Brake lights troubleshooting questions

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Scott in NM
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Location: Tijeras, NM
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Brake lights troubleshooting questions

Post by Scott in NM » Wed Dec 15, 2021 3:43 pm

OK, brakes lights not working (BO and service), got new brake light switch in and bench tested it. Did not make continuity when I depressed the diaphragm with a dowel but decided to install it anyway. Still no brake lights. Although new switch could be bad, I still have a few questions as the wiring diagrams (Wes Knettle 12-29-2006 and those in the TM9-804) are deficient/conflicting:

1 My normal and blackout headlights work with ignition switch OFF. Should the brake lights also? My BO marker lights are currently inop, still need to troubleshoot them, so can't provide any input there.

2 Where is the ground for the BO tail lights?

3 Fig 57 of the 804 shows the wires to the stop light switch are 25 and 75 yet Wes’s diagram shows 75 and 75. Know which is correct? One is for normal brakes and the other is BO brakes (which is which)?

4 Stop light switch questions
a. Should I be able to tape together wire 75 and 25/75 at the stop light switch (bypassing the switch) and turn the brake lights on with the light switch on the dash positioned correctly?
b. Can I just run a wire from the + terminal on the cowl battery to either the 75 or 75/25 wire and turn on the brake lights when I position the light switch correctly and the brake pedal depressed?

5 How can I test if my malfunction is in the light switch on the dash and not the stop light switch?

Need to figure out the wiring diagrams before I start serious spark chasing by pulling out the light switch, and checking for continuities, grounds, etc in the wiring. I REALLY HATE SPARK CHASING!!!

Scott


mdainsd
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Re: Brake lights troubleshooting questions

Post by mdainsd » Thu Dec 16, 2021 6:01 am

Scott in NM wrote:
Wed Dec 15, 2021 3:43 pm
OK, brakes lights not working (BO and service), got new brake light switch in and bench tested it. Did not make continuity when I depressed the diaphragm with a dowel but decided to install it anyway. Still no brake lights. Although new switch could be bad, I still have a few questions as the wiring diagrams (Wes Knettle 12-29-2006 and those in the TM9-804) are deficient/conflicting:

1 My normal and blackout headlights work with ignition switch OFF. Should the brake lights also? YES My BO marker lights are currently inop, still need to troubleshoot them, so can't provide any input there.

2 Where is the ground for the BO tail lights?The body of the Jeep supplies the ground path for all of the lights

3 Fig 57 of the 804 shows the wires to the stop light switch are 25 and 75 yet Wes’s diagram shows 75 and 75. Know which is correct? One is for normal brakes and the other is BO brakes (which is which)?Dont have the manual in front of me, but the two wires going to the brake light switch operate both the BO brake and Service Brake. The light switch determines which of those two lights (BO or Service) the voltage is routed to.

4 Stop light switch questions
a. Should I be able to tape together wire 75 and 25/75 at the stop light switch (bypassing the switch) and turn the brake lights on with the light switch on the dash positioned correctly? YES
b. Can I just run a wire from the + terminal on the cowl battery to either the 75 or 75/25 wire and turn on the brake lights when I position the light switch correctly and the brake pedal depressed?Depends which of the two wires at the brake light switch you tie into. One will do it, the other wont.

5 How can I test if my malfunction is in the light switch on the dash and not the stop light switch? Put a meter or a test light on one of the wires at the brake light switch. One should be hot with the light switch in any position but "off". The other one wont have voltage no mater where the switch is set.

Need to figure out the wiring diagrams before I start serious spark chasing by pulling out the light switch, and checking for continuities, grounds, etc in the wiring. I REALLY HATE SPARK CHASING!!!

Scott
52 M38 2X, '52 M37, '44 WC51, '42 WC 56/57, '50 CJ-V35(U), '42 GPW, '44 M3A1 Scout Car, 2010 M-Gator

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