Lights/Charging problem on 1984 M915a1

[Dig]

M915a1 All lights on tractor except tail lights are dim. Clearance lights on trailer are dim but tail lights are bright. Trailer lights are LED. I assume the back two batteries power the lights because they discharge when lights are on whether tractor is running or not. To troubleshoot, I shut the lights off and pulled the pigtail between the tractor and trailer. Turned on the the tractor lights and nothing came on. Re-installed the pigtail and tried the lights again....They all came on with the dim ones still dim and the bright ones still bright. Couldn't do much checking yesterday because of rain, can you point me to anything simple? Can I check the alternator in place for proper operation? Thanks.

 

[Ferroequinologist]

That blue is very hard to read.

Sounds like you are having a grounding issue. The truck lights are dim and only come on when the trailer is hooked up because they are grounding through it.

Check your batteries as well, I had some issues and found it was just one with a dead cell.

These trucks are sort of like CUCV's, horrible electrical systems. I use mine all the time and every 4 out of 5 repairs is electrical.

Yes, put a multi meter on the back set of batteries when the truck is running, should see 13-14.5 volts. Put multimeter accross the neg on the rear set and the pos on the front set, and you should see 25-28 volts. I'd also disconnect each battery and check them.

 

[Dig]

Sorry about the color (how did I do that?). I couldn't reproduce the headlights on only when attached to the trailer again so I checked the batteries which were in various states of charge (from 50% to 85%) so I brought them all up to !00% and tried again. Still no headlights, in fact no lights at all at first. Upon wiggling the switch all the clearance and all tails worked fine but never the head or the parking lights. My guess is switch. I'm gonna open the panel and see how tough it would be use a jumper to verify my hunch. Still not certain about charging status but it seems better. Again, thanks for the help. I'll try to figure out the text color screw up.

 

[av8or]

The switches are availble at most auto supply houses. Easy to get to and replace. Check grounding again.

 

[MarcusOReallyus]

Check grounding again.

My money is on a grounding problem with the truck.

Test first, buy second. Much faster, much cheaper.

 

[Dig]

Both the headlamps and the running lights are grounded to the frame. The clearance lamps go on but it takes wiggling sometimes. Should both the tractor and trailer tail lights turn on at the first detent in the light switch? Where do the batteries ground to the frame?

 

[Ferroequinologist]

The batteries ground right behind the box. The light switches in these trucks is very inadequate. I replaced mine when it melted with a heavy duty unit, working a set of relays. I also swapped to LED lights.

the headlights are ground behind the bucket off a terminal board. You'd have to pull the buckets to get to them.

The first pull detent of the switch should turn markers and clearance lights on for truck and trailer.

 

[Dig]

I pulled the light switch yesterday and it is crumbling apart. Ordered a replacement. Is the system you have of your design or is it a kit available somewhere? Thanks for the help. Oh BTW, my 1984 has been configured for GPS. There is a magnetic antenna mount on the back of the cab that rises to the top of the cab, a laptop mount where a glovebox might be on a regular vehicle, and a power converter ahead and to the right of the shift lever mounted on the firewall. I think it said on a plate I found on the door that it was done in 2007.

 

[Ferroequinologist]

It wasn't a kit, I don't know of anyone who makes one. I got two 30amp relays from Napa. One is for the headlights, and the other is for the marker, and 24v relay for the trailer lights. I just replaced the switch with a basic heavy duty two position headlight switch. No dimmer function on the switch, but I've never thought they were bright enough anyway. The dash is powered right off the first position of the headlight switch, to take a little load of the marker relay. (Some trailers can have a lot of markers)

 

[Dig]

That sounds like you have to be fairly handy with fabrication to go your route. Was it a huge undertaking? Or is it something a person who can solder pretty well put together?

 

[Ferroequinologist]

I just got two 30amp relays from Napa I think, and made the switch operate the relays. It wasn't that hard at all, I just used crimp connectors as I plan to redo most of the electrical on the truck in the near future.

