I'm new to this Forum. Thanks in advance for the help.

93 first gen Cummings, W250, It was a flat bed farm truck I purchased and rebuilding for a work truck just for my use towing/hauling

The brake warning light and ABS light stay on continuous. I'm working my way thru the testing procedures in the service manual to determine the cause and correct.
I have more than adequate vacuum, and since the master cyl was leaking, I replaced the master cyl and booster. The brakes seem to work well, I just can't get the lights to go off.
There could be several issues, I'm just trying to tackle them one at a time in a logical order. My low vacuum warning switch does not seem to respond to any input. I have removed the switch, and if I apply a vacuum to the nipple with a hand pump, It slowly drops to 0 over a few seconds. I've put a continuity meter between the signal ground and the wire to the light, (knowing there is a switch inside that should either open or close when vacuum is placed on the unit). Then with or without vacuum, there is no signal between the terminals, and no change. I do not know if the switch is NO or NC, but either way shouldn't there be a change?

Is this the correct way to test that switch? Anyone have other suggestions? Once I know this switch is good, or replaced with a good one, I'll continue

Thanks

 

IDK if the switch is NO or NC. However the cause could be the RWAL system is bad because of a bad DUMP valve which is unobtanium. There is a control box for it behind the glove box which has a plug and wires. If you unplug the plug see if the light goes off. That's is the cause of my brake light being on all the time. Could be yours also.

Edwin

 

Thanks, Edwin.

I disconnected the RWAL computer and the ABS light goes off, but not the brake light. I'm fairly sure the low vacuum switch is bad, since it does not send a signal with or without vacuum. I have a buddy with one and a RWAL from an old truck that I am getting. Don't know for sure if either is good. So, just for giggles, I tore into the Vacuum switch. If I can figure out how, I will add some pics. This is what I found FYI. There is a diaphragm similar to those found in small engine fuel pumps. This one has a circular metal disc attached to it by a central rivet. I was surprised at the good shape the diaphragm was in. The diaphragm is kept pushed away from two ~1/4" diameter terminals on the back side of the switch by a central spring. When a vacuum is applied, the diaphragm is sucked in until it connects the two terminals completing the circuit. Therefore, the switch is closed when the engine is providing vacuum, and opens without vacuum. That makes sense because it will cause the brake light to illuminate when starting the truck until adequate vacuum is established.(the self test) My issue is, one of the ~1/4" terminals is not making contact with its corresponding terminal on the electrical connector. One does, the other does not. The back side of the switch is filled with a resin which i suspect completely encases the wires. I probably will try tearing into it further just to know.

Knowing how the switch should operate, I'm sure I can find something in the correct pressure range and rig it to work. I believe my service manual says it the vacuum drops below -8 for 10 seconds, the light illuminates.

Jimmy



Resin filled back side

Diaphragm, spring and ~1/4" terminals

electrical terminals.

how items fit together

 

If your in the USA I can send you a used OEM one for $30 shipped.

 

Let me see if the one my friend has works, if not, I'll take you up on that.