Idiot Temp Light and Idiot Brake Light Not Working

[gschroeder]

I have a freshly restored 1970 CST10. Unfortunately the idiot temp light and idiot brake light will not illuminate while cranking the engine. I have confirmed the bulbs are good by switching them with another light on the dash. By the way, I have the deluxe dash with all the gauges and tach. I tried gounding out the brake light at the master cylinder, and grounding out the temp light at the temp sensor on engine, but still cannot get either to illuminate manually. Any suggestions on how to trouble shoot this is appreciated. I hate these electrical gremlins, and hope someone can point me in the right direction. Thank you

[pjmoreland]

Check out this post from TBone1964 that shows a schematic of the full gauge cluster circuit. You'll notice that the temp light is not connected to anything when you've got a temperature gauge. There is no wire installed in pin 5 of the gauge cluster connector.

http://67-72chevytrucks.com/vboard/s...70&postcount=2

This means your current issue is limited to a nonfunctional brake warn light. Here's what I would check:

1) Power for the brake warn light comes from the pink wire on pin 3 of the gauge cluster connector, which I believe is fed by the Cluster Feed fuse on the fuse panel. You replaced the fuse already. Is the new fuse still good? Use a multimeter to check for 12V on the pink wire on the gauge cluster connector.

2) Remove the brake warn light bulb and check for 12V on one side of the contact on the flexible circuit board for the bulb. Sometimes the circuit board connections get corroded and need to be cleaned up with fine sand paper or Scotchbrite.

3) With the bulb still removed, check for continuity between the ground terminal for the bulb on the circuit board (the one that didn't have 12V on it from the previous test) to the terminal on your ignition switch that has two tan wires running to it.

4) When you turn your ignition switch to the starting position, do the two tan wires on your ignition switch connect to ground? Check this with your battery disconnected so you don't have to deal with your engine starting up. Check for continuity from the tan wires to the metal of the dash with the key in the cranking position.

[gschroeder]

pjmoreland, thank you for the advice. I will follow your 4 steps regarding the idiot brake light trouble shoot, and let you know what I find.

I am surprised to learn the temp light isn't connected to anything. I have 350 engine which has 2 temp sensors. 1 temp sensor on driver side head for temp gauge, and 1 temp sensor on passenger side head for temp light. It seems odd GM would have a temp sensor for the idiot light, and not run the wire all the way to the dash light.

[pjmoreland]

Originally Posted by gschroeder

pjmoreland, thank you for the advice. I will follow your 4 steps regarding the idiot brake light trouble shoot, and let you know what I find.

I am surprised to learn the temp light isn't connected to anything. I have 350 engine which has 2 temp sensors. 1 temp sensor on driver side head for temp gauge, and 1 temp sensor on passenger side head for temp light. It seems odd GM would have a temp sensor for the idiot light, and not run the wire all the way to the dash light.

On a stock setup you would only have one temperature sensor. The sensor type depends on whether it is for an idiot light or gauge.

[pjmoreland]

You could always add the wiring for the temp idiot light if you wanted to. It is the dark green wire that runs from the gauge cluster to the ignition switch and then out to the engine. It is well-documented in the schematics, although you would need to connect it to pin 5 of the gauge cluster connector instead of how it's shown here:

http://67-72chevytrucks.com/vboard/s...d.php?t=185856

[gschroeder]

pjmoreland, I checked my ignition switch, and there are no green wires running to or from it. I will try to trace the wire from the idiot temp sensor into the cab and see where it goes. I do not believe the wiring was ever messed with as everything appeared to be completely original when I restored it. I do know that manufacturing in 1970 was less than precise, and a lot of strange things were done to get trucks shipped out the door to the dealers. I will let you know what I find (after I solve idiot brake light mystery). Thanks again

[pjmoreland]

You shouldn't currently have any green wires going to your ignition switch since you have a temperature gauge. I was just suggesting you could add the green wires if you desired to make your temp idiot light functional (non-factory setup). You do, however, have two tan wires going to your ignition switch for the brake warn light, as shown in the photo you posted on the other thread. Might be good just to focus on the brake warn light issue first.

[68 P.O.S.]

I think the only sensor in the passenger side head would be for the TCS system. What year is your truck?

[gschroeder]

pjmoreland,

Here are results from your recommended 4 steps:

1.) I confirmed there is 12V on the pink wire on the gauge cluster connector.

2.) I confirmed there is 12V on one side of the contact on the flexible circuit board for the bulb.

