Author Topic: SOLVED Need help diagnosing hydroboost brakes on a 93 GMC Topkick (Read 3619 times)

zieg85 · « **on:** September 01, 2021, 09:38:22 AM »

I have spent hours searching the net. My problem is there is an electric motor on the bottom of the hydroboost on this truck that will never shut off. I understand it is the fail safe should the truck loose its power steering belt or the engine quits...kinda important, especially hauling a load. Anyone versed on medium duty trucks, I am all ears. Thanks in advance

ehjorten · « **Reply #1 on:** September 01, 2021, 02:27:40 PM »

I believe that motor is controlled by the brake switch under the dash. Can you pull up on the brake pedal and make it stop? Might need to adjust that switch, or maybe you have a faulty switch?

bd · « **Reply #2 on:** September 01, 2021, 02:29:37 PM »

I agree. Begin by checking the switch adjustment on the brake pedal.

zieg85 · « **Reply #3 on:** September 01, 2021, 02:47:47 PM »

Quote from: ehjorten on September 01, 2021, 02:27:40 PM

I believe that motor is controlled by the brake switch under the dash. Can you pull up on the brake pedal and make it stop? Might need to adjust that switch, or maybe you have a faulty switch?

Can you confirm that? I've pulled up the pedal and no change. I am not finding a relay anywhere in this truck, even for the fuel pump. There are no free manuals on line either

bd · « **Reply #4 on:** September 01, 2021, 03:02:21 PM »

Can you adjust the thumb slide on the bottom of the switch?

zieg85 · « **Reply #5 on:** September 01, 2021, 03:10:20 PM »

I have adjusted that and it made no difference.

zieg85 · « **Reply #6 on:** September 01, 2021, 03:13:50 PM »

As I was out the moving that adjustment I quick came back and edited that photo...great minds think alike

bd · « **Reply #7 on:** September 01, 2021, 03:23:46 PM »

Does the motor stop when you unplug the brake switch?

zieg85 · « **Reply #8 on:** September 01, 2021, 03:34:12 PM »

Quote from: bd on September 01, 2021, 03:23:46 PM

Does the motor stop when you unplug the brake switch?

Duh...I hadn't thought of trying that. Stay tuned Rich. I appreciate the suggestion.

bd · « **Reply #9 on:** September 01, 2021, 03:40:23 PM »

There should also be a series-wired flow switch in the booster head that completes the circuit when the hydraulic flow is insufficient. Check whether that switch is shorted.

zieg85 · « **Reply #10 on:** September 01, 2021, 03:54:29 PM »

Quote from: bd on September 01, 2021, 03:40:23 PM

There should also be a series-wired flow switch in the booster head that completes the circuit when the hydraulic flow is insufficient. Check whether that switch is shorted.

I already ruled that out. I unplugged it and jumped the 2 wires. It did not make a difference, besides that pressure switch is NLA according to GM

zieg85 · « **Reply #11 on:** September 02, 2021, 09:42:09 AM »

Quote from: bd on September 01, 2021, 03:23:46 PM

Does the motor stop when you unplug the brake switch?

I removed the switch and even unplugged it. Motor is still on. Any other suggestions?

MIKE S · « **Reply #12 on:** September 02, 2021, 12:08:43 PM »

I think the flow switch part number is 18020593. I saw one on ebay right now.

zieg85 · « **Reply #13 on:** September 02, 2021, 12:16:50 PM »

Quote from: MIKE S on September 02, 2021, 12:08:43 PM

I think the flow switch part number is 18020593. I saw one on ebay right now.

Do you know if this is a go/no go switch? I've jumped the wires with it unplugged and there is no difference, the motor continues to run

zieg85 · « **Reply #14 on:** September 02, 2021, 03:53:35 PM »

PROBLEM SOLVED...I think. Due to the lack of knowledge/hearing/no wiring diagrams/being hard of hearing/dumb luck. I was just convinced there had to be a relay of some sorts, somewhere. Wind died down and I heard a faint clicking somewhere. I had the owner of the truck swing by to listen outside while I was hitting the brake pedal. Bingo. Found and tested the relay and it tested bad. I have another AC Delco one ordered and will make a final update hopefully in a few days.

Great thanks to all those who helped. I learned a lot, GM calls that fail safe Delco Hy-Power systems. Pictured is the culprit. I ohm'd across A and E and it is connected

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