What type of temperature control do you have? Automatic or Manual.

Does pressing the rear defrost button on either light up the LED indicator?

 

I'd say it was automatic. You can set the temperature and then it just maintains it. I'm attaching a photo of the controls. Pressing the defrost does light up the LED as you can see in the photo.

 

Fuse A (in the PDC). It powers only the rear defroster.

Also, there is a relay in the trunk that controls power to the grid - that could have been removed by someone, or be defective.

If you haven't already, you might want to download the '02 FSM and use the schematics for troubleshooting - section 8W. Section 8W also has diagrams showing physical locations of connectors (including that relay). (FYI, the FSM refers to the rear grid as 'defogger' - that might help word searches in the FSM.)

 

Thanks very much for your help PEVA. Sorry to be inept, but can I download the FSM from the site somewhere? I can't seem to locate a link for manuals anywhere..

 

Hey, found it thanks..

 

It's fixed or you found the manual?

 

I found the manual. Looking at the car now..

 

I took fuse A out of the PDC and it looks fine. Now looking for the relay in the trunk that controls power to the grid. The manual says

"The rear window defogger relay is located in the right rear trunk area behind the carpeting."

so I'm starting to pull away carpeting. I wonder what they mean by "rear". Is that behind the seats or is it next to the light cluster at the opening of the trunk? Anyone happen to know before I pull all the carpet out.. thanks.

 

It's on the right wall of the trunk right under your antenna. Just pull the carpet down from the top and it should be easy to spot.

 

OK, I found the relay in the trunk - just underneath the hydraulic support arm for the trunk lid. It does have a relay attached so I took it off and replaced it and it all looked ok.

I went back to the fuse in the PDC and checked it on my multimeter and it seems to be 100 ohms which surprised me. I'd have thought it would be either zero or infinity if it was broken. A friend of mine around the corner has an LHS so I checked his fuse A and it gives an identical reading so I don't think that's the problem. Do relay's ever go wrong?

I bought the car in the summer and live in Nova Scotia so its only now that I've had reason to know if this is working. I did notice that the interior paneling on the back left roof pillar had been pulled away revealing the connector so looks like this may have been a problem for the previous owner.

I checked the resistance across the heated rear window and it was basically zero which is sort of what you'd expect with heat output of voltage squared divided by resistance so for noticeable heat the resistance must be small.. Suggested the heater is intact though.

 

...I went back to the fuse in the PDC and checked it on my multimeter and it seems to be 100 ohms which surprised me...A friend of mine around the corner has an LHS so I checked his fuse A and it gives an identical reading...

No way. Your meter is defective, or you're not using or reading it right, or both, or the fuse on both cars is bad (unlikely). Think about it: This is a 50A fuse. By Ohm's law, how many amps would the fuse be conducting if it dropped 1 volt? 3 volts? Or put another way, if the fuse dropped 10 volts, how many amps would there be going thru it?

Use your meter to find out if the relay coil receives control voltage across it, and if so, is there voltage to and thru the relay contacts and to the grid. If not, where does the voltage stop? If voltage to grid, is other end of grid grounded?

 

You could always have a helper enable the rear defrost button on the ATC while you hold a finger on the relay to see if you feel the relay contacts engage with a click. If not then there's a control problem.

 

Hi Ronbo, sorry, what does ATC stand for?

 

ATC=Automatic Temperature Control. Your dash controls for Heat and A/C.

 

Great, thanks, I'll try that out tomorrow. Dark out there now

 

Well, I checked it out and you can both feel and hear the relay click as the rear window demister is switched on.

Says to me the signalling side is all working. Seems to leave 2 possibilities; either the switch activation circuit isn't the same as the power circuit in which case power still may not be getting to the relay even though it is switching. Alternatively if power is getting to the the relay then the power either isn't getting to the rear window heater or else the rear window heater itself is dead. The the heater being dead doesn't seem too likely as I found the resistance through it was not infinity, in fact it was low, meaning there's a connection through it suggesting it works.

