1994 Mazda B4000, parasitic drain from audio amp, can I switch it

I’ve traced a 0.3-amp parasitic drain to the audio power amp (fuse #16, 20A). Pulling the fuse, the radio still works (it’s on fuse #15) but then there’s no sound from the speakers. (There’s also no current drain. I’m weighing if it’s more important for the truck to run or to sound good.)

I replaced the radio a month ago. It was definitely not working right, and the new radio definitely is. With the #16 fuse in place, the sound out of front and back speakers was great. “Was” because if I leave the fuse in, the battery gets drained in a few days, so the fuse is now out.

No idea where the audio amp is. My Haynes manual doesn’t say. The amp has no user controls. It just takes the radio’s output and feeds audio to the front and back speakers. The #16 fuse is what gives the amp power. Some online posts say it’s under the glove box, some say behind the rear right speaker. All agree it’s a bear to access.

So a few questions:

  1. Anyone have an idea why the power amp would be drawing current even when the truck and radio are powered off?

  2. How does one troubleshoot this, short of replacing the power amp, which of course I don’t have an extra lying around and wouldn’t want to replace anyway as it’s otherwise working great.

  3. What do you think of installing a switch on the audio power amp circuit so that when the truck is parked, I switch it off, and no current drain, but when the truck is driven, I switch it on, letting me hear the radio or CD. This is obviously a kludge/ghetto solution, but the advantage is that I don’t have to access or replace the audio amp. I want to keep using the fuse, of course, so am looking for a way to wire a switch into the fuse box while keeping the fuse in the circuit. If I knew which wire goes between the fuse and the audio amp, I’d splice a switch there, but I don’t, and again, the Haynes manual is no help.

Thanks for all advice.

– Steve (1994 B4000, extended cab, bought in 1996, apple of my wife’s eye)

There’s apparently something called a “premium sound amplifier” located near the glove box in 94 B4000’s, situated more toward the passenger side door pillar than directly under the glove box. I presume that’s what is causing you the problem.

It could be drawing current b/c one of the power transistors has shorted out. I had a home stereo audio output amp do that one time. That’s one reason. One thing you could do is check your speakers one by one, do they all work the same and sound the same loudness? That might provide a clue to what’s wrong.

It is sort of odd it is powered up with the key out of the ignition. But maybe that’s just the way it works, they weren’t able to find an ignition switched hot wire at that location, so they hooked it up to a battery hot wire instead, and when it works correctly it draws very little current with no input so there’s no problem.

If one speaker isn’t working, it may be b/c when the power transistor stopped working the amp started to put a DC signal on the output. That’s what happened to my home unit, and that’s a big problem b/c if you don’t realize it, and try a new speaker, it will burn the new speaker out in short order. Ask me how I know this … lol … that’s why my other (otherwise working) home unit only has one speaker now.

If everything appears to be working and no DC ouputs from the power amplifier, I think your idea of just installing a switch to turn it on and off is a good one. Make sure there is still a fuse in the switched circuit though. Then as long as you remember to turn it off, you’ll be back in business.

If you’re going to take the trouble to install a switch in the amp’s power circuit you’re going to have to find the power input. Why not go the extra mile and install a relay instead of a switch in that circuit? Have the relay’s coil energized by something from the fuse box that comes on only when the key is turned on. That way your radio is powered instantly when you start the truck, and you don’t have to remember to turn the amp off every time you park the truck.

George, thanks for your thoughtful advice. All four speakers are working. In fact, the sound inside the small cab sounds great. Am glad to find someone else who thinks putting a switch in isn’t dopey.

Jay, your idea of using a relay instead of a switch is great and hadn’t occurred to me. Hard for me to believe but I have an Associates in electronics. Haven’t used it in decades, though. Progressed upwards in management to where I never had to. Old story. I’m going to start looking for parts.

(Back in Car Talk’s early years and up to about 2006, I listened to the program religiously. Not just catching it on the car radio, but making time for it every Saturday morning. With Tom’s passing, I can’t bear to hear it again. Glad to see the online forum is still going strong. Two useful replies to an obscure question in less than 24 hours. Wow.)

I am willing to bet this started shortly after the new radio install, correct? I would guess that the amp is staying powered at all times. The stock amp has a wire that turns the amp on or off with the radio. I bet that wire is connected incorrectly. You need to get a wiring diagram and make sure it is wired correctly. If it is an aftermarket radio it might just have a dedicated “trigger” wire for an amp.

I agree with PvtPublic since I also believe that the amp is being powered all the time.

“I am willing to bet this started shortly after the new radio install, correct?”

Actually, no. The battery was dying a few weeks before I changed the radio. Adjusting the volume on the radio had almost no effect, so I thought the radio was bad. I still think the radio was bad, because a new radio worked great.

At the time I was troubleshooting the dying battery, I pulled the radio fuse (#15) and measured the same 0.3 amps … but I misread it for 30ma instead of 300 ma. The former is acceptable, the latter not.

