Author Topic: *SOLVED* Need help diagnosing hydroboost brakes on a 93 GMC Topkick  (Read 1916 times)

Offline zieg85

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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
« Last Edit: September 08, 2021, 01:12:22 PM by zieg85 »
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
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Offline ehjorten

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« 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?
-Erik-
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6

Offline bd

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« Reply #2 on: September 01, 2021, 02:29:37 PM »
I agree.  Begin by checking the switch adjustment on the brake pedal.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline zieg85

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« Reply #3 on: September 01, 2021, 02:47:47 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
« Last Edit: September 01, 2021, 02:51:18 PM by zieg85 »
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline bd

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« Reply #4 on: September 01, 2021, 03:02:21 PM »
Can you adjust the thumb slide on the bottom of the switch?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline zieg85

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« Reply #5 on: September 01, 2021, 03:10:20 PM »
I have adjusted that and it made no difference.
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline zieg85

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« 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
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline bd

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« Reply #7 on: September 01, 2021, 03:23:46 PM »
Does the motor stop when you unplug the brake switch?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline zieg85

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« Reply #8 on: September 01, 2021, 03:34:12 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.
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline bd

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« 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.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline zieg85

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« Reply #10 on: September 01, 2021, 03:54:29 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
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline zieg85

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« Reply #11 on: September 02, 2021, 09:42:09 AM »
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? 
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline MIKE S

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« 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.

Offline zieg85

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« Reply #13 on: September 02, 2021, 12:16:50 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
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline zieg85

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Re: Need help diagnosing hydroboost brakes on a 93 GMC Topkick C7H042
« 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
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/