Very weird problem on a 1985 Z51 coupe: Defog trips the associated 15-amp circuit breaker. (Late 1985 production. Early 1985 did not have this circuit breaker. It's located below the ECM, on a pigtail.)

I pulled the defog timer switch and checked it out. It runs fine on the bench. The timer runs for a full ten minutes initially, then five minutes subsequently, just as it is supposed to. It can also be turned off manually. So, the electronic part seems to be fine.

The timer's internal power control relay is also working fine. In fact, too much current is flowing when the unit is in the car and the timer turns the relay on. It trips the 15-amp circuit breaker! When this happens, the breaker removes power from the tally lamp on the control and also from the heated mirrors and rear window, but the relay inside the timer continues to hold for the correct ten minutes. (I know this because I've measured the heavy current power source going to the timer module. It goes away when the breaker trips. I've also visually observed the relay inside the timer module and it stays held until the time has elapsed.) The electronic portion of the timer is powered by a different feed, so this makes sense. The problem must be excessive current draw in the actual defog heat circuit.

I’ve measured the resistance on the rear glass and also the rest of the circuit without the rear glass connected.

Disconnecting the harness connectors to both of the gas struts for the rear window, I see about 0.5 ohms across the rear window, including the struts. (Subtracting some extra resistance in the meter probes.) It’s the same if I meter the window directly, going around the struts, so I therefore know the struts are making good contact with the window traces, just as they are supposed to.

I’ve inspected the rear glass very carefully, even using the ohmmeter, and can find no signs that anything is bridging across any of the traces. I measured all around the rear window black paint and also around the latch striker insulator and I don’t seem to be getting any sneak path conductivity at those places that would tend to short across the window traces, lowering the total resistance.

Also just as there should be, there is infinite resistance from the rear window traces to ground with both struts disconnected at the harness connectors. Therefore, I know I’m not getting any bad paths because something is grounded that shouldn’t be on the window assembly. I've opened and closed the window with the meter connected, to make sure.

The ground wire going to the left strut measures less than 0.1 ohm to frame ground, so that ground wire appears to be solid.

As for the power wire going to the right strut, it measures about 3 ohms to ground with the rear strut disconnected. This also sounds about right, as in this case it would include the two heated side mirrors plus the tally bulb on the control switch, all in parallel. When connected to the strut, it measures about 0.4 ohms, which is about what I would expect with the 0.5 of the rear window and the 3.0 of the side mirrors all in parallel.

So, I don’t see an obvious problem with the wiring, but the resulting net 0.4 ohms would draw a whopping 30 amps at 12 volts, not 15. I would assume there would be some drop through the wiring that might get this down to 20 or 25 amps in real life, from the breaker's perspective, but certainly not 15.

Therefore, I am at a loss to explain what has changed to cause my 15-amp defog breaker to now be tripping. It only stays on for about 20 seconds when the defog is activated.

It’s not the breaker. I tried a new one and got the same result.

I certainly don't want to just put a higher rated breaker into the circuit, especially with no explanation of what has changed. This is high current. I can just see my whole car going up in flames!

The system worked fine at one time and nothing has been changed, including the rear glass. I know because I am the car's only owner. (20 years!)

Have these rear window traces been known to DECREASE in resistance over time? Oxidation, perhaps? Mine still seem to have that slightly brown color that I remember from when they were new. The window appears to be in perfect condition. No scratches or foreign material.

What's going on, here?!?!?!

Very strange.

Someone must have seen this sort of thing before. Any GM dealer electrical troubleshooters on the forum, tonight? Help! Any tips, ideas or observations greatly appreciated. I’m going out of my mind trying to figure this one out!!!

Despite Southern California’s sunny reputation, we do get fog, so it’s really nice to have the defogger. I would hate to be without it.

Many thanks!

 

I haven't had the opportunity or the need to check the circuitry of that system, but you mention that the heated mirrors are on that circuit, albiet in parallel. Have you checked for a problem there? Remember, the mirrors are exposed to the elements and the defog isn't. Try disconnecting the mirrors and run the defog etc. I would try to eliminate all else on that circuit and test each loop one at a time. Definately don't upgrade the breaker as the car is telling you there is a problem there.

 

The mirror wiring is pretty hard to get to. I've already got the right door skin off and I still can't find the connector for the mirror heater.

What I know already is that the resistance of the defog heat circuit on my car measures about 3 ohms with the rear window disconnected. That's with both mirrors in parallel. I wish I had a replacement mirror element to measure. If the resistance of a normal mirror element is about 6-7 ohms, two of them in parallel would account for the 3 ohms I'm seeing with the rear window disconnected. If mirror elements were higher than 6-7 ohms, then that would signal a very real problem in my mirrors or associated wiring.

I'm afraid to disconnect anything and then try applying power. If I disconnect the wrong thing and try to operate the system, that may provide just enough relief to keep the breaker in, but burn up something that has a problem by sending the extra current that used to go somewhere else right into the problem spot.

