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I wouldn't jump any wires until you find the short. First, if the fuse doesn't blow with everything connected, but off, then disconnect the purple wire going to the rear defroster. If the fuse doesn't blow, then your short is between the switch and the grid, BTW, there should be at least .5 ohms (1/2 ohm) resistance between the switch and the grid. If the fuse did not blow with everything connected, but off, then that eliminates a short between the fuse and the switch. If the fuse does blow with the switch connected and turned on, then the switch/relay combo is bad. Also, just make sure that black wire coming from the switch is a good ground. This is a pretty simple circuit, with the solid state relay/timer in the switch. Good luck.
I wouldn't jump any wires until you find the short. First, if the fuse doesn't blow with everything connected, but off, then disconnect the purple wire going to the rear defroster. If the fuse doesn't blow, then your short is between the switch and the grid, BTW, there should be at least .5 ohms (1/2 ohm) resistance between the switch and the grid. If the fuse did not blow with everything connected, but off, then that eliminates a short between the fuse and the switch. If the fuse does blow with the switch connected and turned on, then the switch/relay combo is bad. Also, just make sure that black wire coming from the switch is a good ground. This is a pretty simple circuit, with the solid state relay/timer in the switch. Good luck.
Thanks for the additional tips Mrvettenick!
It seems that I'm not getting any power out to the grid from the switch. I've got HOT Orange and HOT Pink with the key on. I'm getting no power to the Purple wire with the switch on or not.
Getting ready to check the puny ground wire next.
UPDATE
Fuse hasn't blown for some reason.
I'm thinking its the darn switch. Correct me if I'm wrong.
The ground checks fine.
I never found a relay.
Now if I could find that new defroster switch. I bought one and can't find the darn thing.
Last edited by Street Rat; Feb 17, 2020 at 10:58 AM.
Hello All,
I've been wanting to use the convenience of the rear window defroster on my '81.I have replaced the operating switch on the console. That didn't fix the problem.
Originally Posted by Street Rat
Now if I could find that new defroster switch. I bought one and can't find the darn thing.
OK, that explains some things. It crossed my mind that the relay might be in the switch, but since you said replacing the switch didn't fix the problem, I dismissed that possibility.
OK, that explains some things. It crossed my mind that the relay might be in the switch, but since you said replacing the switch didn't fix the problem, I dismissed that possibility.
Hello bradleyb66,
For some reason the fuse didn't blow during the testing of the system.
I don't get it. It seems like I'm chasing my tail.
I had previously tried the new switch by just connecting it to the harness and the system didn't work.
I found the new switch this morning.
I'm going out to plug it in and try it again after the weather clears up today.
So here is how I would have drawn the circuit diagram if the relay and switch are combined. Orange should be hot always, and pink should be hot with the key on. If purple is hot, then defroster should be getting voltage also. Perhaps the reason that the light goes out after 3 seconds is that the circuit detects no current flow (bad power connection or ground connection at the window?), and de-activates?
So here is how I would have drawn the circuit diagram if the relay and switch are combined. Orange should be hot always, and pink should be hot with the key on. If purple is hot, then defroster should be getting voltage also. Perhaps the reason that the light goes out after 3 seconds is that the circuit detects no current flow (bad power connection or ground connection at the window?), and de-activates?
That definitely simplifies the circuit diagram. I actually came up with a similar drawing to help me figure this out.
With the switch and key in the on positions the purple wire is getting no power.
Pink is hot. Orange is hot. Black is grounded.
So you are saying that the purple circuit may disconnected from the rear window grid?
Another possibility is the switch is bad and not getting power to the purple wire?
So you are saying that the purple circuit may disconnected from the rear window grid?
Another possibility is the switch is bad and not getting power to the purple wire?
Sorry, I mis-read your comment about the purple wire - I thought you said you HAD power on the purple wire with switch on and off (my bad!). You could jumper the orange and purple wires together, and then verify power at the window, but my vote is a bad switch. Put the new switch in, then if it still doesn't work, post the symptoms and where you have power at.
Today I had the time to do some more testing on the defroster circuit.
I'm using a NEW switch to do this testing. The old switch is still in the console.
The pink wire is hot at the connector with the key on.
The orange wire is also hot.
I jumped the orange wire and the purple wire and the rear defroster got hot. The 10ga jumper wire I made did not get hot at all.
When the NEW switch was hooked up to the connector and activated the following things happened...
The 20 amp fuse would blow. Thus no power to the pink wire. No indicator lamp. No power to purple wire.