So I had the same parasitic drain even before the radio was replaced but I thought I didn’t.

I replaced what might have been a good radio. But the new radio has a CD player and audio in jack, so am not feeling too bad.

This morning, after reading the posts above, I looked for the power amp. Turned out to be easy to find and access. (Thanks, GeorgeSanJose.) Didn’t find any connections that appeared loose or crossed. Worse, I didn’t see any obvious wire that carried +12VDC.

A blue cable goes into the amp that I think carries power. Its sheathing seems tough, like coax cable, but I bet it’s got multiple wires inside. The other end of the blue cable was in the dash when I took the original radio out, and that end is now connected to the new radio (through a Metra adapter).

So I think it works like this: The radio gets +12VDC from the battery. It uses it to power itself, and then sends +12VDC on to the amp. In other words, the radio is the switch for the amp.

Except in my case, it’s not. The amp is drawing 0.3 amps all the time.

Wait a minute. I forgot about fuse #16 (for the power amp). That’s what provides power to the amp. No doubt about it, when that fuse is out, there’s no audio. That circuit is what needs to be switched.

My latest brilliant idea is to get two spade connectors, adding a foot of wire to each. Between the wires I’ll add a 20A fuse and a manual switch. Ghetto to the max, but it keeps me from doing any damage to the truck’s wiring…unless the two spade connectors short in the fusebox.

Maybe I can get a blown fuse, drill into each end, somehow get a wire to stay connected in each end…

Or I could figure out how to keep the amp from drawing current when it’s not powered on.

The stock radio had an amp trigger wire and I bet you get it tied to something that is always hot. I mis-wired the metra adapter for my stereo by putting the trigger from the amp to the power antenna trigger from the stereo. The amp would come on for AM/FM but not for the CD. Had to go back and put it right. If the aftermarket rasio does not have an amp trigger you can use any switched 12v that is on the acc circuit, even the switched power for the radio. At least that way it would only be powered with the key on. I know this is from a 95 but it should be the same.

1995 Mazda B4000 Car Audio Wiring Diagram

Car Radio Battery Constant
12v+ Wire: Pink/Light Blue
Car Radio Accessory Switched 12v+ Wire: Yellow/Black
Car Radio Ground Wire: Black/Light Green
Car Radio Illumination Wire: Light Blue/Red
Car Stereo Dimmer Wire: N/A
Car Stereo Antenna Trigger Wire: N/A
Car Stereo Amp Trigger Wire: Blue
Car Stereo Amplifier Location: N/A
Car Audio Front Speakers Size: 5″ x 7″ Speakers
Car Audio Front Speakers Location: Doors
Left Front Speaker Positive Wire (+): Orange/Light Green
Left Front Speaker Negative Wire (-): Light Blue/White
Right Front Speaker Positive Wire (+): White/Light Green
Right Front Speaker Negative Wire (-): Dark Green/Orange
Car Audio Rear Speakers Size: 5″ x 7″ Speakers
Car Audio Rear Speakers Location: Rear Corners
Left Rear Speaker Positive Wire (+): Gray/Light Blue
Left Rear Speaker Negative Wire (-): Tan/Yellow
Right Rear Speaker Positive Wire (+): Brown/Pink
Right Rear Speaker Negative Wire (-): Orange/Red

Steve_K, what are the positions available for your ignition switch? Varies car to car, but usually there are run (or on), start, accy, lock, off, etc. You might try your current measurement setup in each position (other than start), see if you notice anything unusual happen as you switch from one to the other. It could be your ignition switch is bad and not turning off that power input to the amp like it should. If you jiggle it and the current goes from 300 ma to 30 ma, you know you are on the right track. Worth a try anyway.

I have to ask, have you made sure that power drain is coming from the amp and not something else? Fuses usually supply power to multiple things even though they don’t indicate it on the label. If you have removed the plug going to the amp and seen the current draw go down to a normal level then good, that is solid proof the amp is causing the trouble.

As others stated already, if the trigger circuit for the power isn’t working correctly then that could be causing the problem. I think the trigger from the radio sends 12 volts to the amp in order to turn the amp power ON. I would make sure that the trigger circuit is working correctly.

This happens to MANY factory Amps in vehicles… What occurs is that the Amp is staying on while the rest of the system is shut down. Since the Amp is fed by a constant 12v source…it uses a trigger wire from the factory radio to know when to turn itself on…

Many times the trigger wire from the radio somehow tells the Amp to turn on when the Radio is OFF… What I have done in the past to fix this is to cut the trigger wire from the radio and locate a new 12v switched power source to trigger the Amp.