Therefore, I need to figure out a way to solve this without using power. I really wish I had a known good mirror element and rear window to measure. If anyone has access to such parts and a good digital ohmmeter, I certainly would be interested in your readings.

Also, if there are any GM pros out there who have knowledge of changes that might happen to these rear windows over time resistance-wise, that would be an incredible help, too.

Many thanks.

 

(I'm afraid to disconnect anything and then try applying power. If I disconnect the wrong thing and try to operate the system, that may provide just enough relief to keep the breaker in, but burn up something that has a problem by sending the extra current that used to go somewhere else right into the problem spot.)


I would say not. If the problem area is drawing so many amps from a short, it's going to trip the breaker regardless. You're not going to get that much extra current that would make enough difference to burn up the harness. As far as element resistance changes, I have a Caprice Convertible with an element in the rear glass that is 30 years old and works fine.

 

Well, I've been doing some checking with the Helms manual for my car. It says the circuit should measure "1 ohm or less" with the defog control disconnected. That would include mirrors and rear window in parallel. Furthermore, according to the book, the wiring between the rear window and the defog control is metric #5 (AWG 10) which has a resistance of only 0.00118 ohms per foot. There can't be much more than about 10 feet of wire between the dash and the rear window strut, so the wire would add only about 0.01 ohms. In other words, the added resistance in the wire should be insignificant. This makes sense because they would want to keep that heat out of the wire and on the window!

Anyway, a total of 1 ohm would draw 12 amps. 0.8 ohms would draw 15 amps. So, that means the proper total should be somewhere between those two, since the book says less than 1 ohm and anything below 0.8 would draw more than 15 amps and trip the breaker.

I'm already down to 0.5 ohms right across the rear window, with it disconnected and all by itself, just measuring across the window element. So, it would seem the problem has to be on the rear window. But I can't see anything wrong with it whatsoever. Weird. It's driving me nuts! I wonder if I could have cleaned it with something that caused its resistance to lower. Doesn't seem likely, but I'm running out of ideas, here.

 

The only thing I've experienced even remotely similar was my rear defroster relay became loose inside its connector inside the fuse box.

I presume this happened from bumps and other vibrations resonating through the frame and chassis.

I pushed the relay securely into its connector and mine has worked fine since.

 

Thanks for the idea, but yours is a newer car! (And an improved design.) In the 1985 issue, there is no outboard defog relay. The relay is built right into the defog control switch assembly. Big heavy AWG 10 wire runs to the back of the A/C & heat control panel, where the defog control switch is mounted.

They must have had some problems with this design, because in later years they moved the relay and its heavy current path out of the switch assembly to a more direct route and then controlled the relay remotely. I think they may have also upped the breaker rating to 20 or 25 amps at that point, but I'm not sure about that last part.

In fact, in the early 1985 design (which I forunately don't have) there isn't even a circuit breaker for the defog. In that version, the heavy current path is tied right to a heavy main battery bus that is only protected by a #1 fusible link wire. Those fusible links can handle a staggering amount of current. To give you some idea, the battery charging circuit, the A/C blower, horn, the entire accessory bus (prior to branching into individual circuits and fuses) and the defog loop were all driven from this SAME fusible link in that design. So, in that case, my system would be drawing over 30 amps right now and I would either have a fire or would have blown the fusible link, which would have disabled the battery charging and blower. Owners of early 1985 and perhaps 1984 should take a look at their schematic, consider what could happen if something went wrong in the defog area and perhaps install a circuit breaker for the defog. GM obviously did and even changed the design in mid-year 1985 to add the defog breaker.

Anyway, in my car, there is no relay socket. The relay is soldered right onto the printed circuit board inside the defog control. I've checked the relay and it is working fine. In fact, if it wasn't making good contact, enough current could not flow to trip the breaker.

Many thanks for the idea, though. Keep those ideas coming! And if anyone has access to a digital ohmmeter and another rear window heater element and mirror heater element, I'd really be interested in what those measure. Thanks.

 

.5 ohms for the rear window heating grid.
6.6 ohms for each mirror heating grid.

You can access the mirror wiring by doing the following.

With your hand, manually move the mirror around until you can fit your fingers behind the mirror.

Pull the mirror straight forward until it pops off. The mirror snaps on to a trunion which is mounted to the motor that moves the mirror. You will see the two wires from the mirror go to a harness where the motor leads also attach. You can disconnect the harness plug and measure the resistance of the mirror heating grid.

 

THANKS FOR THE INFO!!! This "second opinion" is extremely helpful. I'll check my car against your measurements. Many thanks!

 

Hmmm. This is very weird. Doing the math, two 6.6 mirrors in parallel is 3.3 ohms. Add a 0.5 rear window grid and you get 0.43 ohms. 0.43 ohms will draw 27.6 amps at 12 volts.

