Constant Power at 12v UNTIL wired to head unit, then... not 12v. Stereo doesn't work!

[TomServo]

Hey, everybody! I've just come into a 1997 Celica GT that I'm trying to fix up for my daughter. It's running perfectly fine, but someone previously tore out the radio (and cut the convertible top, jerks), so I'm trying to put in an aftermarket radio and running into some roadblocks:

1. The radio has a weird problem. I found a matching wire harness on Amazon after some searching, but the radio doesn't seem to turn on at all. So, I broke out the multimeter and tested the bare yellow - 12v with the switch off. Bare red - 12v with the switch on. Bare ground seems to be good. So, the radio should work, right? Well, if I wire the yellow directly to constant power (yellow) on the stereo, then contact it with the multimeter lead, it no longer reads 12v. To restate - Bare yellow = 12v. Wire yellow to yellow = no longer 12v. I don't know why. My limited electrical experience does not lead me to any obvious explanation.

So... is there any fuse-related solution, or obvious solution? Could it be ground related? I have tried three separate radios and have the same problem with all three. I have tried grounding to the metal bracket or the black ground wire.

2. Also, just over the stick shift there's the electric convertible top switch and the rear window switches. The electric convertible switch works fine - the top sets and retracts without incident - but neither of the rear windows raises or lowers. So, I'm convinced that those switches are on their own fuse. Can anybody confirm or deny this? Or know exactly which fuse it is? Any complete fuse layout is appreciated, I've Googled and found some but it's not helping to this point.

Thanks in advance for any help to these questions. I'm looking forward to your responses.

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[winkychevelle 10+ year member]

It's easy to check fuses with a test light or multimeter. Prolly popped the radio fuse in the process of wiring up the head unit. I've done it a few times no big deal just s pain in the butt

 

[TomServo]

It's easy to check fuses with a test light or multimeter. Prolly popped the radio fuse in the process of wiring up the head unit. I've done it a few times no big deal just s pain in the butt

I thought that, but if the fuse was popped, the yellow constant wouldn't still be reading at 12v, right? And it still is, until I hook it up to the stereo. I can repeat this over and over as many times (and tried with three stereos) as I like, but I don't get power to the head unit.

 

[winkychevelle 10+ year member]

Are you sure the yellow wire on the car harness is a constant on? If it is a can bus or data wire it may read 12v but it is not designed to be constant.

When in doubt rerun a new constant on and ignition wire

 

[winkychevelle 10+ year member]

1997 Toyota Celica Car Stereo and Wiring Diagram | radiobuzz48.com

Here is the factory color code

 

[TomServo]

Are you sure the yellow wire on the car harness is a constant on? If it is a can bus or data wire it may read 12v but it is not designed to be constant.
When in doubt rerun a new constant on and ignition wire

Well, as close as I can tell the only wire in the harness that reads 12v with the ignition off is the yellow one, and it matches up with the yellow on the harness I bought, so.... I think so. It's an older system (97) so there aren't a lot of other wires to choose from. I'll compare with the factory colors that you sent to be sure I've got it all in order. Also, I don't know who installed this harness... it might not be factory. Thanks for the help to this point.

 

[wew lad 5,000+ posts]

sounds like you're grounding out the yellow wire

honestly it would be best to start over and get back to the factory wires, someone might have f'ed it up

 

[TomServo]

So I checked all the fuses and tried a few things... same result. I will say that the exact power is about 11.4V on the yellow constant wire and 14.3V on the red switched wire... is that changes anything.

Tried grounding directly to the chassis, using the harness ground, basically... everything I could think of. :\

 

[THATpurpleKUSH 5,000+ posts]

Sounds like there's a problem with the yellow wire elsewhere in the car. Yellow is usually the constant and red goes to the accessory switch. Try running a separate constant wire directly from the battery. If it works then you know what the problem is and can fix it or just wire in an on/off switch and call it a day.

 

[TomServo]

So.... someone else suggested I run a wire directly from the battery just to confirm the head unit was working. Since I had a 12V battery charger just sitting there, I hooked it up and voila! the head unit powered on. A little testing confirmed that the head unit would only power on with power to both yellow and red - I twisted them together and clipped them to the charger's red clamp, and then clipped the ground to the charger's black clamp. The head unit powered on. With just yellow OR red connected to the red clamp, neither.

So... I guess I need to re-check all the wiring and maybe run new wires. I freakin' hate the idiot who tore the old radio out AND cut the convertible top to get to it. People are jerks. :\

 

[TomServo]

Thanks for the responses and thoughts. I solved the radio wiring issue... it turns out there's a built-in factory amp that when I was wiring to the harness, it was the harness coming from the AMP... so when I snaked out the amp's harness and wired into that instead, everything was working.

Well, almost everything. The constant was still jacked, but everything else works great. I ran a new constant from the hazard lights, BUT I did change the hazard fuse from a 10 to a 15 to make sure the 12v needed wouldn't pop that 10. I can't imagine this will have a negative impact in the long run, but if anybody thinks I'm wrong, let me know.

 

[wew lad 5,000+ posts]

Thanks for the responses and thoughts. I solved the radio wiring issue... it turns out there's a built-in factory amp that when I was wiring to the harness, it was the harness coming from the AMP... so when I snaked out the amp's harness and wired into that instead, everything was working.
Well, almost everything. The constant was still jacked, but everything else works great. I ran a new constant from the hazard lights, BUT I did change the hazard fuse from a 10 to a 15 to make sure the 12v needed wouldn't pop that 10. I can't imagine this will have a negative impact in the long run, but if anybody thinks I'm wrong, let me know.

eh.. not really but you should consider running just a simple wire from the fuse panel under the hood and have a 5 or 10a inline fuse on it