1986 MGM 5.0...defog switch

[Toploader]

So nice day today and I put in a new (NOS) rear defog switch in my '86 Tudor.

Question: Is it normal for the switch to be energized even though the ign key is lol...in my pocket?

I thought the switch would only be energized with the ign key in the start position? When leaving he car I made double sure that the little tel-tale light is not illuminated.

Thoughts???

[gadget73]

There is a supply of constant power and a supply of key-on power. The key-on pulls a latch that will let the defrost stay on. When the power is cut, the latch releases and the switch isn't supposed to hold. If you hold it in the up position it will come on but it should go out as soon as you let go of the switch.

one fail mode is they don't release properly and the defrost stays turned on.

[Toploader]

I am praying a bad switch was the cause of my constant dead battery.

[mcb82gt]

Its easy to verify. unhook the battery neg post and put voltmeter inline from cable to neg post on battery. if you have voltage draw, pull the fuse for the rear window defrog circuit and see if it goes away.

There is a small voltage drain that is normal, someone smarter than me will post what that is lol.

Good luck!

[gadget73]

something like 10 milliamps. A stuck defrost switch will kill a battery flat dead in a few hours.

[sick88tbird]

On a modern car, 50mA is about the upper limit of acceptable current draw. On one of these cars, I’d expect to see very little as we don’t have a billion modules all communicating over a CAN-BUS waiting to go sleep. A stuck defrost switch should be pulling MULTIPLE Amps, not milli-amps...like Gadget said, a stuck defrost switch will murder a battery in hurry.

[sly]

On modern cars, I've seen 300-500mA initially after power down. After 20-60 minutes (depending on the car) it drops to 100mA or less. 100mA is not uncommon in more electronics heavy vehicles.

[Toploader]

Gents today marks the beginning of the third day since I put in the new switch. As of this morning the battery has not gone dead.
The waiting continues! I think that after five days or so if the battery has not gone dead, then it was indeed a bad defog switch. Speaking of that switch, even Green Sales has not had one in quite some time..........I was fortunate to find just one on e-thief that was NOS....$20.00. On another note.......so many NYC drivers are scum..........came out and found the drivers side Q-panel (where it ends at the door) was pushed in by a vehicle with a high bumper like on an SUV, no the creep did not leave a note.

[Toploader]

On modern cars, I've seen 300-500mA initially after power down. After 20-60 minutes (depending on the car) it drops to 100mA or less. 100mA is not uncommon in more electronics heavy vehicles.

Sly the thing that puzzled me was the fact that when tested the draw was almost zero (0.000.7) milliamp.

[Arquemann]

Even a digital or analog clock draws more than 7 milliamps (mA)...

[sly]

Yeah, I'm thinking the meter might have a bad fuse in it.

[gadget73]

if it wasn't dead in the length of a good dinner, the defrost switch isn't stuck. I can vouch for the dinner timing, old switch in my Towncar was bad, drove to my aunt's for Thanksgiving, it was dead as a stone by the time I was leaving, maybe 4 hours later. As soon as the jumper cables got hooked up I saw the yellow light on the defrost and knew.

[Toploader]

Yeah, I'm thinking the meter might have a bad fuse in it.

Sly two different meters had the same readout.

[Toploader]

if it wasn't dead in the length of a good dinner, the defrost switch isn't stuck. I can vouch for the dinner timing, old switch in my Towncar was bad, drove to my aunt's for Thanksgiving, it was dead as a stone by the time I was leaving, maybe 4 hours later. As soon as the jumper cables got hooked up I saw the yellow light on the defrost and knew.

Thain the first thing I removed from the car was that switch, with the batt hooked up.........after 5 days I put the new one in, so to me it was the defog switch that was bad.

[sly]

Sly two different meters had the same readout.

I've had 5 meters with blown current fuses at one time... so lack of checking the fuse due to multiple meters with the same goofy reading isn't a good excuse. Check the fuse. It's a sanity check. It could be fine and the mystery continues, but it may be the meters.