How can this be driven by a 15 amp circuit breaker? No wonder it is tripping. Something very strange is going on, here. If someone has a Helms manual for 1986 or later, could you check the defog circuit breaker to see if it is 15 amps, or perhaps a higher value? Just curious.

Also, if there are any parts guys on the forum, do you know if GM made any changes to the mirror heating grids or the rear window grid during the C4 run? I thought those parts were always the same for all C4's.

VERY strange, indeed.

 

Here's how the defog circuit is on an 89.

From the Relay secondary contacts.

One circuit goes to the HTD MIR fuse 10 amp. fuse. It then goes to the Mirror Heater Grids which are in Parallel.

Also from the relay secondary contacts is the Rear Window Defogger Heater grid.

The Circuits are fed by a 30 amp Circuit Breaker.


30 amp breaker|_10 amp fuse 2 Parallel grids_Ground
|
|_________DefrosterGrid__Ground

 

Well, now. Isn't THAT interesting? Looks like it takes more than 15 amps to drive that rear window, after all! I wonder how mine ever worked? Maybe it was always right on the edge?

In my car, the mirrors and rear window hang off the same 15 amp breaker. There is no additonal protection for the mirrors. I can understand how GM wouldn't want to tie the relatively small gauge wiring going to the doors (for the mirror grids) to anything higher than a 15 amp breaker, but 15 amps doesn't seem to be enough to drive the rear window.

I bet this afterthought design on the 1985 was changed a few months later for 1986 cars. The late production 1985 circuit appears to be a band-aid that wasn't fully thought out. I bet they had some fires and felt they had to do something in a hurry. Very interesting.

That still doesn't explain why mine seemed to work originally. Strange.

Anyway, I like the idea of having lower amps protection for the doors.

 

Hooked on Vettes,

If you are looking at a 1989 Helms manual, can you tell if GM still used #5 wire to drive the rear window? If your manual is similar to my 1985 one, the wire size is indicated by the number that precedes the wire color designation.

Thanks for all the great info. It REALLY helps!!!

 

The Helms manual shows the following.

From the output of the relay is a 5 (10AWG) Purple wire.

Then there is a splice. One circuit is a 5 (10AWG)Purple wire which goes to the Rear Window Defog. and then a 3 (12AWG) Black wire to ground.

The other wire at the splice turns into a 3 (12AWG)Purple wire and goes to the HTD MIR 10 amp. fuse.

Out of the fuse is a .8 (18AWG) Pink/White wire that goes to a splice. Out of the splice are two .8 (18AWG)Pink/White wires. Each wire goes to one Mirror grid. The ground wire for the Mirror grid are .8 (18AWG)Black wires.

 

Thanks for the info! This is a BIG help.

 

My 1986 Helms shows a 30 amp breaker and a 15 amp gauge fuse on the circuit. I can email you the page if you PM me. That is is if you still need it. Just let me know if it's manual or electronic.

 

Indeed. My poor memory twinges at the mention of the rear window defogger. I think I saw a service bulletin abut rewiring and splitting the defogger circuit. It has to be over 15 years since I've looked in that book.

I did a quick check of my Helms, and I didn't find the rear window defogger (and mirrors) in any section I looked in. Can you tell me where it is in your 1985 book?

RACE ON!!!

 

Thanks, Artvette. Putting what you wrote together with earlier comments from HookedOnVettes gives me a very clear picture. The 15 amp gauges fuse is present in my car as well. It supplies the power to the electronic section of the defog timer, among other things.

The very interesting part is the other leg, where they went to 30 amps in 1986. My manual shows a 15 amp circuit breaker there. I might think that was a misprint, except that the car actually HAS a 15 amp circuit breaker installed there.

Even more interesting is that if you go to the acdelco website and look up the defog circuit breaker for a 1985, instead of showing 15 amp like the 1985 Helms manual and also what's actually in the car, the website gives you "#D7001 30 amp All Models", not the 15 amp breaker that's actually in the car.

Very interesting. I get the feeling there was a factory...mmm... "situation" in here somewhere.

 

Ah, ha! They gave it a service bulletin for yet another wiring change? That's not too surprising, based on what I've seen in my car.

Now we're getting somewhere! Many thanks to all.

In the 1985 Helms, the early 1985 original defog circuit (with no breaker at all) is shown on page 189.

The late production change (with a 15 amp breaker added, like my car) is only shown on page 189 of a "Manual Supplement" ST-364-85S. This is about 10 pages of updated schematics showing various improvements done by the end of the 1985 model year. All of the improvements seem to be in my car, which makes sense, because I bought it late in the year. All of the "improvements" seem to work fine, except the defog one.

Thanks, CFI-EFI.

 

In the 1985 Helms manual, there's a schematics index printed on page 25 of the electrical section.