Sounds like the transformer/rectifier box attached to the alt has smoked. I would isolate it by disconnecting all the wires going in and out of it and see if the 12 volt part of the alternator is functioning. If so, possibly only the transformer rectifier needs to be replaced. I believe the instruments not functioning is not related to the alternator. You can still use the truck without the transformer/rectifier charging the second bank of batteries, just give them a hot charge every week or so with a shop charger.
Click to expand...
That is what it sounds like to me also.
 
Old Lincoln County Iron

Old Lincoln County Iron

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On my M915A1 My Jake Brake and like gauges all of a sudden quit working also. After looking into it it turned out to be the circuit breaker was bad. After hunting down and replacing the breaker everything works fine. Had a hard time finding the breaker by the way!
 
R

R Racing

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The transformer could infact be bad , but that wont cause your wiring to melt. there is a problem with a current draw causing the issue . we need to find whats causing the draw or you will smoke another transformer.
 
wreckerman893

wreckerman893

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I noticed that the inner fender panels are held in by a few nuts and bolts.....seems easier going in from the side than hanging upside down and doing it from above.

Only one lug will have to come off the starter but of course it it the hardest one to get to.

Since I have an air supply and a 1 inch impact wrench I figured taking off the tires would not be a big issue rather than working around them.

I'll have to find some same guage wire to splice in in place of the fried wires as soon as I figure out what the major malfunction is that caused the issue. Prob use a soldered connection rather than crimped and put some shrink tubing over it.

Thanks for all the help so far.
 
wreckerman893

wreckerman893

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BUMP!!!!

I have the new parts installed on the alternator and my short issue in that area seems to be resolved (no more sparks when I touch wires).

I bypassed two of the batteries (should not have cold start issues this far south) and I am showing 24 volts and 12 volts on the respective sides of the alt/regulator.

This issues driving me nuts now are headlights and guages.

When I turn on the headlights the switch gets hot and the left most breaker gets hot and pops on and off. The wiring to the lights is hidden most of the way so I am having probs looking for short circuits.

When I got the truck the wiring was burnt in two places (starter and Alt...see pics) but I repaired it. I started checking the harness and where it goes around the front of the engine it appears that the protective coating is damaged from within. The only way to know for sure if the wires are not toasted inside will be to remove the protective wrap.

I figure if there is no damage I can replace the coating with plastic loom covering from Advance or similar. If the wiring is fried then I may need a harness. The harness screws into a cannon plug on the drivers side firewall.

All of the guages are ran through this harness except the air pressure ones. I traced the oil pressure, water temp and alt voltage. The Jake Brake wires also run through it so I assume that situation will be resolved (not working) if the harness is FUBAR. There is also some wires from the starter to the alt.

Does anyone have a source for new harnesses? I have heard that AM General will not sell parts to individuals.

My fall back plan is a used harness from a donor truck.

Any ideas from the faithful???
 
NDT

NDT

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OK I get to type this a second time because of the site burp.

On the lighting circuit, you almost for sure have a direct short to ground somewhere, that is causing the circuit breaker to trip. I would isolate the branch circuits from the cab one by one, until the breaker stops tripping. I would start with the cannon plugs that feed the front fender mounted lights.

The engine harness will be more complicated. You must make sure all the wires in the harness are not touching each other or ground. Do this by unplugging the firewall bulkhead connector and disconnecting the opposite ends. Check for continuity between pins and between pins and ground. Any issues here and you will need to remove the harness for repairs.
 
wreckerman893

wreckerman893

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I got the headlight issue resolved (hopefully). I was getting 24 volts through the 12 volt system and fried two headlights. Thanks to Freighttrain for the assist on that.

I still have no guages so my next step is to remove the covering from the wiring harness that snakes around the front of the engine and see if there is a fried wire or short in the bundle of wires.

I replaced the ignition switch just to make sure that wasn't the issue.

I have switched all the relays above the circuit breakers around they seem to be OK.

Maybe the wiring harness will be the culprit.
 
R

R Racing

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Its got to be something dragging the 12v down? Maybe just for grins and giggle un plug the oil senders 1 at a time and see if either 1 is shorted ( there are 2 oil senders the upper 1 by the starter is for the idiot light. second lower 1 is guage. then try the temp sender. then the trans oil temp sender. it also maybe worth unhooking the wire to the voltage guage . and then just temporary jump a wire frome the 12v to see if it reads. also have you used your ohm meter to check all your curciut breaks to make sure all read across and none are toast ? should see voltage on both sides of CB . I had 1 toasted 1 in my truck.
 
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