I'll post a diagram-

 

[MarcusOReallyus]

It wasn't that hard at all, I just used crimp connectors as I plan to redo most of the electrical on the truck in the near future.

There's nothing at all wrong with crimp connectors if you buy quality connectors and do it correctly. If the connection is going to be exposed to the elements, either cover them in self-sealing heat shrink tubing, or use the self-sealing connectors. Don't buy the line that says you must solder every connection.

 

[Dig]

[FONT=Verdana, Arial, Tahoma, Calibri, Geneva, sans-serif]Well,I put in a new switch. No change. What puzzles me is what would seemto be the most delicate of the systems on the truck (clearancelights) is working flawlessly. All that doesn't work are theheadlights and marker lights on top of the fenders. The clearancepart of the marker lights work and both sides also work properly as aturn indicator. Systems that work and things that don't work aresymmetric which should simplify the cure. Are there fuses anywhere? Could the blackout switch be malfunctioning and have a part in this?How about the dimmer switch? When I changed out the switch I wascareful to move one wire at a time but now I wonder if the old switchwas wired wrong and the headlights dimly working at the beginning wasactually a malfunction. Is there a way of knowing for certain whatlead goes where?[/FONT]

 

[Dig]

Well,I put in a new switch. No change. What puzzles me is what would seem to be the most delicate of the systems on the truck (clearance lights) is working flawlessly. All that doesn't work are the headlights and marker lights on top of the fenders. The clearance part of the marker lights work and both sides also work properly as aturn indicator. Systems that work and things that don't work are symmetric which should simplify the cure. Are there fuses anywhere? Could the blackout switch be malfunctioning and have a part in this? How about the dimmer switch? When I changed out the switch I was careful to move one wire at a time but now I wonder if the old switchwas wired wrong and the headlights dimly working at the beginning was actually a malfunction. Is there a way of knowing for certain what lead goes where?

 

[Dig]

How do you delete a post?

 

[Ferroequinologist]

I would jump 12v hot to the wire for the headlights. If they come on bright, the issue is with the switch wiring. IF they are still dim, I would say you have a grounding issue still. There are cannon plugs for the fender wiring on either side of the truck, under the hood between the front of the cab and the fenders. Pull them off, spray in some contact cleaner, clean them up if you can, and put back together. You can also pull the headlight buckets, and directly wire jumpers from the batteries to the terminal block in there, and see what the lights do. Might not hurt to pull the ground wire off under there and clean up where it bolts to the fender.

Have they been dim since you bought the truck? If so it is conceivable that someone might have put 24v bulbs in the front, which would explain the dimness.

To delete go to edit post button down by reply and reply by quote. There is a checkoff to delete post.

I can look under my dash when I get home and give you the wire discription for each function.

 

[MarcusOReallyus]

I told you to check your grounds. Old rusty vehicle = grounds may be bad.

Serial parts swapping is an expensive and time consuming way to fix things. Before you replaced the switch you should have measured your voltages at the lights that aren't cooperating. That's a basic first step; very easy and quick to do.

If you don't have the correct jolts there, THEN you work back from there to see where they are and where they aren't.


If you DO have the correct jolts, you check the grounds.


Like the old carpenter's saying, "measure twice, cut once". But for troubleshooting, especially electrical, it's "test twice, repair or replace once."

 

[Dig]

I have lights! I took a jumper and went to one headlight with 12 volts. They both lit up. So I decided they weren't getting energy from the cab for some reason. I decided the wiring on the old switch was wrong and I transferred the mess to the new switch. The reason I bought a new switch is because the old one was falling apart. Anyway I took all the leads off the new switch and found the one with 12v coming in. After trying the hot lead on each spade and manipulating the switch it was soon evident where the other connections belonged. While at it I did go ahead and clean all of the electric contacts under the hood. Thanks for the help guys.

 

[Ferroequinologist]

Love a happy ending!

Good job!