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Unread 03-18-2014, 09:18 PM   #1
243
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Electric circuit explanation needed (brake warning lamp))

Here is a link to the schematic on Tom Collins website,

http://oljeep.com/gw/82_tsm/Part%204...20Diagrams.pdf

The brake warning lamp in the speedo is a two wire, black and red, red being 12v and the black splitting and running to the parking brake contact and on to the brake failure switch on the proportioning valve, that circuit appears to return to one of two ignition switch plugs on the column.

One, does the brake failure switch create a ground path for the warning lamp when the brakes fail?

And if that is true, if I jumper the connector located at the bulkhead it should cause the bulb to illuminate, correct?

Two, what happens to the ground that goes back to the ignition switch plug, circuit 57A?

I don't see that 57A continues after that reaching the ignition switch plug.

The problem is the warning lamp does not work if I set the parking brake and it appears the two switches work independently. I did try running a direct ground to the parking brake connector and the bulb didn't illuminate.

I don't have the harness that runs from the failure switch to the connector at the bulk head.

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Unread 03-18-2014, 10:05 PM   #2
SLO_Ken
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1) The brake failure switch creates a ground path when there is a pressure differential. So yes, if you jumper the connector at the bulkhead to ground, you should see the bulb illuminate.

2) It's not a ground. Rather, that wire goes to the "start" terminal on the ignition switch. This way, there will be 12V on both sides of the lamp and it will not illuminate while you are starting the engine with the parking brake engaged.

Sounds like you need a new bulb...
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Unread 03-19-2014, 06:01 AM   #3
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Ken, thanks.

I was using a working bulb but I will double check it.

I will try grounding the brake failure connector at the bulkhead; I think I may have a broken ground wire between the bulb and the parking brake connector, I may slit the ground wire insulation at the connector, ground it and see if it illuminates.
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Unread 03-19-2014, 06:43 AM   #4
Mike Romain
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Not sure if it applies to that light, but the dash needs to be grounded for most things to work so if is in your lap....
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Unread 03-19-2014, 08:04 AM   #5
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Mike, it was on the floorboard but I ground the speedo to the battery.
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Unread 03-19-2014, 10:37 AM   #6
SLO_Ken
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That light grounds through either of the switches mentioned earlier, not through the dash.

Here's the schematic from the '84-'86 FSM that includes the brake warning light. It looks like this circuit is pretty much the same as on the '82 schematic you linked above. Just a little easier to follow on the newer schematic.
File Type: pdf doc20140319080534.pdf (290.2 KB, 32 views)
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Unread 03-19-2014, 10:48 AM   #7
John Strenk
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Quote:
Originally Posted by SLO_Ken View Post
1) The brake failure switch creates a ground path when there is a pressure differential. So yes, if you jumper the connector at the bulkhead to ground, you should see the bulb illuminate.

2) It's not a ground. Rather, that wire goes to the "start" terminal on the ignition switch. This way, there will be 12V on both sides of the lamp and it will not illuminate while you are starting the engine with the parking brake engaged.

Sounds like you need a new bulb...
2.) Sorry Ken but it is a ground. The wire that goes to the ignition switch is to illuminate the brake light momentarily when you start your engine so that you know the light is working. Some sort of federal regulation or something at the time.

IT is suppose to illuminate the Brake warning light when the engine is starting.

If there was 12 volts there and the parking brake was engaged, POOF!!! smoke and magic.

[EDIT]
I see you already posted the schematic. See the ignition switch at the very bottom? It grounds out the circuit so the light will light up. The other part og the ignition switch shown there is to illuminate the "HOT" warning light but since the jeep doesn't have one, it's not hooked up.
[/EDIT]
brakewarninglightcircuit.png

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Unread 03-19-2014, 11:10 AM   #8
SLO_Ken
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Ah... Thanks for the clarification.
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