Do you happen to know if the switching circuit to activate the relay is the same as the one that powers the demister? Whether there's a separate switching circuit?

 

...Do you happen to know if the switching circuit to activate the relay is the same as the one that powers the demister? Whether there's a separate switching circuit?

(By demister, you mean the rear grid. FYI, It looks like the FSM uses the term 'demister' to refer to the part of the front duct system that blows air on the side windows.)

The complete relay control and grid circuit schematic is shown on p. 8W-42-2 of the FSM. So, again, using the meter or a 12V troubleshooting light, with the grid commanded on, you need to trace voltage from Fuse A, thru the relay contacts, all the way to the grid - where is the voltage lost?

Start at the grid if you want and work your way backwards. If all your observations up to now are correct, you should not see voltage there when it is supposed to be. If that is the case, then work backwards to find where you do have voltage. Obviously Fuse A is good, the output to the relay contact is good, and you have voltage up to splice S306, or the relay wouldn't click.

There are less than a handful of possibilities at this point, mainly: Bad relay contacts, broken wire, or loose connection in relay socket. (I've never heard of a single case of a bad splice on these cars.) You might swap in another identical relay from the PDC as a quick test (I don't recall if that relay is the same as some others in the PDC).

 

I followed these instructions in section 8G - 2 of the FSM

(3) Locate the ground lug that connects the negative
lead of the rear window grid to ground. Remove
the screw holding the ground lug and isolate it from
any vehicle ground.

(4) Connect one lead of an ohmmeter to the negative
ground lug of the rear window grid (with the lug
still not connected to a vehicle ground), and check
the resistance to a known good vehicle ground. If the
ohmmeter indicates less than 10 ohms, then the rear
window defogger grid is shorting to the vehicle
ground, then go to Step 5. If the ohmmeter indicates
more than 100 ohms of resistance, then go to Step 6.

The resistance over the rear heater on my meter was about 280 ohms so looks ok there. Step 6 says:

(6) There is a problem with the circuits that connect
to the rear window defogger relay. Refer to the
appropriate wiring information for circuit descriptions.
Check wiring with an ohmmeter to verify
proper wiring connections to the rear window defogger
relay. If no problems are found, then replace the
fuse and try running the system.

 

Yeah, totally agree with you PEVA about the resistance measurement. Resistance through a fuse should be practically zero if it isn't dead.

I've not used this meter on resistance before - mostly just for checking life of 9v DC batteries for my smoke alarms and its fine on that. I noticed there's an ohmage adjustment wheel on the meter so adjusting that to have zero ohms when the probes touch makes the ohmage over the rear window 80 ohms and the fuse to be zero. Sounds more sensible..

Still, the result is that we still get to (6)

 

Hi PEVA, thanks very much for the guidance on this, I really appreciate it. It's going to have to wait though unfortunately, social engagements call..

 

So, weekend's come around again and I'm back looking at my rear window defogger. Thanks very much for the help so far..

I have an intrepid which my son normally drives - he's away at college but I had the opportunity to swap round the relay and fuse just in case (they are identical), but still no defogger. The relay is still functioning and I the fuse is fine. I again checked that switching on the defogger activates the relay and it does, I heard and felt it click when activated.

So, I checked the voltages over the relay with my voltmeter. Inside the relay I have 5 pins marked 30, 87, 85 and 86 and one in the middle also marked 87.

From the fsm, I think 30 and 87 (non central pin) are for the defogger power circuit (30 is to power circuit +ve, 87 is on to the window defogger) and 85 and 86 are for activating the relay (86 is +ve, 85 is ground). Getting my voltmeter, switching on car and activating the heated rear window I get 12v between 85 and 86 so the relay is activated (as we know - above). I checked the voltage between 30 and 87 and no voltage, I also checked the voltage between 30 (+ve) and 85 and I got 12v, so the power circuit feed to the relay seems to be functioning. The problem seems to be between the relay and the rear window defogger.

