1998 Cherokee Sport - I'm looking for a bit of help diagnosing a short circuit. My rear defroster did work normally last winter and I've not made any changes to my XJ since. When I turn on the rear defroster, the 30 amp blade fuse in the engine compartment fuse block blows. And I found out the hard way that this fuse also provides power to the starter. It seems weird to me that they are on the same fuse, but here we are.

I've checked as much as I can and here are the results:
1. With the rear defroster switch disconnected, passenger side of the rear defroster is grounded (black wire), the driver side is not (12 volt side, black with white strip). To me this indicates that the power wire from the dash switch to the defroster is not shorted.
2. a. The black\white wire at the switch harness plug does have 12 volts with the key on.
b. Black wire is grounded.
c. Other two wires are also grounded and I think one is for switch illumination, the other I don't know.

I'm not sure how to test the switch to see if it's failed\shorted. It seems to move normally, a bit clunky. I attached a couple pics, one of the back of the wire harness plug (showing the wire colors) and one of the back of the switch. None of the contact points are corroded at all. This XJ spent it's life in dry climate New Mexico.

I've looked through the wiring diagrams that I could find at XJJeeps but I could not find anything pertaining to the rear defroster switch or wiring. I can order a new switch, but I thought I'd check here with the experts before dropping $30+ on a new switch. Any help would be appreciated! Thanks

 

Section 8N-4 of the 1997 or 1999 FSM have the procedure to test the switch and the relay.

 

It may be a while before I can access the wiring prints. Just for clarification, it only shorts when you select defrost? If you disconect the connectors at the defroster...same result? Does it still short if you open the hatch and try defrost? The wires through the hatch boots break over time and short together

 

Thank you Saudade - I was able to find the info and will be able to do the testing. Perfect!

 

Thanks for the reply. I have not tested with the grid disconnected and that's a good idea. Open hatch? I will have to try that too. I did inspect the wiring through the hatch. They are still wrapped in the factory tape under the boots, and appear to be in good shape, but I understand that they could have cracked as I've dealt with that in the front door wiring harnesses already.

I'll have to buy a big pack of fuses, after I test the switch and relay of course.

 

From the prints....ONLY the defrost relay internal send power switch contacts 30-87, defrost grid and wiring to it from relay pin 87 come directly off the 30 amp fuse in engine bay. ALL other components get power thru smaller fuses, that would have blown first if any of those components were bad or shorted. As a note...the engineer who designed this should have been shot

 

Thank you for your read on the diagrams bluejeep2001 - I agree on the engineering. I'm thinking that after I determine the cause for the short, there's an opportunity to fix that poor engineering. I may just disconnect the defrost grid from that 30 amp starter fuse and replace it with an inline 30 amp fuse.

 

In the real world, the physical design of the defroster grid makes it extremely difficult to short to ground. I would suspect most likely that wires in or near the boot in the hatch hinge is the culprit .It is possible that it is the defroster relay itself but they're mostly plastic so difficult to short.

 

Agree. I’m still leaning toward the switch as it sounds clunky and gritty. I may get it tested today.

 

As always..do what you think best but from what I can tell...the 30Amp fuse does not power the switch...it passes through 1 or more 10A fuses in between the 30A and the switch. They should have blown first and saved the 30A if your switch was the issue. I cant load diagrams directly to this site but will attempt something as soon as I can

 

Ah... A misunderstanding on my part. NOW I'm buyin' what yer sellin'. I'll still bypass that 30 amp fuse just because it's such a bad idea, and peace of mind. Thanks again!

 

In your first post you mentioned the black/white wire isn't grounded. Was that the 20 gauge or 12 gauge black/white? The 20 gauge is at the switch, 12 gauge is at the heater.
Your 12 gauge white/black is shorted between the junction/relay block and the rear hatch. Could be a connector, in the engine bay, or somewhere that sucks.
Unplug the defogger relay and check resistance to ground on pin 87. Should be close to 1 ohm. Anything less than 0.5 ohms is a short and will definitely blow a 30 amp fuse.

 

I didn't think of that wire size difference! I only noted that it was black\white, same as at the switch plug. Neither black\white wire was grounded (at the switch or at the grid heater). But I did check it with the hatch open and when it blows the fuse, the hatch is closed. All good clues and I will add it to the list of checks. Thank you for the info!

 

No update yet - It hasn't been above zero in about a week! But I'll report back when I find this gremlin.