I've got one that I cannot figure out.

There is a Brake Warning Light up in the Main Instrument Panel (the one with the Circle ! and Circle P) that is operating correctly. It illuminates if/when the Parking Brake is set, and I assume would illuminate if there was a Brake Problem.

There is another Center Instrument Panel (I believe it is called the "TELLTALE MODULE") with warning lights located just below the Radio and above the Heating and Air Conditioning Controls. This is where the Exhaust Brake Engaged light is located. On the left side of this panel there is a light that says "BRAKE" with a wrench to the left of the word BRAKE. Since I don't know what it is, I'll call it the "Service Brakes" light.

This light NORMALLY illuminates for a few seconds when the key is turned on, and then goes out. The other day, the light illuminated when I turned the key on, and then stayed illuminated with a warning Chime. The light stayed on after I started the engine, put the vehicle in Drive, and rolled out of the garage.

I stopped, turned the engine off, and checked for Codes. There were none. After about the third time I went through the start procedure, the light went out and remained out for several days.

Today, while driving, on three occasions this light came on and the Chime sounded. It does not inhibit engine RPM or transmission operation. I pull to the side of the road, turn the engine off, restart, and it's back to "normal". I do not have to be applying the Brakes for this light to come on, it came on once while I was accelerating fro a stop light.

Does anyone know what this light indicates? I have been through my manual at least a dozen time, and this particluar light is not mentioned in the manual. I have also checked the Electrical Diagrams in the Upfitter drawings, and it is not shown.

Any help would sure be appreciated.

George

Been there, done that. Here is a copy of the reply I received a while back from Dirtdog on the very same matter....

"As far as brake light switches, the problems I have seen on the GMT-560 (2003 and up) have been primairly due to electric brake controllers being hooked up to the brake light switch for its signal. Can't do that anymore, but the brake controller companies haven't kept up with the new technology and still tell the installers to use the switch for the brake on signal. Using the switch over loads it and will cause them to fail. You must hook up to the backside of the brake light relay.

Check out the GM Upfitter website (www.gmupfitter.com) for the correct way to hook up electric brakes. Click on "Publications" and then "Technical Bulletins". Bulletin # 66 is the one you want."

I sent this to my mechanic and sure enough, that was it! He has seen this in several other 4500 and 5500 MDT's since. I actually carry an extra switch in the truck as I am sure it will happen again and is very annoying! Good luck,
Rodeome

Something else that I have recently seen happen, is this warning light and chime coming on because of a failed Brake Booster Fluid Flow Switch. I think it sets a C0285 code.
Also, if the Hydromax relay goes bad and sticks closed, it can cause the same warning light, but the electric emergency booster motor would probably run all the time when you press on the brake pedal when the engine is running.

Unless you have a DTC reader that can read chassis related codes you probably can't see the code. The relay is right behind the P/S reservoir and would set a C0272 code.

Let me make sure that I'm heading in the right direction.

First you are correct in that Western Hauler did tap the Brake Pedal Switch for the Brake Controller input signal. I even spoke to them about doing that, and they stated "we've never had a problem doing it that way". ( No they don't have the problem, I DO!)

Second, in looking at the Upfitter Wiring Diagrams, I see that the Light Blue Wire at the Upfitter Connector on the right side of the Frame comes from a 10 Amp BRAKE ISO Fuse located in the IP DISTRIBUTION/RELAY CENTER under the hood. This is the same location that the BRAKE LAMP RELAY is located.

My thoughts are that I should be able to tap into the output side of the 10 Amp BRAKE ISO Fuse and route this into the Cab for the Brake Controller. This will provide fuse protection for the wire, as opposed to tapping into the BRAKE LAMP RELAY itself.

Does this make sense?

The thing that has me baffled is that it has worked as wired for several months, and that the failure (Lamp and Chime) are occuring even when the brakes are not being applied.

I'll also take a look at the Brake Booster Fluid Flow Switch and Hydromax Relay.

I'm using the Predator to read the codes, so I probably do not see the Chassis Related Codes.

Thanks for the help. I'll report back.

George

rodeome,

When yours failed, did it just stay on all the time or were you also having intermittent problems?

George

It was just the same as you are describing. I had the brake box installed and in use for several trips before anything happened. Then out of the blue, while driving, the light came on and the chime sounded without any application to the brakes. The new switch fixed the problem and it has not happened since.
Rodeome.

Oh, and I forgot to add that it did not happen all the time. And when it did come on, if I stopped, turned off the motor and restarted, the chime and light were disabled and the truck would go along fine....for awhile.

rodeome/dirt dog,

I just finished checking the signal going to the input of the Prodigy Brake Controller.

Western Hauler DID use the output side of the 10 Amp Fuse that is powered by the Brake Light Relay.

SO, I'll have to look elsewhere for the reason that the light is coming on.

Of note, during my testing there were several times that I pushed the Brake Pedal and DID NOT get 12 Volts at the input of the Brake Controller, nor did I get the Brake Lights on the vehicle. This may indicate an intermittent Switch on the Brake Pedal Arm. I moved the switch and harness around, and have not been able to duplicate the "no signal" symptoms.

Ah! The wonderful world of electronics. Don't you just love it! And that has been my trade (USMC, Hughes Aircraft) since I was 18 (now 64).

Thanks again.

George

Humm, the more you describe what is going on, the more it sounds like a brake pedal switch problem. You really need to know what code is being set. Know anyone with a TECH II?

Dirt Dog,

I'm thinking the same thing, Brake Pedal Switch.

My situation is that there is no source here in Yuma, and I'm headed out next week for the Summer Rodeo Circuit and will be on the road until October.

If you happen to have the Part Number for the Switch, I'll check in each major city with a MDT Service Center and just purchase one and replace it when I can.

I'm also still looking for the Low Coolant Reservoir Switch (P/N 19130014) that failed, but there are none in the GM Inventory and the last time I checked there was no Estimated Availability Date.

Let's close this thread, and I'll create a new Thread and update if/when I find something out.

Thanks again for all the help.

George

I went through the same problems, turned out to be a circuit breaker or what I would call a fuse.

I went through the same problems, turned out to be a circuit breaker or what I would call a fuse.

Certainly worth checking out. The hydromax relay gets power to pin #85 from the EBCM (pin#22) on a tan wire (ckt# 2660). A bad connection at the relay or at the EBCM would cause a warning light.

The EBCM (pin# 14) gets the stop lamp switch signal to it on a white wire (ckt#17) from the pedal switch which is fed power from the stop lamp fuse (orange wire, ckt 140) in the IP fuse box (under the grab handle on the passenger side of dash). That fuse is fed by ckt#142 (red wire) from the secondary underhood fuse box by a 60amp (Batt/Haz) fuse.

Bad connection in any of those places would cause the warning light too. You will have to wiggle the wires and see if you can trigger the light and warning chime.

Dirt Dog,

EBCM = Exhaust Brake Control Module ?????????

George

Sorry about that EBCM, "Electronic Brake Control Module". some would also call it the ABS Module, but it does more than anti-lock brakes these days.