Help! 85 Rear Window Defroster Blows In-Line Fuse

[gibbsrc]

The GM service manual schematic supplement shows a 15 amp fuse added to the later '85 models in-line under the right side dash. I have increased this to 30 amps and still it blows the fuse. The rear window and side mirrors defrost fine until the fuse blows, but the wiring gets hot and the hatch struts also get hot.

Today I disconnected the rear window and the mirrors heat without blowing the fuse. The resistance across the window measures 0.4 ohms. With a normal charging system voltage of 13.8 volts, 34.5 amps would be required (not including the mirrors). This would explain the blown fuses.

Looking at the window, I see 14 lines wired in parallel on each side. The two sides are wired in series. There is a strange braided cable soldered directly against the window at the top and bottom.

I can't understand what is wrong unless (1) this is an after-market window with an incorrect defrost grid, (2) The soldered braided cable is something that a previous owner has added, or (3) this is a factory design problem.

Can anyone verify that the soldered braided cable on the top of the window is factory design and not some type of after-market mod.

 

[Schrade]

Maybe don't increase the amp rating of the fuse anymore. It's in there so that when a circuit gets too much electricity flowing through it, it blows the fuse so that the wire doesn't get hot and start a fire and turn your vette into toast.

Of course, since 30 amp is the highest, so next size up is a piece of angle iron Give it a try - see if it blows the angle iron. If it doesn't, it's fixed!




j/k gibbs...

You're going to have to physically follow the circuit, to find where the wire is shorted to ground - probably starting on the ground side of the circuit...

Best way to explain this is to look at it like water going through a hose. Source - water wheel, and place for the water to go out. If the water wheel gets rusty, and 'blocks' too much water from flowing out, the supply keeps coming, and blows a hole in the hose.

 

[boomdriver]

Maybe don't increase the amp rating of the fuse anymore. It's in there so that when a circuit gets too much electricity flowing through it, it blows the fuse so that the wire doesn't get hot and start a fire and turn your vette into toast.

Of course, since 30 amp is the highest, so next size up is a piece of angle iron Give it a try - see if it blows the angle iron. If it doesn't, it's fixed!




j/k gibbs...

You're going to have to physically follow the circuit, to find where the wire is shorted to ground - probably starting on the ground side of the circuit...

Best way to explain this is to look at it like water going through a hose. Source - water wheel, and place for the water to go out. If the water wheel gets rusty, and 'blocks' too much water from flowing out, the supply keeps coming, and blows a hole in the hose.

My guess as well...the harness is shorted to ground...somewhere near the window, otherwise it would blow faster. Is there supposed to be a resistor in that circuit? Sending 12V thru a glass heater element seems a little extreme to me...knowing what the amperage is. That sure seems like it should be reduced so the heater element does not glow in the dark like the stove top...

To the OP...
the braided cable is OEM. Its not unusual for the short wire from the window plug to get broken off the glass and need to be resoldered. If one or the other breaks off, that usually breaks the circuit. Its possible someone repaired this one and somehow created a short. They may have tried to glue the contact, creating poor connections. Soldering glass upside down is challenging at best...too often its expensive !

Next would be the body harness. Without looking at the book, I remember that harness section running under the passenger door sill around the outter body shell then up the cockpit backwall to the glass support lifts where those act as grounds. You may want to see if those are OEM or if they are part of the problem. I can see resistance building if one of the lifts was not making contact and all the current had to travel thru the opposite. In fact, the more I think about it, those gas lifts are more than likely the problem...thats where the power/grounds are dangerously close.

If you just GOT to have the rear defrost and cannot find the short,. go buy a cycling breaker the same size as the original fuse. That will break the circuit as soon as the circuit heats up and it will close again when it cools down. be sure to get the type-I cycling breaker. Type-II opens and breaks but will not reset until its physically pulled and reinstalled.Type-III opens but will not reset until a button is pressed on the breaker. Either that, or go buy more comprehensive coverage....

 

[gibbsrc]

I'm fairly certain it is not the glass lifts or wiring harness. At this point I have the braided cable disconnected from the lifts and am measuring the resistance in ohms across each end of the braided cable at 0.4 ohms. This is practically a short circuit.

There is no resistor shown in the shop manual wiring. If I add a 1 ohm resistor, I will prevent the fuse from blowing, but then all the heat will be at the resistor rather than the glass.

I suspect the problem has to do with the two sets of fourteen vertical lines being soldered in parallel by the braided cable. If they were soldered in four sets of seven, it would measure 1.6 ohms.

No, I don't need the defrost. I won't attempt soldering against the glass. That would be too risky. I was just thought I might be able to fix it since I had the dash apart to fix a couple of other minor problems.

 

[Hot Rod Roy]

Is your defrost timer working properly? It's not supposed to run long enough to get anything that hot, certainly not the strut rods! My '84 would pop the fuse occasionally. I couldn't find any reason, so I disconnected the defroster in the passenger side mirror. Decided I could do without that extra electrical load. No more problems.

