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 installing new rear springs and a set of new AT tires... but there's one problem that troubleshooting seems to show isn't a problem at all (but it definitely 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 from the grid traces shows that it voltage drops smoothly on all traces as I move 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 year but now is not. My heated mirrors work fine (on same circuit, controlled by same switch.) The only thing I can think to do is to remove and clean the terminals but that's actually harder than it sounds as I'd have to remove the hatch trim which is held on by a bunch of those damn Christmas trees - the trim is actually already cracked from when I had to remove it to fix the handle.

Any other ideas before I do that? If that doesn't work, what's next? I had to run and help a friend dig out of the snow yesterday (after roommate and 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

On Fri, 14 Feb 2014 06:18:09 -0800 (PST), N8N >
wrote:

>Got my old Jeep Cherokee just about in "drive anywhere without fear" shape... have fixed literally everything that bothered me about it including installing new rear springs and a set of new AT tires... but there's one problem that troubleshooting seems to show isn't a problem at all (but it definitely 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 from the grid traces shows that it voltage drops smoothly on all traces as I move 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 year but now is not.

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

On 2014-02-14 9:04 AM, Bill Vanek wrote:
"> On Fri, 14 Feb 2014 06:18:09 -0800 (PST), N8N >
> wrote:
>
>> Got my old Jeep Cherokee just about in "drive anywhere without fear" shape... have fixed literally everything that bothered me about it including installing new rear springs and a set of new AT tires... but there's one problem that troubleshooting seems to show isn't a problem at all (but it definitely 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 from the grid traces shows that it voltage drops smoothly on all traces as I move 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 year but now is not.
>
> If I'm reading this right, if you're getting 12V on both sides, the
> ground circuit is open.

Except for this:

> Measuring voltage from the grid traces shows that it voltage drops
> smoothly on all traces as I move the probe from left to right on the traces

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

No, I meant that if I put a probe on each side of the grid, I read appx. 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 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 not enough 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 know what I'm doing this evening...

On Fri, 14 Feb 2014 09:50:08 -0600, Mark Olson >
wrote:

>On 2014-02-14 9:04 AM, Bill Vanek wrote:
>"> On Fri, 14 Feb 2014 06:18:09 -0800 (PST), N8N >
>> wrote:
>>
>>> Got my old Jeep Cherokee just about in "drive anywhere without fear" shape... have fixed literally everything that bothered me about it including installing new rear springs and a set of new AT tires... but there's one problem that troubleshooting seems to show isn't a problem at all (but it definitely 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 from the grid traces shows that it voltage drops smoothly on all traces as I move 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 year but now is not.
>>
>> If I'm reading this right, if you're getting 12V on both sides, the
>> ground circuit is open.
>
>Except for this:
>
> > Measuring voltage from the grid traces shows that it voltage drops
> > smoothly on all traces as I move the probe from left to right on the traces
>
>There's no way that could be happening if one side was open or if the trace
>being measured was open.

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.

On Fri, 14 Feb 2014 08:29:45 -0800 (PST), N8N >
wrote:

>No, I meant that if I put a probe on each side of the grid, I read appx. 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 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 not enough 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 know what I'm doing this evening...


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

On 2014-02-14 10:29 AM, N8N wrote:
> No, I meant that if I put a probe on each side of the grid, I read appx. 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 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 not enough 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 know what I'm doing this evening...

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.

On Friday, February 14, 2014 12:37:17 PM UTC-5, Ashton Crusher wrote:
> On Fri, 14 Feb 2014 08:29:45 -0800 (PST), N8N >
>
> wrote:
>
>
>
> >No, I meant that if I put a probe on each side of the grid, I read appx. 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 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 not enough 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 know what I'm doing this evening...
>
>
>
>
>
> What would have made the CHMSL wires fatigue? Do they move every time
>
> the lift gate is open.

Yup, same bundle. CMHSL is in the liftgate not the body of the vehicle. The 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, and 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 can'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

On Friday, February 14, 2014 12:55:36 PM UTC-5, Mark Olson wrote:
> On 2014-02-14 10:29 AM, N8N wrote:
>
> > No, I meant that if I put a probe on each side of the grid, I read appx.. 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 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 not enough 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 know what I'm doing this evening...
>
>
>
> 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.

yeah, one of these days I am going to buy a nice test light with an incandescent 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 more 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

N8N wrote:
> Got my old Jeep Cherokee just about in "drive anywhere without fear"
> shape... have fixed literally everything that bothered me about it
> including installing new rear springs and a set of new AT tires...
> but there's one problem that troubleshooting seems to show isn't a
> problem at all (but it definitely 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 from the grid traces shows that
> it voltage drops smoothly on all traces as I move 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 year but now
> is not. My heated mirrors work fine (on same circuit, controlled by
> same switch.) The only thing I can think to do is to remove and
> clean the terminals but that's actually harder than it sounds as I'd
> have to remove the hatch trim which is held on by a bunch of those
> damn Christmas trees - the trim is actually already cracked from when
> I had to remove it to fix the handle.
>
> Any other ideas before I do that? If that doesn't work, what's next?
> I had to run and help a friend dig out of the snow yesterday (after
> roommate and 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

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


--
Steve W.