GMC anti lock brakes.

John-MI · Nov 17, 2014

I have been told that anit lock brakes turn off when you are in 4 WD. Does anyone know if that is true. Thanks. John

JMOR · Nov 17, 2014

(quoted from post at 11:05:07 11/17/14) I have been told that anit lock brakes turn off when you are in 4 WD. Does anyone know if that is true. Thanks. John

Drive on dry road and jam brakes on hard & you will feel the pulsing if it is on.

MarkB_MI · Nov 17, 2014

The old rear-wheel-only ABS system was disabled while in 4WD. I don't think that's the case with the newer systems.

Why do you ask?

John-MI · Nov 17, 2014

Just a table discussion the other day.

Sprint 6 · Nov 17, 2014

Anti-lock DOES NOT turn off in 4wd from at least 95 and up. It does use a modified program for 4WD and 4Low.

Allan in NE · Nov 17, 2014

No they weren't.

Allan

MarkB_MI · Nov 17, 2014

Allan, I know you used to work on Chevys, but I have to disagree with you on this. I'm pretty certain the Kelsey-Hayes rear wheel ABS system was disabled in 4WD. The info at the link below backs me up with a simple statement: "On trucks with 4WD, the antilock system only works in the two-wheel drive mode."

If you think about it for a second, it would make no sense to leave the system in operation while in four wheel drive. The K-H system has no wheel speed sensors, so it can't tell if a single wheel has locked up. (It uses the speedometer transducer to detect wheel lockup.) It doesn't make much sense to modulate the rear brakes and leave the front brakes engaged when both the front and rear axles are locked together by the transfer case.
Kelsey Hayes rear wheel ABS

GMC anti lock brakes.

John-MI

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JMOR

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MarkB_MI

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John-MI

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Sprint 6

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Allan in NE

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