 

[gmjunkie]

Is your defrost timer working properly? It's not supposed to run long enough to get anything that hot,

 

[Hib Halverson]

The braided wires and soldered connections are OE.

When you disconnect the defroster at the back window, what happens with you measure across the hot and ground wires in the harness with an ohmmeter?

 

[gibbsrc]

The window when disconnected from the hatch lifts measures 0.4 ohms. I think the resistance may increase as it heats so this may not be the problem.

The timer works fine. Slightly less than 10 minutes the first time I turn it on. Slightly less than 5 minutes the second time.

I ran the defrost today and noticed that the right side mirror was much warmer than the left and the mirrors were warmer than the window. Maybe this is due to the distance from the connector.

Hot Rod Roy: Do you remember where the connector for the mirror heater is? The shop manual calls it "C1" with a purple and black wire. I have the RH under dash open, but haven't found it there.

 

[Hot Rod Roy]

When I was trying to figure out why I was occasionally blowing my rear window defroster fuse, I was wondering the same thing: Is this a marginal design? Since mine is an early '84, this didn't totally surprise me. How about it guys? Is this a common problem?

Since I'm in Southern California, I didn't feel too bad about loosing the heat on my passenger side mirror, but you're in BrrrRhode Island! And you drive more than just July?? It wouldn't be that difficult to connect the two mirrors in series, so you'd still have some heat, but not such a heavy current load. I'm obviously presuming there is a design problem here, rather than a short circuit in your mirror heaters! Those wires will be a challenge to find in that major tangle behind the dash!

Hib was asking you to measure the resistance of your mirror circuit with his procedure. Obviously with the rear window disconnected and the timer OFF.

The suggestions of some "helpful" members sure is entertaining!

Save The Wave!

 

[gibbsrc]

I'm going to try out a current limiting circuit:

Current Limiting Circuit

This can be added in series with the fuse. It seems like something that should be built into the dash switch and probably is on some cars. I just have to find a power transistor and heat sink big enough to handle the load.

 

[boomdriver]

I'm going to try out a current limiting circuit:

Current Limiting Circuit

This can be added in series with the fuse. It seems like something that should be built into the dash switch and probably is on some cars. I just have to find a power transistor and heat sink big enough to handle the load.

is this by chance an electronic a/c control ????? opt-68?

 

[Hot Rod Roy]

gibbsrc, since you are a new member (5 posts), you may not realize that you are apparently rejecting the assistance of an outstanding Corvette expert, Hib Halverson (4,260 posts). There are thousands of '85 Corvettes that work just fine without current limiters on their window heaters! My FSM says the resistance across your rear window should be .5 ohms, but it doesn't show the resistance of the mirrors. You have a very good DVM if you can measure .4 ohms accurately! By the way, I'll bet you don't have 13.8 volts at your rear window terminals. (I am presuming you have the Factory Service Manuals (FSM). If not, they are must-have books!)

 

[gibbsrc]

gibbsrc, since you are a new member (5 posts), you may not realize that you are apparently rejecting the assistance of an outstanding Corvette expert, Hib Halverson (4,260 posts). There are thousands of '85 Corvettes that work just fine without current limiters on their window heaters! My FSM says the resistance across your rear window should be .5 ohms, but it doesn't show the resistance of the mirrors. You have a very good DVM if you can measure .4 ohms accurately! By the way, I'll bet you don't have 13.8 volts at your rear window terminals. (I am presuming you have the Factory Service Manuals (FSM). If not, they are must-have books!)

You are right on all points. Thank you for this advice.

Answer to Hib Halverson's question:

+ Wiring harness measures 2 ohms with the window disconnected.

Additional information:

+ With the window disconnected, the system draws 6 amps on initial on, then settles down to 3 amps after 10 seconds.

+ With the window connected, the system pins my multi-meter in the 10 amp position. (It does not have any higher positions.)

+ With the window connected, the system blows a 15 amp fuse after 20 seconds. A 20 amp fuse will not blow for at least 2 minutes. After 2 minutes the 20 amp fuse is red hot.

+ With a jumper in place of the fuse, both mirrors and the glass heat fine. The mirrors get very warm. The glass gets only moderately warm.

+ The voltage at the window with the system on, engine off, and fully charged battery is 9 volts. (I don't have enough ventilation to run engine.)

+ I have the 1985 Factory Shop Manual (FSM). It has supplement inserted into it which claims to be for later production. The supplement shows the 15 amp in-line fuse. The original FSM w/o supplement shows no fuse, only a fusible link near the starter.

+ The in-line fuse in my car is spliced into the wiring harness with electrical tape as if it was added after the harness was manufactured. The in-line fuse is a glass barrel type fuse. All the other fuses in the car are plastic plug-in types.

+ My multimeter is an analog Simpson 260 series 5. It has not been calibrated so all measurements are subject to error.