ABS Light on 92 K5 Blazer

[MI0706]

I have a 92 Full Size Blazer. The ABS Light comes on when speed exceeds 15mph. As soon as the speed is below 15mph, the ABS light goes out. I checked/cleaned the speed sensors earlier today (took voltage measurements on the speed sensor back at the plug on the ABS unit, volts varied between .5 volts and 4.5volts AC depending upon speed of the drive shaft).

I have a scanner/code reader (Auto X-ray 3000) but it does not pick up ABS codes (wasn't showing any problems). I do not get the any other warning lights (check engine, parking brake, etc.)

Is the next step trying to figure out which ABS code is being sent? Or is there some other preliminary things to look at prior to figuring out the code?

Thanks for the help.

[bradrhale]

yes. get the codes ran at a shop. once you find out what the code is, it should be alot easier to diagnose the actual problem.

[crabtruck]

That 92 is the second year they had the 4wh ABS on Blazers.
You can pull the code yourself by jumping A to H on the DLC/ALDL
and counting the flashes of the anti-lock light.
Codes are different for 4wh ABS and the more common (then)
2w RWAL ABS.
When jumping A-H you may hear the ABS motor run.

Regarding the measured voltage: "varied between .5 volts and 4.5volts AC
depending upon speed of the drive shaft" I think that's a problem.
At hand spinning speed they should be around 0.350v
and might drop to 0.200v if dirty. Maybe you should test them differently.
Unplug each front sensor to measure while spinning at hand speed.
This isolates problem and each acts as reference for other side.
Seems that super low 0.5v has to be a problem.
The back I think just shares a common sensor from the rear
that goes through the speed buffer first before reaching antilock.

That "H" contact at DLC is connected to the wire from buffer to ABS
and if you were to measure voltage there you should see about 1-5v.
A much high read suggests bad ground for ABS.

Problem may be coming from the buffer. See if you have problem with
any of these: cruise control, shift points, speedo reading right.

[MI0706]

crabtruck: Now that you mention it, my cruise control doesn't work. However, the shift points seem fine and I believe the speedo is reading correctly (how do you verify your speed, radar gun?!?)

Makes sense what you said about unplugging each at the sensor. I just unplugged both of them at the ABS unit (common plug). I will isolate each and test individually. But correct me if I'm wrong, the speed sensors are nothing more than a PMG (permanent magnet generator). Therefore, spinning the shaft faster will generate a higher voltage.

Thanks for the help. I will test each speed sensor isolated and check the codes and post back here.

[gmctrucks]

My ABS light came on once on my 93 F/S GMC well I just unplugged it at the harness and plugged it back in and it cleared up and it never happened again this was about 15 years ago.

[crabtruck]

The sensors are not a complicated design, but must isolate to know
if one of them is guilty or perhaps the wiring to ABS.
You tested a system as a whole and without a benchmark.
Yes, the signal voltage would increase with speed, but
what voltage at what speed ? Therefor disconnect.

If the other things I mentioned are having problems,
ABS error could be buffer or VSS related. If speedo is right,
reasonably accurate not scientific, VSS would seem ok,
and bufffer probably ok, pointing to ABS system as concern.

In what way does the CC not work ? Nature of failure
might help point out the common source of error.

ps-thks brh

[MI0706]

gmctrucks: I've tried unplugging every connection on the unit. My luck is not that good... LOL...

crabtruck: The CC will not turn on. I've checked the switch at the brake. I've checked for power at the CC unit. I have a new turn signal lever that I'm going to install to rule it out (I was replacing this anyway, old one is very worn).

Still haven't had a chance to test the sensors and get the ABS code. I've got the entire interior ripped out right now (sound deadening and repainting all the plastic panels).

A new thing started happening today with it though. At idle, the ABS light will come on and then go off. It's not "flashing". This happens when in park. As soon as I put it in drive, the light goes out (until I reach >15mph).

Thanks for the help.

[crabtruck]

gmc has a good point about checking connections,
can make you nuts.

A couple good sites to check for you would be the
park brake switch and the ABS ground (shared I think)
on the frame below driver door area.

I don't recall if you have the 4 wire or 6 wire brake switch.
If you have the 4-wire there should be another 2-wire for CC.
It is not needed to crawl down there if you check function at
module itself, and also will check fuses if you haven't already.

Check at the module connector (near brake master):
Make sure meter is set for right type reading as you go.
May want to "map" the 10-wire connector along the way.
If I have the wrong pins, you can correct as you go.
Always do a voltage check before ohming for safety.

