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Rear window defogger is stumping me.

Got my old Jeep Cherokee just about in "drive anywhere without fear" shape. .. have fixed literally everything that bothered me about it including ins talling new rear springs and a set of new AT tires... but there's one prob lem that troubleshooting seems to show isn't a problem at all (but it defin itely is a problem!) The rear defog just doesn't work. I'm getting 11.5V at the terminals and also at both sides of the grid. Measuring voltage fro m the grid traces shows that it voltage drops smoothly on all traces as I m ove the probe from left to right on the traces as you would expect. But it does absolutely nothing against fog, ice, etc. It was working fine last y ear but now is not. My heated mirrors work fine (on same circuit, controll ed by same switch.) The only thing I can think to do is to remove and clea n the terminals but that's actually harder than it sounds as I'd have to re move the hatch trim which is held on by a bunch of those damn Christmas tre es - the trim is actually already cracked from when I had to remove it to f ix the handle.

Any other ideas before I do that? If that doesn't work, what's next? I ha d to run and help a friend dig out of the snow yesterday (after roommate an d I had to dig out five vehicles at the house, I'm one hurting unit) and I couldn't see a damn thing out of the back window driving home.

n
Reply to
N8N
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If I'm reading this right, if you're getting 12V on both sides, the ground circuit is open.

Reply to
Bill Vanek

Except for this:

There's no way that could be happening if one side was open or if the trace being measured was open.

Reply to
Mark Olson

No, I meant that if I put a probe on each side of the grid, I read appx. 12 V across them. However you were right anyway; on a hunch I pulled the trim and pulled the wiring out where it goes through the bellows seal at the to p of the liftgate; ground completely broken. So apparently that grounds to the hatch somewhere and I had enough of a ground to show voltage but not e nough to make it work right. Also one of the wires for my CHMSL is hanging on by a strand and three more wires show signs of chafing, so I guess I kn ow what I'm doing this evening...

Reply to
N8N

What you're saying is true, but his measurements are contradictory. how can the voltage drop as he moves across the grid, and them jump back up to 12V at the end? At least, that seems to be what he said.

Reply to
Bill Vanek

What would have made the CHMSL wires fatigue? Do they move every time the lift gate is open.

Reply to
Ashton Crusher

Ah.

Which is why you need to be very careful when using a high impedance voltmeter. Sometimes it's best to do this sort of thing with a instrument lamp bulb with a couple of pieces of wire soldered on.

Reply to
Mark Olson

12V across them. However you were right anyway; on a hunch I pulled the t rim and pulled the wiring out where it goes through the bellows seal at the top of the liftgate; ground completely broken. So apparently that grounds to the hatch somewhere and I had enough of a ground to show voltage but no t enough to make it work right. Also one of the wires for my CHMSL is hang ing on by a strand and three more wires show signs of chafing, so I guess I know what I'm doing this evening...

Yup, same bundle. CMHSL is in the liftgate not the body of the vehicle. T he harness that I pulled out has the rear defog, tag light, rear wipe/wash, CHMSL, and something else unused (power locks?) all in the same harness, a nd it goes through a hole in the body with a sharp edge and only electrical tape for protection (yes, the harness is apparently wrapped with adhesive tape.) Thanks, Chrysler! I think I will buy a stick of door edge molding to cover that sharp edge so it doesn't happen again in another 15 years.

Seems to be working now, I can't test the CHMSL until it gets dark and I ca n't give a 100% thumbs up on the defog, but since the trim is now off I was able to pull the connectors off the window and I was reading 17 amps so I assume that's good.

nate

Reply to
N8N

. 12V across them. However you were right anyway; on a hunch I pulled the trim and pulled the wiring out where it goes through the bellows seal at th e top of the liftgate; ground completely broken. So apparently that ground s to the hatch somewhere and I had enough of a ground to show voltage but n ot enough to make it work right. Also one of the wires for my CHMSL is han ging on by a strand and three more wires show signs of chafing, so I guess I know what I'm doing this evening...

yeah, one of these days I am going to buy a nice test light with an incande scent bulb... probably ought to have one already, but I don't.

The value of all my tools, if I had to replace them with new, is likely mor e than my car, or even a modest house in a less affluent neighborhood. And yet there are so many tools that I still don't have.

n
Reply to
N8N

Bad ground. You have voltage but not enough current to make the grid work. Run a jumper to a good ground to verify.

Reply to
Steve W.

Welcome to the club.... I'm not sure it's a good thing to be on a first name basis with 3 different tool truck operators...

Reply to
Steve W.

Maybe it had a grommet that long ago disintegrated.

Reply to
Bill Vanek

Seems like there was a TSB on that harness. Something to the effect of new wire/wrap with harness tape and a clip to hold it in place.

Reply to
Steve W.

I wouldn't drive any Cherokee anywhere. Those Quadra-Drive ones make me nauseous just pulling around the shop parking lot.

GW

Reply to
Geoff Welsh

"Steve W." wrote in news:ldm9m4$cb6$ snipped-for-privacy@dont-email.me:

I second that.

Bad grounds can create the very weirdest effects. I once had a problem with my Integra's brake lights that I traced back to a bad ground. My reading of the EWDs told me that the problem I had was not possible, but it did happen.

Reply to
Tegger

XJ 'box' Cherokee, not Grand. Other than just general noise, it's rather p leasant to drive, at least given that it's an el strippo 4WD vehicle. Driv es pretty well - exceptionally well when you remember it's got a straight f ront axle and has a rather high COG compared to a regular car.

But my understanding is that the grand is very similar mechanically other t han transmission, xfer case and rear suspension...?

Reply to
N8N

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