This 2001 Chevrolet Silverado came in the shop with the complaint of no stoplights. First I confirmed that none of the stoplights were working including the CHMSL (center high mounted stop light). After this I went to the underhood fuse box because the manufacturer has made diagnosing the stoplights very easy at this location. By going to the red, 10 amp, vehicle stoplight fuse, #17 and checking for power while some pushes on the brake pedal I can tell easily if the stoplight switch is good and I do not have to lay under the dash to do it. I identified the fuse location using the legend.
This one was really easy because when I located where the fuse was supposed to be I found that it was missing. I installed a new fuse and tested both sides for power while someone depressed the brake pedal. I found that the owner had been swapping fuses trying to keep his power outlet working for his cell phone charger.
If I had found power on one side of the fuse but not the other it would have indicated a blown fuse. If I had found no power on either side with the brake pedal depressed I would have suspected a faulty stoplight switch or no power to it and more testing would have been needed.
Luckily, the power supply fuse for the stoplight switch is the yellow, 20 amp fuse, #14 is right above the fuse I was testing. Testing here would have shown one of two things. Power on fuse #14 with no power on fuse #17 would indicate a faulty stoplight switch. If fuse #14 had been blown it would have indicated a short between the fuse and the stoplight switch and back to fuse #17. Also a blown fuse #17 would indicate a short between the fuse and the rear of the truck.
Your 1.4 ohm reading on the one terminal is probably reflecting the resistance of the stoplight bulbs. Fuse 14 supplies power to the stoplight switch. When the switch is activated it supplies power to fuse 17.
I assuming that we are looking at the fuses in the underhood fuse box. Fuse 14 will be powered at all times and fuses 16 CHMSL and 17 Vehicle Stop lights only have power when the brake pedal is depressed. Depressing the brake pedal completes the circuit at the stoplight switch between the orange “B” and the white “A” wire. The BCM would not be the cause of no stoplight condition. There is another connection in the stoplight switch harness about 10-12 inched from the switch that may have an issue as well. The other complaints sound like separate problems as well.
I followed brake switch harness up a good two feet and found no other connections. When I set my meter on ohms, connect one lead to ground and the other to fuse 17 I get 1.4 ohms with brake pedal switch on or off. Should this be telling me something? I can jump power to fuse 17 slot on one side and get brake lights on bed and to the other pin for fuse 17 and get just the CHMSL. thanks for the response.
Have power to fuse 14 but none to fuse 17, indicating faulty stoplight switch…Replaced stoplight switch for 13 bucks still no power to fuse 17. Dang man. Could a bad BCM be the issue? Also have no tach, bright lights always on, fuel guage jumps around, and every light on the dash is on. Looks like a darn chrimas tree.
Hi jhunter_123,
Do not waste your money on a multifunction switch at this time! The orange wire is power supply for the brake lights and it’s source is the fuse box and not the multifunction switch. The brake light switch sends a signal out on either a blue or a white wire to the multifunction switch. Check your voltages at the stoplight fuse and see if you have the same drop on either side of the fuse. If you do correct the condition. If you do not see the same voltage drop I would suspect a faulty connection at the brake light switch sub harness connector . It is about 12 inches up the harness from the switch itself. You could also locate the orange wire at the junction block, just to the left of the column and check your voltages there.
jhunter_123 asks,
ok sparky i got one for ya replaced the brakelight switch have power coming in on orange when pedal is depressed voltage drops to 0 have been told it is the multi function switch or in my case the multi malfunction switch all other signal light,headlight and taillight communication works fine new one will be in today but i’ve also heard that the chmsl would still work if the mfs is shot can you verify any of this cause its starting to get pricy sounds to me like a short but there are none visible