94' GMC Reverse Lights Issue Please Help!


#1

Ok so here is where I am now. My reverse lights do not come on when I put my truck in reverse. I got out my multi meter and there was voltage at the switch, so I put a jumper in the connector and the lights lit up. Thinking it was a bad switch so I replaced the switch, still no lights. So the big question is does anyone know what to take apart to find whatever it is that pushes the switch. The truck details are : 1994 GMC K1500 350TBI 5 speed, the reverse light switch is on the drivers side of the transmission. Please help I need to fix this for my state inspection.


#2

Take the reverse switch out; hook it up; and see if the reverse lights work. Push the plunger in until the lights go out. Now you know the switch is working.

Screw the switch back into the transmission with the shifter in Reverse. Use the multimeter to determine when the continuity breaks. Back the switch out 1 or 2 turns and lock it some way. That should do the trick.


#3

The wiring diagram shows the switch as being normally open. So the switch contacts don’t close and turn on the reverse lights on unless the transmission is put into reverse.

Remove the switch and plug it back in. Now push on the ball at the end of the switch to see if the reverse lights come on. If they do, whatever pushes on the switch inside the transmission when put into reverse is no longer there.

Tester


#4

Good advice from both @Researcher and @Tester. I seem to remember that my son fixed his truck by getting the right switch after he bought the wrong one. They sell a switch for the right side and the left side and they will not interchange because of the length. Here are the pics:


#5

I have actually already replaced the switch, and tested the old one with my multimeter. Both the old switch and new one are working switches, the problem is that the switch isn’t being hit by whatever is inside the transmission. I’m thinking that to fix that means having someone take it apart and would probably cost about $1500 considering that it would cost me about $500 just in labor to do the clutch. I think I’m going to see if I can rig some other switch to get the lights to work.


#6

The switches dont look too deep. Try shining a flashlight into the mounting hole as someone shifts in and out of reverse to try to see what is moving or not moving. At the very least, you can confirm the problem is internal.


#7

If the switch is moved mechanically and not hydraulically, and if the pin isn’t moving far enough to activate the switch, you might be able to get it working again by adding in a thin spacer between the pin and switch plunger.


#8

All excellent ideas above. Assuming the new switch is known good on the bench, is the same exact geometry as the old one, and the old one at one time worked.

hmmm … well, I don’t mind kludges, so a kludge fix is easy to come up with. Install a switch on the dashboard to manually power up the backup lights . Works for me. It’s a 94 after all, some kludges at this age are to be expected. But that might not pass the safety inspection.

hmm … well, I think using a flashlight to look into that hole may be the thing to do, maybe you’ll get an idea from that. If you can’t see much using a flashlight, maybe invest a few buck in one of those Harbor Freight Bore-hole cameras. I think some are less than $100, and you’d likely find other uses for it in the years ahead. Like next time changing the spark plugs, figuring out if you accidentally dropped that paperclip you were using to hold the wire out of the way into the spark plug hole or not!

Frustrating problem. Other than above, if the state requires this to work the way the manufacturer intended, you may have no choice but to remove the transmission. Before doing so it would be wise to get an assessment from a transmission specialist. Ask you mechanic who he uses for transmission work. The transmission shop may have already figured this one out for you.


#9

I ran into this situation trying to help a colleague with his 1970 Plymouth Fury some years. The transmission had been rebuilt and the reverse lights were stuck on. My colleague assumed it was the switch. I got under the car and changed the switch which did absolutely no good. He then took it back to the transmission shop. they dropped the pan of the transmission and replaced some part. The job took less than an hour and the transmission did not have to be removed.


#10

When the transmission is shifted into reverse, the shift lever is in a certain position.

I would try to attach a switch to the shift lever under the boot. Then fabricate a bracket that can be mounted under the boot so that when the shift lever is put into reverse, it causes the switch to come in contact with the bracket, thus closing the switch to turn the reverse lights on.

Just make sure the contacts for the switch can meet the demands for the current required for the reverse light circuit.

Tester


#11

Yeah I’m thinking I’m going to go with something like Tester suggested. there is no pan on my transmission so to get into it it would need to be removed and opened up and it is actually a pain to remove this one, not to mention having to have it taken apart to fix the issue. as for shimming the switch, well its a ball bearing that’s used as a contact plunger so I don’t think there’s any way to shim it. A bore scope is a pretty good idea for looking into it to see what the real issue is but I can’t see myself being able to fix the problem so the $100 cost is a bit steeper than the external switch idea. I think this switch would work ok its listed from radio shack, the part number is NTE54-402 and I’m attaching an image, I’m good with electronics but not real familiar with the loads involved here, some advice might help.


#12

Radio Shack also sells a 40 Amp capable relay designed for automotive use for less than $5. I use one on my Corolla to switch the “START” signal to the starter, rather than forcing that current (which can exceed 10 amps) through the ignition switch. If the current rating of the switch you’ve selected is in question, or you just wanting something more reliable, you can use the switch to power the relay rather than the lights. And use the relay to power the lights. Then you’ll be set. The relay is only a 30 ohm load, 500 mA is the most the switch would have to handle if done this way. Best of luck.


#13

If you get a switch with contacts rated for 20 amps it’ll work.

Tester


#14

@garbagegasman, that switch you show is rated for 15 amps, so you should be able to use it without a relay. Just wire it right into the reverse light wiring. Good luck.


#15

The reverse light circuit is a 15 amp circuit. When installing a switch in a circuit, you want to install a switch with an amp rating higher than the circuit amps. This allows the contacts in the switch to handle any in-rush current when the switch contacts close.

Tester


#16

Some of those microswitches can handle a fairly beefy load. I have some that are rated at 5A. That being said, you’re driving at least one lamp, probably two. If it were me, I would use the switch to drive the coil on a relay and wire the contacts of the relay into the reverse light circuit. Then you can be sure it will last a long time.


#17

While I don’t know what the wattage rating is of the reverse lights really is I would guess they would be no more than 40 watts combined. Headlights are usually 55 watts so even using a switch rated for 5 amps should be no problem without having to add a relay.


#18

I agree with @Cougar.
It’s been many years since I’ve had to replace a backup light bub, but a type 1156 bulb which is what I remember, is rated at 2.1 amps each at 12.8V. Even allowing for a higher draw at , say, 14 volts, there is plenty of margin for a switch rated at 15A.


#19

2.1 amps@12.8 volts = 26.88 watts.


#20

A headlight bulb is only 55 watts? Just by a seat of the pants estimation of the brightness compared to a home light bulb, I would have guessed an incandescent headlight bulb – like the kind used on my early 90’s Corolla or the sealed beam unit on my 70’s Ford truck – would be more like 150-200 watts (on bright). Maybe it is less watts than you’d seem b/c a headlight beam only goes forward, not backward, so it seems brighter.

I know the one I took out of my Corolla, one of the filaments measures 0.5 ohm at room temperature. I know that b/c I use it as a dummy load to test my AA batteries. Power is v^2/ r, right? So P = 144/0.5, or 288 watts. r goes up with temp, so the “on” power would be lower than 288 watts, but 100-150 watts isn’t out of the question is it?