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Arench
Arench, Master Technician
Category: GM
Satisfied Customers: 1461
Experience:  28 years experience auto and truck sales and service.
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No brake lights on my 1998 GMC Sierra 1500 4wd, have changed

Customer Question

No brake lights on my 1998 GMC Sierra 1500 4wd, have changed the brake switch and the multi function switch, the tail light scircuit boards and all fuses. Any ideas? Thanks
Submitted: 4 years ago.
Category: GM
Expert:  Arench replied 4 years ago.

Greetings from Mikey at Just Answer!

 

First get your voltmeter or 12v test lamp out. Check the 30 amp Brake (#2) maxi fuse in the under hood junction box. See diagram below. Fuse should have 12v at both sides at all times.

 

If okay proceed.

 

Go to the brake light switch. The orange wire should have 12v at all times. The white wire should have 12v when stepping on the brake.

 

If okay, proceed.

 

Next thing is the brake lamp relay in the underhood fuse box. Swap the relay with one that looks just like it and recheck the brake light operation. If the relay is no good replace it.

 

If the relay is swapped and still not working, see if you can feel the relay clicking when someone else steps on the brake pedal.

 

Let me know how you make out with this.

 

 

graphic

 

 

Customer: replied 4 years ago.
We have 12v constant at relay and fuse, but no click what so ever at the relay when pedal is pushed in.
Expert:  Arench replied 4 years ago.

voltage at brake light switch orange wire at all times?

 

voltage at brake light switch white wire when applying brake?

 

Swap brake relay with another?

 

 

 

 

 

 

 

 

 



Edited by Mikey on 11/14/2009 at 11:59 PM EST
Customer: replied 4 years ago.
12.1 v
Expert:  Arench replied 4 years ago.

The diagnostic procedure for no brake lights is long, involves removal and opening of the underhood relay box and opening the wire harness, steering column and bulkhead connector. Understanding of basic troubleshooting, use of a voltmeter and general knowledge of wiring is going to be necessary to proceed.

 

12.1v at what? when? you are going to have more specific in your information in order for me to be of any help to you.

 

 

 

 

Customer: replied 4 years ago.
we have 12.1v constant as well as 12.0v when pressing the brake pedal
Expert:  Arench replied 4 years ago.

and swapping the relay...

 

 

 

Customer: replied 4 years ago.
yes
Expert:  Arench replied 4 years ago.

Remove brake lamp relay.

 

Get a paper clip and open it so you can measure voltage at each terminal of the brake relay one terminal at a time.

 

I need to know how many terminals have power at all times, then I need to know how many have power while stepping on the brake.

 

 

 

 

Customer: replied 4 years ago.
there are four terminals 1 has constant power the other have nothing
Expert:  Arench replied 4 years ago.

when you apply the brake you still have only 1 terminal with power?

 

The relay termials are layed out in the diagram below.

 

batt is the one with power all times

 

sls is the white wire from brake light switch

 

gnd is body ground

 

mfs is the load wire back to the multifunction switch.

 

terminal labeled sls should have power when stepping on the brake. thats your white wire from the brake light switch.

 

 

 

graphic

 

 

 

Customer: replied 4 years ago.
which ones should have power and which should be ground and does the mfs wire have a signal for the ebcm or the pcm
Expert:  Arench replied 4 years ago.

 

batt is power from the maxi fuse 12v all times

 

sls is the white wire from the brake switch 12v with brake applied

 

gnd is body ground 0v all times

 

mfs goes to the multi function switch 12v with relay activated

 

 

the circuit is simple—power from the brake light switch white wires turns on the relay which then sends power back to the multifunction switch for distribution to the rear lights.

 

if you jump terminals batt and mfs the brake lights should come on.

 

If you have no power at terminal sls with the brake applied then you have a wiring fault white wire between the brake light switch and the relay.

 

 

 

 

 



Edited by Mikey on 11/15/2009 at 12:59 AM EST

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