Over the last month, my rear defrost has been working intermittently. The symptoms are as follows.

Some days:
Push the switch and it works (amber light), window defrosts, great.

Other days:
Push the switch and nothing happens; no amber light, no defrost.

I tried the MFU test today when it was not working but the test didn't seem to work. I held the defrost switch and turned the key to position 2 and no beeps.

So, I tore apart the fuse box to get to the MFU, took it out and didn't see anything unusual. Then I put it all back together.

Then I turned the car on and pushed the defrost button and it worked! So I turned it off and tried it again and it did not work. Grrr!!!

It worked/didn't work a few more times and now it isn't working.

All the other functions tied to the MFU appear to be working fine (intermittent wipers, interior lights, auto dim, etc.)

I checked the connections to the rear window and all is well as far as I can tell.

I'm wondering if.

A. The MFU could still be flaky and need replaced
B. The defrost relay on the fuse box is flaky/bad
C. The switch is bad/dirty
D. Something else
E. All of the above

Thoughts?

 

The switch is dirty. Clean it.

 

I second this...

Most switches in my truck have been replaced at this point.

 

Cleaned the switch and the connector with some CRC electronics cleaner, dried it out, reinstalled. Doesn't work.

Funny thing is, on my way to get the CRC cleaner, I pushed the button and it came on. But, it wouldn't go off until I turned the key off. After that (and cleaning) it doesn't work; at least not when I want it to!!

So, I guess next steps are.

1. Buy a spare switch and replace it.
2. If #1 fails, buy a spare relay and replace it.
3. If #2 fails, buy a spare MFU and replace it.
4. If #3 fails, give up.

Additional troubleshooting tips welcome!

Happy Holidays!

 

Replace the switch. Just what I'm going to have to do with my turn/headlight switch. They just get progressively more flaky until they just cease functioning. Are the switches made by Lucas by any chance?

 

oh yea, they're lucas switches.

 

Ordered a switch from Rovahfarm.

Also ordered some of this since I was out.

Lucas Replacement Smoke Kit

 

Installed the new defroster switch today and while I now have a working (green) dash light in the new switch (so it must be getting power), it does not activate the defroster. The old switch worked this morning, but then did not work this evening.

So it must be something else. I did notice that defroster went off automatically after it worked a while this morning, but I was able to immediately turn it back on.

Perhaps auto-shut-off is normal, and maybe that is related to the intermittent behavior?

I supposed next steps would be to confirm power to the defrost grid, then consider replacing the relay and then the MFU even though everything else tied to the MFU is working without issues.

Is the relay readily available? I haven't seen it at the usual parts suppliers.

Would appreciate additional suggestions if you have them.

Thanks!