Brake Light Problem on 1993 Dodge W350 w/Cummins & 5 Speed

[Racer Brown]

I’ve got a 93 Dodge W350 LE, 5 speed, Diesel with 132K miles on it. The truck has been kept in my shop since I bought it in 2000 with 78K miles on it and I only used it to tow my race car trailers over the years. Sold my last race car back in 2015 and the truck hasn’t been used much since with an average of around 200 miles annually.

So about a month ago I drove it to town (about 35mile round trip) and on my way home I stopped at a stop light and noticed that the brake warning light on the dash was on? Didn’t feel anything different in the brake pedal and it stopped normally on my trip so I decided to do some testing when I got home. Oh, and that light only comes on with the brake pedal applied at idle. Rev it up and it goes out?

I’m also an HVAC guy so I rigged up some fittings to connect my A/C compound gauge to various items in the system and also used my electric HVAC vacuum pump to test the booster.

The vacuum pump on the engine was showing me 25 in of vacuum at idle, which I figured was OK. Then I connected my 110v electric vac pump to the booster and pulled it down to around 28 inches then closed the valves on the pump to show what it did with that pump off and it eventually did bleed off after 10 minutes or so.

So, does anyone have an idea what is going on with this brake light problem? Like I said, I only see it when I’m sitting stopped with the foot brake applied and feeling “normal". Oh and the cruise control works fine also as I used it that day going to town and back.

Just saying...................

 

[RSchwarzli]

First question. What was ambient temperature?

 

[Racer Brown]

First question. What was ambient temperature?

Don't remember the exact temp but it was probably in the mid to upper 40 degree range when it first started doing this? I've driven it several times in the past couple of weeks and temps then were in the mid 60s and it was doing the exact same thing.

So are you thinking this is temperature related??

Just saying................

 

[sdeeter19555]

Are you sure it is the booster, the HVAC controls also use vacuum?

My guess is either you have a leak, the vacuum sensor for that light is going bad, or you have a weak vacuum pump. Although, 25 inches is about normal, so I'd suspect the sensor.

 

[Racer Brown]

Don't know if it matters or not, but when the brake light in the dash comes on there is also a yellow ABS light on as well.

Maybe it's time to sell it!

Just saying...................

 

[sdeeter19555]

Don't know if it matters or not, but when the brake light in the dash comes on there is also a yellow ABS light on as well.

Maybe it's time to sell it!

Just saying...................

ABS, either the module, the RWAL sensor, or the wiring...

 

[Racer Brown]

ABS, either the module, the RWAL sensor, or the wiring...

Well, you just lost me cuz I got no clue what half of that stuff is. I guess I need to get my service manual out and see if I can find out.......

Just saying.................

 

[sdeeter19555]

Well, you just lost me cuz I got no clue what half of that stuff is. I guess I need to get my service manual out and see if I can find out.......

Just saying.................

The ABS is causing both lights to illuminate.

It is caused by either the Rear Wheel Antilock (RWAL) deal on the rear frame, the RWAL sensor in the rear axle, or wiring in between.

 

[Racer Brown]

Thanks for the info. Looks like I'll be taking it to the garage this coming week. I don't need it since I sold the last car and trailer I had in March of 2022 so plan now is to fix it and sell it.

I'm heading for 78 years old in September and with bad health it's hard for me to work on it any more. I did do a bunch of stuff to it early on, ie, replaced the Getrag with an NV4500, changed from an 18 to a 16 turbo housing and install a set of marine injectors all of which netted me more boost and also almost 14mpg TOWING a 24ft enclosed trailer. That and it still gets 20mpg highway empty.

Ennywho, like I said......thanks for the info.....................

Just saying...............

 

[Racer Brown]

FINAL UPDATE: Well, FYI that brake warning light problem I had now belongs to the guy that offered me $14,000 CASH for my truck last week before I had it worked on! Needless to say I was happy to sell it for that as that was more than what I paid for it when I bought it in December of 2000.

I will miss it though and when I closed that drivers side door for the last time, I laid a BIG KISS on it for all the good times I had with it!

Just saying................

 

[Topzide]

@Racer Brown, I hope that it has found another good home! Time to start looking for a replacement. You know that it won't be to long before you'll wish that you hadn't parted ways. It's all the best and beats beating your head on the steering wheel trying to figure it out.