creating a bench test device to check if car parts work.

I am not very good at making homemade electrical devices, So, I’m hoping someone out here can help me out.

Here’s what I’d like to do… I’d like to be able to hook up a horn, radio, or anything else that runs on electric current from the car’s battery when the car is put together properly.

I believe the horns, for example, need 12 volts. Could I go out and buy myself say 2 six volt batteries, hook them up together and run a wire from the power source (batteries) to the individual horn to check to see if it was in working order?

Right now, the only way to check anything myself is to put it in the car and test it by using it as you would in the normal fashion the device would be used… blow the horn or turn on the radio. It would be great if I could check these parts outside of the car. So if I have a dead radio in my car, and I find a radio that could fit, I could quickly check the radio outside of the car and without the car’s battery?

Anyone have an idea on how I could do this?


you could use two equal 6V batteries (of adequate size) in series, you could use a single 12V battery (of adequate size). you could use the battery from your car.

you could buy yourself a 12V motorcycle or alarm system battery that could work. you could buy yourself another auto battery. you could buy a 12V DC supply of adequate size.

Don’t use batteries - they are too expensive, and too low power. Get a 12v power supply like this.

I usually just use a 12volt car battery charger to check to see if things like car horns or window motors work.

I guess what I’m asking is how would it look hooked up? What wires go where? I understand the concept of 2 6 volts = 1 12 volt battery, but beyond that, I’m at a loss as to how I’d run wires from a horn or radio to the battery and back again to either hear the horn blow or hear static on the radio.

Maybe I’m not explaining this correctly. I’ve searched on the web (figuring this kind of bench test setup would be already out there, and I could either buy it or make one myself. But I’ve not found anything like that at all.

Thanks for the replies so far!

It’s pretty simple.
There will be a “hot” terminal on each device (horn, radio, etc). This goes to the + terminal on your power supply (or battery). The case of the device is “ground” this goes to the - terminal on your power supply. For devices without case ground (like lamps), one wire will be hot, and one ground.

The radio may have a power input wire. It will generally be red or orange, and may have an in-line fuse. If it’s a factory radio, it may not have a separate power cable; you’ll have to find a wiring diagram for the car or radio to find out which terminals on the plug is for power. The horn, generally, won’t as the car’s wiring harness will plug directly into it.

Some accessories will only have a power lead, the accessory is grounded to the frame that that provides the return path. This was almost universal on older cars, but with the widespread use of plastics, this may not work, so they may have two leads, one +12v and the other ground.

Also, with the modern use of computerized electronics, if you don’t know which lead is for +12v, you should probably do some research, since you can fry some things if you give it +12v and it wasn’t designed for that.

excavating (for a mind)

to add to answers

a device will have a wire labeled ‘hot’, PWR, POWER, 12V or similar that goes to the positive (maybe also called + or plus) terminal on your battery or power supply.

a device may have a wire labeled ‘ground’, ‘negative’, ‘neg’, ‘- (that is a minus sign)’ that goes to the negative (maybe also called, neg or - (that is a minus sign)). if there is not a wire like this then the mounting point (screw or bolt hole, bracket) is the negative or ground connection.

this for USA auto, some nonUSA autos will have the hot and ground be different.

Why not just stick the leads of the device to the posts on the battery that is in the car, that’s how I test shit.

Depends on the age (and brand) of the car. For example, even the previous generations of BMWs used a bus called i-bus (and sometimes others, depending on the model), and if a device didn’t see the bus, it wouldn’t do anything even if it had power. For some of the dumber devices, a resistor to +12v was enough to get them to at least light up (for example, the IHKA climate control box). Other subsystems were more complex - for example, the radio needed to talk to the seat memory module at startup (for a very convoluted reason - you really don’t want to ask why). Newer BMWs use a fiber optic bus which is even harder to fake.

Even the less-expensive brands had a bunch of weird interactions. One common problem was that a car maker would integrate the keyless entry/alarm system with the radio, so installing an aftermarket radio involved stuffing the original radio behind the dash so the doors would still work.

This is a good point.

The good news is, I’m dealing with a 60’s muscle car, so I shouldn’t run into any of these technological advances.

I’m kind of confused what the issue is here. If you want something ready to go to test 12 volt auto accessories all you need is this. Wasting expensive batteries on this would be crazy.

or this