Need some Guru assistance with an electrical issue

[Irish Reiver]

Background
A few weeks back I had to go and rescue my daughter from a flat battery. She was at school and typically arrives early (06:45) to avoid the traffic and so sits in the truck with the AC running to listen to music and wait on her friends. Since she always drives with the lights on auto, my assumption at the time was that she somehow managed to mess up the "ignition off door open" sequence which would have turned the lights off.

Fast forward to Friday and the battery was flat again. This time I was the last one who drove the truck and so I know the headlights were not on so not in the mix. I let the battery charge back up but with the rain it was hard to do any troubleshooting until later in the evening when it was dark. This was somewhat fortuitous because when I got in the truck I noticed the "Brake" light and "Seatbelt" light were dimly lit. The keys were still in my pocket at this point. I then noticed that all the appropriate lights in the HVAC control module were also dimly lit. With the ignition on all these lights come on with full brightness.

My initial assumption was that I must have a bad ground somewhere so I went through the EWD and according to the page 181 of the Ground Point chapter both the Brake and Setbelt lights are grounded on the passenger footwell at point "ID". I thoroughly cleaned up this connection and just for s***s & giggles did the same on the equivalent ground point in the drivers footwell ("IE") since the wiring diagram shows they are connected.

Anyway, none of this has made any difference to the issue.

The EWD shows these functions run through connector J2 which appears to be located up behind the instrument cluster so I thought I would reach out to the hive mind before digging in deeper in case I am missing some basic logic or something obvious.

Thanks in advance for your help.

 

[Malleus]

Since Toyota operates circuits by grounding them, my guess is that you have a short, not an open ground. My initial thought ran to the courtesy circuit that keeps the lights on and window switches operable, until you open the door, but that probably isn't the problem, because that function doesn't have a direct connection to the brake light or seatbelt indicator on the combo meter.

IME, the interior connector housings usually aren't teh root cause of electrical problems, it's the wires at the terminals on the back ends. Just teh opposite for external, sealed connector housings. They tend to grime up inside where the terminals join.

There are a number of ways to find a short, but probably the easist in the 80 is just to start pulling fuses from the dash panel. That won't indicate where the problem is, but it'll tell you where it isn't. Once you find the circuit, you can then trace the wiring in that circuit.

FWIW, my daughter's 80 had a funky problem when her alternator regulator died (and killed the battery) recently. The throttle position sensor lost its mind when I jumped it –the tach went to 3500 and kept climbing. I had a loose ground. Took forever to find it.

 

[Mat Jadamec]

Check voltage while running with no load. Could be alternator or battery

 

[Irish Reiver]

@Malleus thanks for the thoughts on the internal connectors - good call. I moved onto step 2 and began pulling the fuses 1 by 1. Hit a yahtzee when I pulled the "Defogger" fuse. I heard the relay click (off I assume) and the Brake & Seatbelt lights as well as the HVAC control lights all went out. Re-seating the fuse did not make the issue return.
I checked the defog switch and it was activated. I have not been able to replicate the issue however, since I haven't fixed anything, the potential for it to happen again remains.
I will spend some time this week trying to follow the EWD to see if there are any more clues. Thanks again for the sound advice.

 

[60 toy ota]

Background
A few weeks back I had to go and rescue my daughter from a flat battery. She was at school and typically arrives early (06:45) to avoid the traffic and so sits in the truck with the AC running to listen to music and wait on her friends. Since she always drives with the lights on auto, my assumption at the time was that she somehow managed to mess up the "ignition off door open" sequence which would have turned the lights off.

Fast forward to Friday and the battery was flat again. This time I was the last one who drove the truck and so I know the headlights were not on so not in the mix. I let the battery charge back up but with the rain it was hard to do any troubleshooting until later in the evening when it was dark. This was somewhat fortuitous because when I got in the truck I noticed the "Brake" light and "Seatbelt" light were dimly lit. The keys were still in my pocket at this point. I then noticed that all the appropriate lights in the HVAC control module were also dimly lit. With the ignition on all these lights come on with full brightness.

My initial assumption was that I must have a bad ground somewhere so I went through the EWD and according to the page 181 of the Ground Point chapter both the Brake and Setbelt lights are grounded on the passenger footwell at point "ID". I thoroughly cleaned up this connection and just for s***s & giggles did the same on the equivalent ground point in the drivers footwell ("IE") since the wiring diagram shows they are connected.

Anyway, none of this has made any difference to the issue.

The EWD shows these functions run through connector J2 which appears to be located up behind the instrument cluster so I thought I would reach out to the hive mind before digging in deeper in case I am missing some basic logic or something obvious.

Thanks in advance for your help.

**EDIT: I did not see any of the previous replies between the time I started and posted this**

I think you're on the right track and have gone about this is a methodical way. Nice job. I'd agree that you have a floating ground. You may try duplicating the issue, you'll likely be able to turn the dash lights off by pulling one of two fuses: the gage fuse, which circuit is presumably the path back to ground, and some other fuse whose circuit has a compromised ground. Sequentially pulling your fuses to find this second one is an easy way to narrow down on your problem. I'd begin with fuses that don't require the key to be powered