Reverse Switch (backup lights)

[jmanng]

Just bought a 2000 Mustang this week and took it in for the safety test today. Well, she failed because the backup lights did not light up when shifting into reverse.

So I took her home and started troubleshooting. To make a long story short, what I found is that if I jumpered the backup/reverse switch connection, then my backup lights work. So I then removed the switch for further testing... and this has me puzzled. The switch checks out on the meter, no resistance (appears to be a piece of wire as it should) when the ball bearing is pressed in. So I then using two jumper wires with aligator clips to hook the switch back into the circuit... just to check for a connection issue on the plug. Well, with the switch in place, no backup lights. Switch out, so that the aligator clips are just connected together - lights on. There was also one other interesting note, with the switch in, I notice that the voltage drops from 11.64v to basically zero, like the switch is drawing all the current. But again this has me stumped as I checked the switch legs for short to the casing (no continuity), and the fuse isn't blowing..

Just doesn't seem to add up... am I missing something?

Your insight and help would be much appreciated.

Jaime

 

[Darkhorse845]

Welcome to Stangnet!
Congrats on your new purchase.

 

[n0v8or]

The orange wire should be at ~12V whenever the ignition key is in the "run" position. The black/pink stripe wire should be ~12V in reverse, 0V otherwise.

I don't understand your observation regarding the volage measurement; where are the 2 meter probes connected?

It is possible that the internal actuation mechanism is worn, therefore not pushing in the ball far enough. Perhaps you could improvise with a shim.

 

[jmanng]

Thanks for the Welcome.

Regarding the voltage measurement, I measured from the +12 reverse lead (black/pink wire) at the tail lamp connector (power side) to ground. I was still expecting to see ~+12 volts to ground with the bulb lit.

I pulled the switch off and operated it manually and got the same results. In fact I'm sure mechanically it is fine because of the drop I'm measuring at the lead by putting the car in and out of reverse gear. It's the electrical side that seems to have some sort of high resistance... but I measured for resistance across the switch and (from each terminal to unit body), can't find anything wrong electrically. But I always come back to the fact that if I simply jumper the connector in place of the switch, the reverse lights light up... Just doens't make sense.

I I've ordered the part and with the Holidays, unfortunately it be Wednesday before it arrives... cant believe none of the local ford dealers have one on hand. I guess I'll know for sure then but I'm sure I'll won't get any rest until then and will redo the measurements a few times more.

Merry Xmas all.

 

[n0v8or]

OK, let's recap to see if I've understood correctly:

You removed the connector from one of tail lamp assemblies and measured ~12V from the blk/pink wire to ground . . . with the tranmission not in reverse. Then moving it ro reverse changed the voltage to 0?

When the voltage was ~12V, the other reverse light should have been On, since they are wired in parallel, and the opposite side connector was still in place. So that's the first condition to verify, unless I've interpreted your statements wrong.

 

[jmanng]

No, not quite. With one of the tail lights connected, I get around .4v to ground from the black/pink while in reverse. Now if I disconnect the other taillight, then I measure 12 volts (circuit is not complete to ground). then if bypass the transmission switch, it all lights up properly.

I was talking with my brother last night and he's offered what I believe is the solution, the tranny switch maybe a solid state device (the electrical portion). I was thinking it would be a simple switch and wasn't measuring any resistance, but when active, has enough resistance to drop the voltage as it will be in series with the two tail lights (that are in parallel). I'll get under the car today and measure for a voltage drop across the switch, I'm expecting to find the ~11.6V drop there and will prove this out (don't know why I didn't already measure for voltage drop there).

I'll post my findings, and that should put this issue to bed.

Thanks

 

[n0v8or]

OK, that makes more sense. I have the factory wiring diagrams . . . they show a standard switch, not a solid state device.

Here's what I think is happening:
You have a high resistance (worn contacts) tranny reverse switch. With no bulbs, you measure 12V because the only current required is microamps to operate the meter. Put a bulb in, and the switch needs to pass >2 amps to light it. Instead, you only get 0.4V because the remaining ~11.6V is being dropped across the internal switch resistance.

A new switch should do it.

 

[jmanng]

It makes sense now. Now I just have to wait until Wednesday to pickup the switch.

Thanks,

 

[jmanng]

Part came in yesterday, replaced unit - all is well.

Thanks for the help.