With key off, CC switch off,
Ohm the grouind connection for black/white on pin-E.
Batt voltage only with brake pedal on yllw,wht or purple on pin-G or L
(there were changes, make note where you see it,
should be only hot to show up at this time,)

With key on, CC switch off,
Batt voltage at pink/black pin-F
Batt voltage only without brake pedal on purple on pin-G or L
(This second brake wire acts opposite to one above, which still works)

With key on, CC switch on,
Batt voltage at gray pin-A
Batt voltage at brown pin-D should toggle off / on with pedal.
(if manual use the clutch pedal)
Batt voltage at dk-grn pin-C when pressing R/A
Batt voltage at dk-blu pin-B when pressing S/C

Other contacts, no tests:
Wht pin-J PCM signal
Rd/wht pin-K VSS buffer signal

I know you haven't checked the front speed sensors yet.
Here's why you don't want to have to go back to the buffer
for speed signal checks. It can vary, and requires good equip.

For my 245/75/16 tires with a 3.73 rear I would expect the VSS
to turn at about 2,469rp/mile generating about 98,800 pulses
per mile, which would translate to 1,647hz to the buffer and speedo
and about 824hz to the PCM, CC and ABS.

275/85/16's would change that about 11.5%
and depending on rear could change another +/- 10%.
You would need a good o'scope or freq counter to read these.

Again I say double check connections, LOL.
That would include the pug-in's in the conv center near fuses.
Parts of ABS, CC and brakes jump through that block.

[MI0706]

crabtruck: I did some testing using your posts as guidelines. Bare with me, this will be a long post.

Checked voltage at pin H on the DLC: 4.2V

Performed the ABS code check (flashing lights) and this is what I came up with:
code 13, 25, and 68
If my research is correct (Kelsey-Hayes 4WAL ABS) then:
code 13: system normal
code 25: left front speed sensor problem
code 68: locked motor or shorted motor circuit

Performed the isolated speed sensor tests (at the speed sensor plug) and both sides produced 2.5 volts AC at what I would call "normal hand speed".

I couldn't locate the ground wire for the ABS unit or parking brake switch.

As for the CC system:
No physical connection to pin G nor do I have a pin L
With key off, CC switch off:
Pin E to ground: 0.5 ohms

With key on, CC switch off:
Pin F voltage: 12v

With key on, CC switch on:
Pin A voltage: 12v
Pin D voltage: no voltage present with the brake pedal pressed/depressed
Pin C voltage (when pressing R/A): 12v
Pin B voltage (when pressing S/C): 12v

Reverified connections at the ABS unit and at the convenience center.

If my troubleshooting was correct, I would say I have the following problems:

1. Drivers side front speed sensor (I don't understand this one because both speed sensors checked the same).
2. Problem internal to the ABS unit (motor locked or circuit shorted). I need to further investigate this one. Any advice?
3. Brake pedal switch doesn't seem to be functioning properly afterall.

Just to clarify the brake pedal switch, it is a 6 wire switch (assuming two for the brake lights, two for the CC, and two for the ABS?). Keep in mind, the brake lights have always worked properly.

[crabtruck]

You're worried about giving ME a long post ?
With all that typing I was bound to make a error and I did.
Error was I trusted a part of my factory drawings that I hadn't checked.
Bad move, on closer inspection, they showed 10 wires
going to a place with only 9 connections.
And of course there's no "L" - that would make it 11.
So I got off my tail and checked my truck for the truth.


Seems you are ok on A,B,C,E,F
D and G could probably be disconnected and it still work,
since they arte for turnibg it off. Needed for safety.
They imply a problem with brake switch wiring though,
might want to look for damage caused by the addition of
trailer brake controller, someone might have cut them there.
If the wirig checks out, look for mechanical reason for no cruise
such as disconnected or jammed linkages.

When you jumped A-H for ABS you didn't mention hearing motor.
Do you have hot for motor on big wire red A of 2-wire that
goes to a fusible link (doesn't go through fuses).

Your first code 13 is not "normal"
that code is "normal in 2wd w/ brake" so suggest same place as
above on brake switch check. If you jump again A-H while on brake
see if you get 12 which is for no brake.
BTW: 14 is no brake in 4wd and 15 is brake in 4wd. (also normals)

For that code 25 you might want to ohm the sensors to be 1-3k ohms.
That's B and G on the 8-wire connector for left wheel sensor.

For code 68, can be caused by poor connection at 2-wire
with A-batt and B-ground. This should be checked for good
batt and gnd voltages. Clear the DTCs and hope you get it ,
because the other causes for 68 are not fixable -must replace ABS.
There are two grounds on the ABS box, B is really big and
the other (J) is kinda big wire, very important there in good shape.

Note that the "ground wire" for the park brake switch is the metal mounts

Seen together,all this stuff makes me suspicious of wiring, but when all else fails,
you're looking at the expensive one, so be double sure.