I replaced the 20 amp fuse numerous times. Each time the activation of the switch would blow the fuse.
Power to circuit breaker was good. The power to the orange wire on fuse block marked defroster was good all the time.
So I hook everything back up to the switch that is still in the console.
The old switch lit up and the purple wire got power.
The fuse did not blow with the old switch connected. The fuse had blown previously with the old switch so I don't understand why it's working now.
At this point I have yet to actually use the defroster since it is working again. Tomorrow morning should be a good test for it.
So here is how I would have drawn the circuit diagram if the relay and switch are combined. Orange should be hot always, and pink should be hot with the key on. If purple is hot, then defroster should be getting voltage also. Perhaps the reason that the light goes out after 3 seconds is that the circuit detects no current flow (bad power connection or ground connection at the window?), and de-activates?
OK. Let me make sure of what you're saying, The 20 amp gauge fuse is the one that's blowing?
There are the gauge and tell tail lights,seat belt buzzer/ light, A/C compressor, and the defogger on that one fuse.
That being said, any one of those devices can trip that fuse. I would disconnect all of those devices, and isolate the defogger circuit on that fuse.
Then connect them one at a time. When the fuse blows, the last circuit that you connected has the short. You might be looking in the wrong area.
My first choice would be the seat belt buzzer circuit.
Last edited by Mrvettenick; Feb 22, 2020 at 06:49 PM.
OK. Let me make sure of what you're saying, The 20 amp gauge fuse is the one that's blowing?
There are the gauge and tell tail lights,seat belt buzzer/ light, A/C compressor, and the defogger on that one fuse.
That being said, any one of those devices can trip that fuse. I would disconnect all of those devices, and isolate the defogger circuit on that fuse.
Then connect them one at a time. When the fuse blows, the last circuit that you connected has the short. You might be looking in the wrong area.
My first choice would be the seat belt buzzer circuit.
Yes the 20 amp fuse marked CTSY and CLK is the fuse that keeps blowing.
It has many circuits on it but not all that you mentioned.
Most of those circuits are already disconnected.
Dome, horn relay, power windows, and defroster are still connected.
It's easy enough to disconnect the windows by pulling the circuit breaker.
Pulling the horn relay and dome light bulb should exclude those circuits.
Am I on the right track?
I'm looking at the service manual right now. The 20 amp fuse that I understand is blowing is the gauges fuse. That being said, that fuse powers gages/telltail lights, seat belt buzzer , the defogger, and the interior light delay module. And nothing else. You need to look at what that fuse powers. There may be something that's borderline, and the defogger circuit just puts it over the top. The courtesy clock fuse has nothing to do with the defogger. Let's make sure you have the correct fuse. We're on different pages. Can you verify the correct fuse on the box?
Last edited by Mrvettenick; Feb 23, 2020 at 12:43 PM.
That chart is not accurate. If you go to page 8a-31, you will see that the courtesy fuse powers a lot, except the defogger circuit. Take a look at the fuse box to see which fuse is blowing.
The gage fuse is in the middle at the very left GAGES, while the courtesy fuse is in the middle almost at the bottom. CTSY CLK. So let's make sure which fuse is blowing by actually looking at the fuse box.
I have circled the fuse that is blowing in the attachment on my last post.
The fuse box is marked CTSY / CLOCK.
The fuse is a 20 amp yellow fuse.
I see the connection S260 on the other page that you mentioned. Yes there are a few more items on that circuit.
No wonder the fuse is blowing.
OK great. Just a note. If the gage fuse was blowing as in your original post, then one thing that both fuses have in common is the interior delay timer. That might be your issue. One other item could be the visor mirror light. Depending on how much usage it gets, the harness gets pinched. Let us know how you make out.
I've been studying the wiring diagrams in great detail.
The page 8A-31 does not show the defroster connected to the CTSY/CLK fuse. I see your point Mrvettenick.
The page 8A-56 shows the defroster supply power connected to the 35 amp CB and the GAGES fuse which is also a 20 amp fuse.
Somewhat confusing indeed because the defroster switch activation blows the CTSY/CLK fuse now.
So my understanding in a simple man's mind, is that something else is blowing the CTSY/CLK fuse besides the defroster switch. As the defroster switch is not even in that circuit.
This is what you have been trying to explain to me Mrvettenick. I think.
Somehow the switch, light, and defroster grid are functional. For now.
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It crossed my mind that the relay might be in the switch, but since you said replacing the switch didn't fix the problem, I dismissed that possibility. 