I took off the fabric and followed the cable from the relay and was able to move the back of the rear right passenger seat forward about 9" and see the cabling below. It divided into 3 sets of cables.

1) going down the bottom of the door under the seat

2) going up the roof pillar

3) a cable with what looked like a torpedo shaped fuse receptacle that went through a grommet through the trunk floor.

The cable from the relay to this division seemed fine, no obvious breaks or abrasions.

I wonder about the the torpedo shaped fuse receptacle. I see in the manual - bottom of right column on page 8G - 1 that

"Circuit protection is provided by a fusible link,
located in the charging circuit, for the heated grid
circuit and by a fuse for the relay control circuit."

and I wonder if what I think looks like a fuse is the "fusible link". Does anyone happen to know? Otherwise, I guess we're looking at a problem up inside the roof pillar. Maybe an abrasion through the cable somewhere..

Any thoughts would be much appreciated..

 

Take a look at page 8W - 80 - 76 of the manual. That shows the pin out of the relay connector socket. With that you should be able to figure out what to measure and where..

 

"Circuit protection is provided by a fusible link,
located in the charging circuit, for the heated grid
circuit and by a fuse for the relay control circuit."

The fusible link they're referring to is the one inline from the battery to the alternator. The second part regarding the Fuse refers to Fuse A in the Power Distribution Center under the hood. The only thing that fuse supplies is the rear Defroster grid in an Intrepid.

 

Good troubleshooting! As you said, you proved that you have voltage everywhere needed for control and grid power up to and including that pin.

Don't rule out that the relay socket terminal for relay pin 87 could be loose or overheated (not making good contact with the relay pin). Just one possibility.

"Circuit protection is provided by a fusible link,
located in the charging circuit, for the heated grid
circuit and by a fuse for the relay control circuit."

The fusible link they're referring to is the one inline from the battery to the alternator...

Weird the way they worded that, but it makes sense after you know their meaning.

FWIW, for relay pin labeling, between the two ways in the FSM (2nd way is by letters - see p. 8W-10-2) and the markings on the relay itself (http://www.the12volt.com/relays/relays.asp and many other similar pages on the internet), the corresponding relay pin numbers are:

2 = 87 = D (n.o. contact)

4 = 86 = A (one side of coil)

5 = 87b = E (n.c. contact) <-- not used in this particular circuit

6 = 85 = C (other side of coil)

8 = 30 = B (common contact)

(I may have A and C swapped, but I think they are correct.)

 

The cable going through the floor under the rear seat cushion is for the fuel pump.

 

Hi,

I live in Nova Scotia and we have a warm day coming along on Sunday with temperature 45f compared to seasonal average of 28f so I thought I'd have another go at resolving my rear window defogger problem.

The problem seems to be that there is a break in the connection between pin 87 of the relay in the trunk and the +ve to the defogger. I'm thinking my best bet may be to drill a hole in the rear shelf near the bottom of the rear roof pillar and run a new wire through it from the relay to the defogger. I'd cut the wire running from pin 87 maybe a foot from the relay and cut off the wire in the roof pillar maybe a foot from the defogger connection and then solder a new piece of wire between them threaded through the hole in the shelf. If I'm careful and drill the hole close enough to the pillar, I might even be able to conceal the new wire almost entirely within the pillar.

This sounds a reasonable approach to me. Does anyone have any thoughts on either a better way or why this would maybe not be a good idea?

Thanks very much for the help so far, really appreciated..

 

Hi,

just wondering if anyone has any thoughts on my plan I just posted above, I'm planning on tomorrow as the day to do it, but I really don't want to make a screw up. Thanks for any thoughts..

 

You could do it that way and will know if you're right by the results. To avoid possibly hacking things up unnecessarily if your conclusion is wrong, you could gain access to the connection at the grid and (using multi/ohmmeter) check for lack of continuity between relay contact output socket terminal and the grid terminal. I'm just suggesting proving conclusion before hacking.