I would see if the relay inside the Amp isnt just stuck in the ON position. Meaning it cannot shut itself off if it tried. The way to figure this out is to find the trigger wire…and see if the trigger is supplying power while all else is off…if it is…then you need a new trigger wire source… IF the wire does NOT have power…then the relay inside the Amp is stuck ON… When this happens…it gets a little more complex, but is still repairable. You just need to wire up a new external relay to supply 12VDC to the Amp in a switched capacity.

Let us know which flavor of failure you have… Its either a bad trigger wire for the Amp staying energized when it is not supposed to… OR its a stuck internal relay inside the Amp not letting it cut power when it should. Two different failure modes with two different solutions.

I installed Car Alarms, High End Audio and Remote Start systems Professionally for over 15yrs in addition to being a Pro Mechanic and a Field Engineer for Robotic Data Storage Silos…


George, next time I have the hood popped I’ll try your idea of testing the switch. (But am not optimistic it will be this simple.)

Cougar, I’d bet a paycheck it’s just the audio amp controlled by that fuse. But I’ll try pulling off the blue connector at the amp next time I have the hood open, as you suggest.

Blackbird, your information is great but the two glasses of wine at dinner is making it a little hazy around the edges. Let me read it again tomorrow morning.

Thanks to all for your interest and advice.

OK, here’s the latest troubleshooting (with 10 photos and a PDF if Car Talk will let them be uploaded).

First I wanted to find the wire going to the amp that carried the 12VDC that was draining the battery. Found it. It’s yellow, thicker than the other wires, and loses its 12VDC when I pull the fuse. It’s in the large black connector going into the lower part of the amp. If I need to install a switch, that’s where it will go.

Second, I pulled out the Kenwood radio’s instruction manual. (I know, that’s cheating, but I’m getting desperate.) Found two interesting things on the page showing the wiring connections (which I’ll try to upload).

  1. There’s a blue/white wire labeled “P. Cont” (“Power Control wire”) that, according to the manual, goes “to the power control terminal when using the optional power amplifier.” I had NOT connected this wire. (We’ll come back to it in a minute.)

  2. Still on the instruction manual, we see a red wire coming from the battery through the fuse box and the ignition key. We see a yellow wire (see above) coming from the battery through the fuse box AND BYPASSING THE IGNITION KEY.

So it looks like there’s ALWAYS current going to the amp.

That made me think there must be a way to switch the current off inside the amp, as one of you has said (but whose name doesn’t appear on my monitor as I write this post).

The way to switch off the current, it finally dawned on me, might be with the blue/white wire labeled “Power Control wire.” Duh.

So I connected the radio’s blue/white wire to a blue/white wire in the truck’s harness. Why didn’t I originally? Because I didn’t think I was using an amp. I thought the radio was powering the speakers directly.

But did it help? No. With the blue/white wires connected, there’s absolutely no change. With the truck powered off, the key out, the amp fuse in, the parasitic drain is still 300ma. With the fuse out, the drain drops to 30ma.

Damn, I thought I’d fixed it.

AutoZone has a good 20A toggle switch for about five bucks. That looks like the next step in this saga.

(Trying to upload photos, it looks like only a couple can be uploaded at a time. Will use separate posts for the others.)

More photos.

More photos.

Last of the photos.

So, did you remove the connector going to the amp to verify that is truely where the extra current draw is being consumed?

Many cars use the radio system for door chimes etc. so it may be always powered on. Possible the kenwood has not the same power usage as oem when ignition is off. Check crutchfields for your radio and car, there might be an additional part you need.

Second, I pulled out the Kenwood radio's instruction manual. (I know, that's cheating, but I'm getting desperate.) Found two interesting things on the page showing the wiring connections (which I'll try to upload).

Should have been the first thing before attempting the install.

2. Still on the instruction manual, we see a red wire coming from the battery through the fuse box and the ignition key. We see a yellow wire (see above) coming from the battery through the fuse box AND BYPASSING THE IGNITION KEY.

This wire is for the radio time/station memory, nothing to do with amp.

So I connected the radio's blue/white wire to a blue/white wire in the truck's harness. Why didn't I originally? Because I didn't think I was using an amp. I thought the radio was powering the speakers directly.

But did it help? No. With the blue/white wires connected, there’s absolutely no change. With the truck powered off, the key out, the amp fuse in, the parasitic drain is still 300ma. With the fuse out, the drain drops to 30ma.

Just because there is a blue/white wire from the radio, does NOT mean it goes to a same colored wire in the trucks harness. The info I have shows a plain blue wire from the amp. This whole thing may be a moot point. Because it looks like you are running full speaker power into the amp. You need to use the pre-amp outs to the truck amp. You radio only has 2 and you need 4 since the trucks amp powers 4 speakers. Full speaker power will kill the trucks amp. I would just completely bypass and disconnect the trucks amp and run wires for all 4 speakers directly to the speakers.

Did you do as @Cougar mentioned first? It’s really easy to do. Simply disconnect the wire harness to the amp, put the fuse back in and see if you still have a 300mA load. Quick and easy before you cut any wires.