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MG MGA - Brake switch wires - no resistance

Hi guys,

I'm just moments away from dropping the engine back in, but I'm having a problem. When I went to hook the brake light switch wires back up, there was no resistance measured between them. I thought it was a faulty switch, but it turns out that after disconnecting them, there is no resistance between the green wires at all.

How can this be possible? I'm pretty sure the brake lights worked 2 months ago before I pulled it apart. I also checked the battery cable (no battery in the car right now) and its not touching anything.

Help!
-Rich
R Stokes

rich, maybe i'm missing something here, but what exactly are you checking for? i don't understand "no resistance between the green wires at all". the easiest way to tell if your brake lights work would be to connect the battery, turn the ignition, and hit the brakes, if the state of your car allows you to do that. or, have someone hit the brakes and test for resistance on both sides of the switch. if the switch is working, then test for resistance bewteen the green wire on the fuse block and the green wire down at the switch. if that's working then check for resistance between the wire on the other side of the switch and the connection back at the tailights. that should allow you to pinpoint the source of any problems.

hope that helps,
mark
mark

It requires a battery to complete the circut. If you are testing without a battery, you would have to make contact between your battery cables in order to complete a circut.
Ed Bell

Guys,

To repeat - the switch is working fine. If the two wires leading to the switch are disconnected from the switch and an ohmmeter is placed between them, there is no resistance (0 ohms). The battery is disconnected, so it sounds like my suspicion is correct - something is shorted out somewhere, but where?
R Stokes

Rich - If by not reading any resistance, you mean that the reading on the ohm meter is infinite (or no reading at all on a digital meter), the reading you are getting is correct. You will not measure any resistance between the two wires at the switch (which should be green and green/purple). Ed is correct, you would have to connect both battery cables together (or connect the negative cable assuming a positive ground vehicle, to ground) to read any resistance at all. The proper way to check your wires is to read from the switch end of the green wire to the fuse block where all the green wires tie together. That reading should be 0 ohms (this is the true reading of no resistance). Then check from the switch end of the green/purple wire to ground. This reading should show a low ohms reading (somewhere around 1.5 ohms) as it has two 21 watt filiments in parallel. By connecting the meter leads to the two wires as you are doing, you are reading an open circuit, the results of which is infinite resistance, not a short (this is the same thing same as when the two meter leads are connected to nothing). Cheers - Dave
David DuBois

No, no resistance = 0 ohms = the lines are shorted together somewhere.

A reading on the ohm meter of infinite would mean infinite resistance, which means two entirely disconnected wires. This is not the case.
R Stokes

Hi Rich. Connect the battery, and then connect the 2 brake switch wires together. If the brake lights come on, then the brake light switch is defective. If the brake lights dont come on, then there is an electical system problem, perhaps with a bad ground wire, or a problem with the wiring harness, or blown brake light bulbs, or dirty contacts at the bulbs, etc. Cheers! GLenn
Glenn

I like Glenn's idea. It is the KISS system. Keep It Simple Stupid!!!!
JEFF BECKER

Rich - Ok, you are reading 0 ohms when you hook the meter to the two wires that are disconnected from the switch. Sorry to have jumped on you about what no resistance means, too often people refer to no meter movement as no resistance, which was what I was trying to clarify. Next question, what kind of a meter are you using? Is it a relative high end digital that will read very low resistance or a cheap analog meter (I like that kind), that can't discriminate between 1 or 2 ohms and 0 ohms? My thought here is that you are reading through a sneak path, like a side light or even the ignition light, the regulator or generator. A lot of these things are in the circuit all the time and as with the brake lights (2 parallel 21 watt filiments), have a very low resistance that is hard to read with one of the cheaper analog meters (even my Triplet digital meter has limited accuracy down that low). Look in the electrical drawings and see what all the green wires go to, including the other side of fuse that they all come off of. I think that a sneak path to ground that is leading you down the garden path is a very real possibility here, particularly when you say that the brake light circuit worked before and I would assume that you haven't done any other work that would affect the wiring. A way to start would be to Read from the green wire at the switch to ground. I suspect that you will read 0 or a very low resistance. If this is true, disconnect the far end of the green wire to the brake light switch and then read between the two wires at the switch. If you now get a reading of infinite, then go to the common tie point at the far end of the wire and read that connection to ground. Again, I suspect that you will read 0 or a very low resistance. Good luck - Dave
David DuBois

KISS worked! I hooked up the battery, double-checked the connections, and the brake light worked fine. So much for an ohmmeter test.

Thanks for your help, guys!
Rich

This thread was discussed between 13/08/2005 and 14/08/2005

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