Hey everyone,

I have a backup light switch that doesn't appear to be working. My current diagnosis has shown that 12v does indeed flow through the switch, but the lights will not come on. I've tested every point I could along the run with a 12v hot from the battery. Every time the lights lit up. I'm fairly confident the switch is bad, but I wanted to know how I can test how many amps are actually flow through the switch when its tripped.

I have a volt meter, and I tried to do it by attaching the hot wire to the switch, then measuring with the + lead and the - to ground. I got nothing. So I know I'm doing something wrong. Basic electrical knowledge missing here!

Thanks

MJ

To measure current you put your meter in the appropriate mode (usually some sort of selector switch pointed to the A, ~A is ac current, you want the other A with horizontal lines, thats dc current).

I doubt its the switch from what your saying, I'd double check the grounds on the bulb itself.

Yeah I had it in the right mode, I just wasn't interrupting the circuit and letting the current flow through the meter. Not so smart here!

However, I did check the ground first thing. Running a wire straight from the negative side of the battery to the mounting screw on the bulb housing still didn't light it up. The second I switched that wire to the positive side of the battery and to the feed on the light, voila! LIGHT! Tracking this all the way back along the circuit to the switch, the lights worked right up until the point I got past the switch. Once the current had to flow through the switch the light stopped working. However, at the bulb I still was getting 12V and 0V depending on whether the reverse push button was depressed or not. So I know current is passing through, but clearly not very much!

After taking the switch out, I've cleaned it up a little bit and just wanted to test it.

Thanks!

MJ

Well, then....if you've got the switch out, place one meter probe on the terminal, and the other on the switch body. Put the meter into ohms, or resistance measuring mode, and manually activate the switch. The ohm reading you get should tell you whether to trash the switch or not. If it's anything over one ohm, throw it away and get a new one.

Roger

You dont want to measure amps. The second you jump your meter between 12+ and ground youre going to blow your meter's fuse. You need to connect your meter in series with a load to measure the amperage going through that circuit.

Here's how to diagnose the reverse lighting circuit. So the bulb doesnt light. Did you check to see if it's burnt out? Yes. OK, check voltage at the housing. You would have had 12V between the pin at the base and the socket housing itself, since it lit when you connected the terminal at your switch to battery positive. Which would have been your next step.

Now, here's what you do. With the ignition key in the RUN position, you need to measure the voltage between the other wire going to your switch and a good ground. Do you get 12V? Yes, then continue to the switch. No? Find out what happened to your 12V feed. If you arent getting 12V to your switch, it certainly isnt going to come out of it and go to your lights. This is where I think your problem is. You said you fed +12V to various points in the circuit, including the input to the switch, and your lights came on. So measure voltage at the input wire and go back from there because the rest of the circuit is fine. You can also measure resistance across the two terminals of your switch with it installed in the vehicle and the reverse gear engaged. You can do it on the bench if you want, but what if your trans isnt pushing on the button hard enough? Doing it in the car is the best way.

Understanding DC electrical systems is really not as hard as it seems. Your local community college will offer night school classes on basic electrical systems for less than $100. Once you understand the terms used, and learn how electricity flows through circuits (much like water flows through pipes), you will have confidence.

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

Right, so interupting the switch circuit would tell me how much is flowing through the switch. Right?
Sorry mistyped there. I get the lights to come on at every point up TO the switch. The exit of the switch, no problem, lights come on. If I put the +12v through the switch, I get nothing. This is why I wanted to measure how much current the switch is letting through. I've always gotten 12v with the key in and the reverse lever depressed. Yet, I never got the lights to light up. After tracing all the way back to the switch, I concluded that although the switch is letting the 12v through, the current level is so low its not lighting up the lights.

I tested the feed coming to the switch--I have 12v (which we knew since I had 12v at the bulb socket). I just didn't know how to measure the load.

Thanks!

MJ

You really dont need to concern yourself with current/amps. I almost NEVER have to measure current as part of diagnostics, and I do it every day.

Using a water hose as an analogy:

The flow of water can be compared to amps. Amps is the amount of electricity that flows through a circuit. Amps is like water flow, like gallons per minute.

Voltage is like pressure. And resistance (ohms) is just what you think it would be. You put your finger over the end of a running garden hose and all of a sudden the pressure (voltage) goes up, flow (amps) goes down, and your finger is creating resistance (ohms). Make sense?

This reverse switch is like a valve. It opens and closes, turning on or off the flow of electricity. If it's corroded and creating a restriction, your lights will be dim. Right now it's off. First thing you need to do is put your trans in reverse, and measure resistance across the two switch terminals. The switch should be on and you should read little to no resistance (less than an ohm). So what you need to narrow down is whether the switch is bad or whether the trans isnt pushing the button down far enough. If you replace the switch and the problem persists, tell me and we'll get into that. Dont panic.

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

Whoa, hang on a second, I just read this again:

ok those last two paragraphs Im having trouble with---first, how did you measure 12V with the key on? Did you put your red meter lead to the switch input wire and the black lead to ground? It's not that I dont believe you--just making sure.

How did you conclude that the switch is letting 12V through if it wont light up your lamps when you jump battery positive to the input terminal? Even if it's letting out less than an amp, the light filament will still glow a little.
I dont get that... how could you have 12V at the bulb socket? You said that unless you jump battery positive to the switch output terminal, the lights won't come on. Voltage always travels from positive to negative. If the lights come on when you jump it, but dont when you have the wires hooked up properly, youre not going to read 12V at your lights if they aren't on.

Really, I think it's just a bad switch, or maybe the trans mechanism, but I doubt it.

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

Exactly right. Turned on the key, put it in reverse, + lead to the wire coming from the switch at the tail light. I unplugged it. - goes to the bulb body or any body ground. I have 12v. Turn the key off, I have 0v. Bulbs do not light at all. Run the wire from + battery to any break in the circuit before it gets to the switch, lights light up.
I can still measure 12v past the switch at any section along the way, I just don't get any lights.
I think the switch is also the culprit, but I was just trying to expand my understanding and see if cleaning the contacts on the switch would solve the problem vs. replacing the switch. I'm sorry that my explanations seem strange or not thorough enough =) I appreciate the help!

MJ

A bad switch will easily do this. The meter draws almost zero current. A few mA. The bulbs draw about 2.1A apiece. The same phenomenon is at work when you have an internally-broken battery cable or a severely corroded battery terminal: You get in the car and the dome light comes on, but you turn the key, all the lights go dead and the starter relay chatters.

yes, but you should still see a voltage drop when the circuit is on. How about this; measure voltage across the switch; that is, put your red lead on the switch input and the black one on the output terminal with the trans in reverse. If the switch is bad you will read 12V or close to it.

If however, as Dan says there is a bad connection somewhere then when you put the car in reverse the voltage will drop right off.

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I've been calling it as i see it for my entire life and that's not about to change. Take it or leave it.

Step, would you read 12v because the switch isn't drawing any voltage? I was confused as to the logic here. If you've got the trans in reverse, the switch is open, and voltage is flowing through it. So wouldn't I expect to find 12v there when it does work? Could you enlighten me a bit more on the specifics of what I'm testing there?

Sorry, learning 'lectricity here so excuse my ignorance.

MJ

Voltage isn't "drawn". Also, the switch doesn't draw current (unless its internal resistance is improperly high, in which case it will also heat up and cook), it passes or blocks current.

Also, very important, when the switch is "on", it is closed, and when it is "off", it is open.

Spend half an hour with the electrical chapter of your Stockel book.

bedtime reading

thx