View Full Version : Radio power problem. stumper.

01-15-2010, 12:06 PM
Always on voltage drops when aftermarket radio switches ON. It is as if it is trying to pull power from the always on power instead of the switched power.

1989 4runner. Aftermarket radio. Was working fine.
Picked up truck from storage lot. Started right up, but the radio will only show the demo screen, as soon as I turn it "ON" (any audio source) it flips on for 1/2 second, then shuts down. Repeat....

Some days it would work a little better than others, but more volume would trip it to off. Tried turning off various speakers thinking shorted speaker wires, but no better. Last couple days= nothing.

Into the dashboard.
Try new head unit. Same problem. Unplug all speakers. Same problem.
Down to 3 wires. Always on power, ignition switched MAIN power, and ground wire. Same problem! Hazards work, (always on ckt that the radio is hooked to) lighter works fine too (switched 12V power for the radio.)
Ground directly to chassis or through harness = same problem.

Rip out aftermarket alarm.
Rip out all non stock wiring for CB, fans, etc.
Get out voltmeter.
OK, now we're getting somewhere. 12V switched is always fine, BUT when the radio tries to turn on, the always on voltage drops to <10volts and the radio shuts off due to low power! No relays click, no fuses blow...What the heck! and why did it "just start happening." Bad relay somewhere?
I found one broken ground wire under the hood, but I think it is emmisions related.

Great, even the wiring on my truck is worn out!

01-15-2010, 12:27 PM
Sounds like a ground somewhere.

Great help, eh?

01-15-2010, 12:29 PM
why did it "just start happening." Bad relay somewhere?
Partially broken wires, cracked insulation, corrosion on terminals, etc. It's a higher impedance path someplace so as soon as you start drawing more than minimal current the voltage drops. Remember V=I*R, voltage equal current times resistance, so at low current a resistance (and corrosion is just like electrical resistance) will have smaller voltage drop than higher current. So a 3 ohm corroded connection someplace at 100mA (like just running the radio clock) will drop 0.3V drooping your battery from 12.4V to 12.1V. But at 2 amps (the radio actually doing something) will be 6V, dropping your supply from 12.4 to 6.4V. Obviously it does not take much corrosion to get you 2 or 3 volts of droop with any decent current. Make sense? What you need to do is trace all the circuits that feed the radio and figure out where the rust or dirt or broken harness happens to be. Just tedious, not catastrophic. Also check for a partially blown fuse. If you stress a fuse near it's rated value long enough it increases resistance without ever melting open, but that can also just happen over time and temp.

01-15-2010, 12:47 PM
hehe, yeah thanks, Red Chili. That really narrowed it down. ;)

Thanks DaveInDenver too. It has to be a wire or connection downstream of where the always on power branches from the hazard ckt. The hazards must draw more than the head unit, so I doubt fuse. Can't really test with the horn. Horn has gone from works to sometimes works to never works over the last 10 years. (I think the contact in the steering wheel wore out. At least it doesn't make that scaping noise when I steer any more!)

I'll look for the connection at the branch from the hazard ckt. Not sure where that is, but I have the real factory books including the electrical volume at home.

01-15-2010, 12:52 PM
maybe the hazard relay is being triggered on and of by the load of the radio since they are on the same circuit.. is the load before or after the flasher/relay?

wire it to another always on power source instead of the hazard ckt... i dunno...

01-15-2010, 01:08 PM
Marco: The hazards work fine. That was just my test of the always on power/fuse.

I'll try to figure out where the radio always on power taps into the hazard ckt.
I'm already thinking about "plan B." There is convient always on power on my air locker switches down there.

Maybe I accept the fact that I have aging wires and the ol' truck requires an electrical bypass. I guess I'm lucky it hasn't affected the EFI (yet.)
Besides, I always thought those wires were a little small for my bumpin' system.
word up yo.

01-15-2010, 01:14 PM
Stop listening to rap and listen to classical music. It'll stress the wiring less...


01-15-2010, 01:41 PM
I solved a bunch of problems in my 40 when I ran fresh wire to the radio and to the CB. Stopped blowing fuses and the CB seemed to work better too. I installed a new fuse block on the firewall. Easier to fix with new wire than to find the location(s) of the problem in the old harness.

01-15-2010, 03:02 PM
I run my stuff from a new fuse box. May be an idea for you to wire all the radio stuff and ARB to a new fuse box directly from the battery.

01-15-2010, 03:14 PM
I really like all the input.
DaveInDenvers point that it only takes a few Ohms to cut my voltage in HALF is the most interesting point!

Looking at the Toyota radio harness, the "always on" grey wire is a really light guage. while the switched power red with green dots is nice and chubby.

Looking at the Sony end of the harness, the "always on" yellow wire is nice and thick while the switched power wire is a lighter guage.

I think Sony intends to power the radio over the always on ckt while Toyota designed the radio to run off of the switched power ckt. I have been abusing the always on wire for 5 years with my penchant for rap music caused some latent damage to it.

My plan is to use the always on ckt I pulled to the center of the dashboard (at least for now!) I tested out this configuration and everything works again.
As for the fuse box, I have a near stock solution for aux power on my rig. On an 80's Toyota with factory A/C, I found a factory add-on fuse box that bolts right onto the main battery connection. Fits like...well stock! This gave me 3 additional ckts with nice big fuses right at the battery.
I should probably isolate the radio/cb/fan switch with a fusebox in the cab, but I'm really pressed for space under the dash as you other mini-truckers know. Maybe someday...

01-15-2010, 03:41 PM
Use some of that ample engine compartment room for the fuse box.


01-15-2010, 11:56 PM
Biggity-bam. All rewired. Much better than it was before, but far from perfect. Lots less extra wire back there. Less extra wire means more room too.

We'll see (hear?) how well I did tomorrow.
Thanks for all the input.

Peace out.

01-16-2010, 01:02 AM
excellent work rudy,