Fixing the rear defroster: Blows fuses on high
 

Interesting problem. Rear defroster never worked, and finally got around to checking it out. Replaced the control relay and the fuse with a new 25 amp fuse.

On low setting (turn the knob) the light in the knob now comes on which is good.

On high setting (press the knob) the light came on for a few seconds, then off. Blew the fuse.

What's the difference between the settings, does it engage a second set of wires on high, duty cycle, something else? And does it run the wing mirrors on both settings?

Side thought, the button sticks when pressed. Can it be overhauled/lubricated without taking the entire dash pod off?

On my '78 pod, the knob can be pulled out of the pod switches, and then the switch body pulled by reaching in and depressing the two latches and pulling the switch toward you.
It would be difficult if you don't have access under the pod, but might be possible to pull the switch forward with the bottom cover still installed.


I'm not sure on the functionality of the defroster system, hopefully someone more versed in the proper operation behavior will chime in.

Jetson8859 melted or burnt wires from the CE panel all the way to the DS coil when used the defroster for the first time - so, please be careful. We had to pull the engine to replace the harness.

No problem, the circuit is rated for a 25a fuse (pretty big) and I'm not putting any higher value in there. May be that one of the hot wires for the high power heat is shorted to ground, the question is which one.

When I turn it on low with the car off I can see the voltmeter move a bit, so something is happening. Will have to wait for the next foggy day (which should be any day around here) to see if anything defrosts.

Kevin, any idea what the source of the short was, and was Jetson using proper fuses?

C

The rear defog setting (turning the knob) sets the dual heater grids in series, and defrost (pushing it in) puts them in parallel, along with the mirror heaters.
The Defrost circuit could be shorted somewhere. I would check resistance between ground and pin 87 at the defrost relay.

Never did figure it out. Correct fuse, melted the CE panel at the fuse block. He pulled the fuse after we put the replacement harness in.

Hm, that helps. I'm looking at the schematics and it looks like this:

The rear coils (there are two) have two sides. The first goes to pins 1,6 which then merge to form pin 1.Then the other sides go to pins 2 and 3. Pin 3 goes to "A16" which is probably a ground. Pin 2 goes to the first relay (87b) and pin 1 goes to the second relay (87).

So my guess is when you set the switch to turn, relay 1 (87) is closed the path goes to the second coil, then the common, then the first one which goes to the second relay (87b) which is open so it goes to 31 which goes... somewhere. It also appears to go to the mirrors, which makes sense.

When switch 2 is closed (pressed) the path goes to the first coil, the second relay goes to the second coil, and then to ground. Both sides are positive so the ground is the center (neutral). I think....

It's possible if one relay is stuck it could blow things up, hm.... Need to look at this diagram more....

The schematics can definitely be confusing. It took me several hours staring between all the pages trying to figure out the whole thing. 31 is actually a ground.
So here's my more in-depth explanation:
The first setting (turning the knob) closes the first relay. This sends power to 87b, the second grid in the rear heater. The two grids are connected together at pin 87 so the second relay is left open. The power from the second grid goes through the first grid, then to ground, so the voltage drop through each grid is about 6v.
The second setting is actually timed. Pushing the button tells the circuit to instead close the second relay for a set period of time. Leaving the first relay "open" connects 87b, which is the second grid, to pin 31, which is CE ground. This gives full 12v through both grids and the mirrors, to ground.

...I hope this helps make a little more sense :D

It does, thank you. The hot question then is where is the short coming from, either it's in the mirrors or something else. Can't be the rear defrosters as they are connected in series without blowing up. I may just have to wait for the next fog/frost to see what happens, then start tracing wires. Maybe one of the side mirrors is shorted..

Much more complicated than the 944S, where the switch just turns on the hatch and the side mirrors. Quite a complex little system.h

Finally got some fog the other night and tested the defogger.

On low the bottom half of the rear window would defog but not the top. Looks like that might be where my short is, explains why it blows the fuse at high setting (the top loop would be in parallel).

Any thoughts on what to look at next?

Though I haven't yet encountered ground shorts on mine, I would start unpluging until there's no more short. Or use a multimeter to test for resistances to ground.
Maybe the easiest to start with would be the doors? Then the rear hatch, and working back to the electrical panel. Getting to the rear heater connectors requires taking off trim panels and, although it's relatively easy, you should take some care in it. Through my current experiences the little clips are easy to break and the trim panels can bend out of shape.

According to the manual the functionalities are as follow

Turn – Push – Action
Off – Off – off, that was an easy one :roflmao:
On – Off – low power
Off – On – full power for about 15 minutes
On – On – full power for about 15 minutes then low power
As long as defogger resistances are powered, the light in the button lights up at full brightness

There are two relays inside relay I-II
Both relays off, power off :evilgrin:

When r1 is activated by the electronics, both resistances are in parallel, read full power.
R4 via: “30” > fuse 19 > relay I-II 30 > relay I-II r1 > relay I-II 87 > T12 > R4 > T13 > relay I-II 87b > relay I-II r2 > relay I-II 31 > ground
R5 via: “30” > fuse 19 > relay I-II 30 > relay I-II r1 > relay I-II 87 > T12 > R5 > MP VI

When r2 is activated by the electronics, both resistances are in series, read low power.
R4 via: “30” > fuse 19 > relay I-II 30 > relay I-II r2 > relay I-II 87b > T13 > R4 > R5 > MP VI

When both relays are activated only R5 will generate heat since R4 is shorted by both relays r1 and r2. So I don’t think this a normal operation case.


The mirrors are always powered, full power when r1 is activated, low power when r2 is activated since in series with R4.
Pin 50 deactivate the defroster when the starter is activated.
Maybe a bit too late for the OP, but maybe helpful for others… …

Still helpful and quite amazing job of documenting the flows. Been a bit too cold/busy/etc to get things done on the car, but will look at this and see if I can find the short.

What's the software used to draw those images, I could use that in a few other projects.

C

Hm. Thinking about it, the side mirrors were *NOT* defrosting even with defrost on low. Maybe the problem is one of the mirrors is shorted, that would explain why one rear element is out on low. Hm. Anyone know what their resistance should be in ohms?

I think it's possible to remove the side mirror without breaking it, I remember having to replace the driver's one, will check the passenger side one......

I use this software : https://www.expresspcb.com/expresssch/

I think you better check at connectors T3 & T4, they are inside the car, no need to take off the mirrors, yet… …

I'm pretty sure none of the relays inside relay I-II get stuck.
If that was the case you should have a major current draw because the defroster would always be on.

Most likely, like you said one of the mirrors is shorten.
When in high power, fuse blown due to short circuit via r1 and one of the mirrors
When in low power, R5 is shorten by one of the mirrors, only R4 is working.

So, have a look at connectors T3 & T4, pins 5 & 9, you can measure the resistance over there.
With a bit of luck one is ok and you will know the resistance value.


WATCH OUT:
All above connector number & pins are for a ’89 car!
You have an ’86, so for sure numbers are different!
But principle is the same.