3.) I could not get 12 volt on multimeter between the ground terminal for the bulb on the circuit board, to the terminal on ignition switch that has two tan wires running to it. Not sure I did this right as I am not sure what you mean by continuity.

4) Sorry I could not understand how to do step 4. Specifically how to run/test the two tan wires for continuity. I understand how to disconnect the battery, turn ignition switch to the starting position,... but how do I test the two tan wires for ground to the metal of the dash? Use multimeter somehow?

I am not sure if this means anything but I was able to get the brake light to illuminate manually by running a ground from the ground terminal for the bulb on the circuit board to the cab. Does this simply mean my ignition switch is bad (not creating the ground for the brake light when cranking engine)? Thanks again for all your help. Thank you

[pjmoreland]

You can use the resistance setting on your multimeter to check continuity. You touch one probe on one end of a wire, and the other probe on the other end of the wire. A low reading means you have a connection, similar to the reading you'd get if you touched the two probes of the multimeter together. You can use this technique to determine if you have a good connection between the ground side of the brake warn light and the two tan wires on the ignition switch. Testing in the resistance mode should be done with the truck electrical system turned off.

Instead, though, you could use the same technique you used with a grounded wire to test the connection from the ignition switch to the light. If you short the two tan wires on the ignition switch to ground and the warn light turns on, then the connection is good. This approach is a little more risky than using the multimeter because you could accidentally damage something if you connect ground to the wrong spot.

Either way, the goal is to verify you have a good connection between the ground side of the brake warn light and the ignition switch at the two tan wires. This connection also includes where one of those tan wires connects to the back of the gauge cluster, so there could be an issue at that connector as well. Watch out because there are two separate tan wires on the gauge cluster connector. You want the one that is NOT next to the empty terminal slot on the connector (that one is for the fuel gauge).

[gschroeder]

pjmoreland, I confirmed there is good continuity between the ground side of the brake warning light, and the two tan wires on the ignition switch. I also was shorted the two tan wires on the ignition switch to ground, and the warning light did turn on.

So does this mean my ignition switch is bad, and needs to be replaced?

Thanks again for all your help.

Thank you

[pjmoreland]

The ignition switch connects the tan wires to ground through the housing of the switch to the dash when in the start position. Maybe you don't have an adequate connection between the switch and the dash. Did you recently paint both the front and back of your dash?

[gschroeder]

pjmoreland, the front of the dash was recently painted, but not the rear of dash. I did try to manually ground of the ignition housing in a couple of different places to the cab, then crank the engine. I could not get the brake warning light to illuminate by doing this.

[pjmoreland]

It sounds like your ignition switch has issues. That would only explain the light not turning on while cranking.

Have you tried shorting the sensor wire out by the brake master cylinder to ground since you replaced the cluster feed fuse? The light should turn on if the ignition is in the run position. This should work regardless of the issue with your ignition switch.

[gschroeder]

pjmoreland, I shorted the sensor wire out by the brake master cylinder to ground, and the brake light illuninated.

Should I go ahead and replace the ignition switch? Do I need to replace the ignition cylinder/keys as well, or just the ignition switch? Thanks again for all your help.

Thank you

[pjmoreland]

If you want the testing feature to work, and you're confident the ignition switch housing has a good ground, then replacing the ignition switch should fix it. You can transfer your lock cylinder from the old switch to the new. LockDoc has described how to do this in other threads, so you should be able to do a search to find out how to do the swap.

[Chevys4life]

I have the same problem with my brake warning light so I'm going to run through these test this weekend to see what I find out for my issue of not having the brake warning light illuminate either.

Thanks!!!

[gschroeder]

pjmoreland, I replaced the ignition switch, and my brake light illuminates as it should when cranking the engine. Problem solved. Thanks again for all your help. Thank you

[Chevys4life]

I will troubleshoot mine and see if my ignition switch is bad too which I think it is based on how hard the cylinder was to turn when I first tried to start the truck.

[pjmoreland]

Originally Posted by gschroeder

pjmoreland, I replaced the ignition switch, and my brake light illuminates as it should when cranking the engine. Problem solved. Thanks again for all your help. Thank you

It would be interesting to take your old switch apart to see why it failed.

[1970cstblazer]

I had an inoperative cluster brake warning lamp on my 1970 Blazer. I removed the ignition switch from the dash, and somehow one of the ground terminal spades was bent over at a 90 degree angle. Everything else worked fine, just not the lamp during crank. Once all of the terminal spades was cleaned, and the bent terminal fixed, the brake warning lamp illuminates during crank.