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I screwed up bad... PLEASE HELP

Old 05-13-2019, 05:25 PM
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Default I screwed up bad... PLEASE HELP

So long story short, we installed a light bar. Everything was working great, except the rocker switch didn't fit well in the console where we installed it. We ordered a different smaller switch, this is where everything went to hell. We did something very wrong when we plugged the wiring harness into the new switch, it instantly got super hot, the wires started smoking and melting. We quickly disconnected it and turned it off. We figured, not a big deal, will order a new wiring harness and switch and be more careful this time. Not so easy. Here's where the big problem is. After removing everything and unplugging the light bar, the jeep is now revving strangely and I'm getting check engine code P0121, Throttle/Pedal Position Sensor A Circuit Range/Performance. Anyone have any idea what could've happened here? Is it most likely the wiring harness heat melted or damaged part of the oem harness going to the throttle position sensor? We had it run through the firewall grommet. Or did it likely somehow short out the throttle sensor and I need to replace that? Wasn't expecting it to damage anything on the jeep as it was only connected to the light bar and the battery. ANY HELP WILL BE GREATLY APPRECIATED.
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Old 05-13-2019, 05:28 PM
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Also... It is a 2000 Jeep Cherokee XJ

Was just trying to help my friend wire up his light bar now I'm extremely terrified that we caused an expensive repair, I'm hoping to be able to figure it out myself without needing to take it to a mechanic
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Old 05-13-2019, 06:48 PM
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Where was the rest of the harness connected, power and ground? It sounds like you might have melted a common ground wire under the hood?
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Old 05-13-2019, 06:53 PM
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Originally Posted by lawsoncl View Post
Where was the rest of the harness connected, power and ground? It sounds like you might have melted a common ground wire under the hood?
It was connected to the negative and positive of the battery. The only wires that melted were what went from the relay thru the firewall and to the switch in the dash. The wires going to the battery and the wires going to the light bar were fine
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Old 05-13-2019, 07:04 PM
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As lawsoncl mentioned, knowing exactly how the switch and actual light bar were grounded is key to helping.

Was the light switch you installed connected to the existing electrical system at all? This is very important to know. For example was it connected to the dimming circuit so that it would react to dimmer switch adjustments inside the vehicle?
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Old 05-13-2019, 07:22 PM
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I attached the wiring diagram, everything grounds to the negative battery terminal I guess? I'm far from an expert. It's possible we had the positive and negative backwards or something on the new switch because it didn't have any wiring diagram that came with it. I never thought it would ever cause the wires to melt. It was not hooked up to anything oem. The wiring for the switch went from the dash, under the pedals, thru the firewall to a relay mounted to a bolt on the hood hinge. The wiring from the relay went to the pos/neg of the battery and to the light bar.
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Old 05-13-2019, 07:36 PM
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Here's the exact wiring diagram of the one we were using, except ours was 1 lead not 2 (1 light bar)
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Old 05-13-2019, 07:42 PM
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Swapped out the original switch for this one, I'm now realizing a big oversight was that the original was 5 pin and this one was 6 pin
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Old 05-13-2019, 08:43 PM
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Was the battery connected or disconnected while you were doing the work?

Did you try disconnecting/reconnecting the throttle position sensor at the throttle body?

Based on all the information your provided (thank you), there is no clear reason that the two problems would be associated with each other.

You definitely created a short circuit. By this, I mean the positive going from the battery was going right through the new switch and directly back to the battery with nothing slowing it down in between. This is a short circuit, and with switches that small can melt them pretty quickly. Sometimes vaporizing their little pins. But even so, this would have just fried the switch and damaged the wiring to/from the switch. If this wiring was laying right on top of other wiring, there would be some possibility that it could melt other wires as well. However, you haven't provided much information on that. Plus this should be relatively easy to inspect, as there are only so many places it could have touched other wires right?

If the battery was disconnected while doing the work, this leaves the two most reasonable possibilities being:
1. Absolute coincidence. There is another problem going on, and it chose now to surface (thanks universe!).
2. You bumped something or jostled the factory wiring harness when creating/using the grommet through the firewall.

If the battery was connected while doing the work:
1. It is possible that something random happened, that may never be fully known (touched the wrong thing, to the wrong place)

The good news, is that since the vehicle cranked, and started. That it does not sound like a system wide problem with the OBD2 sensor circuits or PCM (hopefully). This typically would result in more serious starting troubles.

If the throttle position sensor is sending bad signals, the computer can mess with the idle RPM (thinking your foot is on the pedal, moving it around). This might account for why the idle was unusual. (unless that is not what you meant...you might want to clarify).

Last edited by jordan96xj; 05-13-2019 at 08:52 PM.
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Old 05-13-2019, 08:52 PM
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The wiring getting so hot that it melted the plastic sheathing, could this have damaged other oem wiring in close proximity? Does the wiring for the throttle sensor go thru the firewall grommet? I will try what you suggested. I guess I need to go thru and check all the oem wiring that was close to the wires that melted and see if it effected anything else? I also don't understand how they're related.
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Old 05-13-2019, 09:01 PM
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Originally Posted by 2000_jeep_cherokee_xj View Post
The wiring getting so hot that it melted the plastic sheathing, could this have damaged other oem wiring in close proximity? Does the wiring for the throttle sensor go thru the firewall grommet? I will try what you suggested. I guess I need to go thru and check all the oem wiring that was close to the wires that melted and see if it effected anything else? I also don't understand how they're related.
It would not have melted other wires by just being near them. It would probably have to be physically touching other wires to do any damage (if any). You should know where the new wires were routed and be able to visually inspect pretty quickly.

I don't believe the TPS wires traverse through any firewall grommets. I have a 96, so my wiring is not the same as yours, but I doubt the TPS wires are coming through the firewall. That harness is typically just under the hood. Perhaps someone with a 2000 and a wiring diagram would correct me.

You can get a PDF service manual for the 2000 XJ at the following link at a reasonable price. It has wiring diagrams if you need to double check things:
http://www.pacificcoastmanuals.com/j...e_manuals.html
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