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Discussion Starter · #1 ·
Recently did a cab swap on my 98. Kept all 98 harnesses. The problem I’ve run into is that my blower stays running with ignition off/ key removed. The blower will not operate when key is in accessory or run position. I have replaced the ignition switch, resistor, tried a different head unit for the climate controls, and tested my blower, all with no success. I have been picking through the wire harness at pdc and have been going over grounds with out very much luck. I’m sure there is something I’m missing, it would seem to me there is a short to ground somewhere. Anyone have any advice or insight? Thanks, Josh.
 

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2016 2500 CCSB
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What year is your truck? 96-98 have a couple differences in cabin harnesses vs 94-95 trucks. I have the same stuff going on, IMO, best thing to do is use the old harness. I havent gotten around to replacing my harness yet, but its on my list.
 

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Couple Q's. Does the speed of the blower change at all if you change the fan controls? And does the blower stop if you A> remove the blower relay, B> remove the resistor pack, or C> disconnect the climate controls?
 

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Discussion Starter · #5 ·
the engine harness is still original to the truck. The cab/dash harness is from another 98 doner truck. All plugs are the same and everything else works as it should. The blower will not run without the relay, resistor, or climate controls. It has all the speeds as it should. I also unplugged the ignition switch to test something and found that the blower still ran. I have since removed the engine harness and have started peeling back the loom amd tape to get a look at everything much clearer. I’m certain I have a short. I also found that the 10 amp ignition fuse in the cab was blown and blew again when I replaced it.
 

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I think your blower relay is faulty, or potentially the wrong part? - on when it should be off and off when it should be on. The blower gets 12v from the relay and is grounded through the resistor pack and then through the climate switch. If the speeds/switch are all working as they should, the ground-side of the blower is all correct. You've got no short to ground or anything like that. And a rubbed wire to the chassis is going to ground things, not somehow throw 12v in. So the relay kinda has to be the point of failure.

There is some possibility of a problem in the circuit that controls the blower relay. But I'd verify the relay itself is right before digging in further there, since swapping a relay is easy enough.
 

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Discussion Starter · #7 ·
I have tried a new relay as well. With relay removed, in each cavity I have: post 30 hot all time, as it should. this is the B+
on post 86, ign on is hot with key on, off with key off, as it should. pin 87 goes to blower and has no voltage with relay removed, which is also correct, and post 85 which is ground, is hot at all times which does not make sense to me. Between the hot ground terminal and the 10 amp fuse for ignition blowing in the cab when the ignition is in the start/run position I'm left believing there is something in the harness before the relay. Just a matter of finding it i suppose. I appreciate everyone's input on the topic at hand. This problem has me pulling my hair out.
 

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Post 85 which is ground, is hot at all times
Yikes. Well I think you're on the right track there at least. Makes sense - the control side of a relay will probably work just as well with -12v as it will with 12v, as it's just an electromagnetic coil. If "ground" side is hot always, "trigger" side goes to 12v with the key on (as it should), which means 0v across the coil, which means relay/blower off. Then key-off, it grounds the "trigger" line and you've got -12v across the coil, relay engages, blower comes on.

No idea how you might be getting 12v on the ground into the PDC/relays though. I'm fairly sure (though I won't swear) the PDC ground bar doesn't come from the cab - should be a strap directly to the body? so I got no idea how a cab swap might have messed with that...

Best I can think of is that the "proper" ground to the PDC came loose AND one of the other relays failed and is putting 12v into the ground bus inside the PDC? Anything else and you'd have a direct path from 12v to ground and there'd be sparks and burnt fuses and/or dead batteries.
 
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Discussion Starter · #9 ·
Yes, it’s a strange occurrence to say the least. I did find one burned out ground wire that had fused itself into part of the ignition circuit which would explain some of what was going on. I would think that with a short like that, that there would be more issues then the blower being on and a blown 10 amp fuse. I would think that the entire ground bar would be hot? Something I never actually checked out before removing the harness from the truck. Anyway, I will be putting the harness back into the truck over the course of the next few nights after work and will update on the progress. How the ground wire from the junction bar got to be so hot and crusty is a mystery as well, one that hopefully won’t reappear. I appreciate your insights into the matter and taking the time to aid in the trouble shooting.
 

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If you pull some other random relay (trailer tow, horn, etc) is the ground on those relays hot too? I'm fairly sure it's just one big ground bus in the PDC so if it's hot for the blower relay it should be hot for all the relays. But I guess there's some possibility it's just on the blower.

I appreciate your insights into the matter and taking the time to aid in the trouble shooting.
No problem, I love the head-scratcher problems most of all :D And this is certainly a doozy.
 

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Discussion Starter · #11 ·
If you pull some other random relay (trailer tow, horn, etc) is the ground on those relays hot too? I'm fairly sure it's just one big ground bus in the PDC so if it's hot for the blower relay it should be hot for all the relays. But I guess there's some possibility it's just on the blower.



No problem, I love the head-scratcher problems most of all :D And this is certainly a doozy.
Come to think of it, I do recall my trailer relay clicking at one point. I removed the relay not thinking it was anything more then it being bad and hadn’t replaced it. Chances are that had I checked, I would have found it to be hot as well. It’s too late to investigate anything further as the harness is out of the truck and sprawled across the garage floor. And In case you were wondering, the short was in the pk/bk ignition wire which explains the blown 10 amp fuse as well. Strange it didn’t blow the ignition fuse in pdc. I guess reinstalling the harness will tell weather that was the issue.
 
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