Intermittent ABS Light 1994 Chevrolet K1500 Blazer Repair

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H2814D

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Hi All. Hopefully this will help someone else with this problem. History: I have a 1994 Chevrolet Full Size Blazer with a 5.7 Liter (350CID) V8. The ABS System is a Kelsey-Hayes 4WAL. About a year ago, I started getting an intermittent ABS light. Once the truck was shut off, it would go away, and then, for no particular reason, the light would come on sometime later and then sometimes not at all. As you know, when the ABS light comes on, it turns off the ABS System.

I had followed a recommendation to replace the front wheel hubs, as that seemed to be a common cause. The truck only had about 90 thousand miles on it when it started happening, and I thought that amount of miles would necessitate front wheel hub replacement anyway. I did the replacement. It seemed to work for a little while, and then I was again getting the intermittent ABS light again.

I then did an OBD1 test on the ABS using a wire jumper in the connection port. I got these codes (I didn't get them in this order, but I put them in ascending order for this write-up): 12, 41, 42, 45, 46, 51, 52, 65, 67.
12-Sytem Normal - (This reading is to confirm the computer is transferring the trouble codes correctly)
41-4WAL Control Unit Fault Isolation/Dump Coils shorted or open-internal EBCM
42-4WAL Control Unit Fault Isolation/Dump Coils shorted or open-internal EBCM
45-4WAL Control Unit Fault-Control Valves
46-4WAL Control Unit Fault-Control Valves
51-4WAL Control Unit Fault-Control Valves
52-4WAL Control Unit Fault-Control Valves
65- Open or shorted pump motor relay
67- Open motor circuit or shorted BPMV output

Not seeing anything, code-wise, that would indicate a bad sensor, my next guess was that the problems were due to electronic issues and most likely in the ABS Control Module under the hood. These trucks have a history of electronic issues due to circuit board solder failures. So I decided to remove the Aluminum Cap from the Control Module and check the electronic circuit board underneath it.

Note that if you decide to attempt this repair, as I did, I do not assume any liability for your success or damage to anything that may result from your actions. Following any of the steps here is entirely up to you.

Remove the negative battery cable.

The module is located on the driver's side inner fender wall just forward and below the brake master cylinder. There are seven security torx screws holding the cap on. See the photo for module and screw locations. I removed the three electrical plug connections first, then removed the screws. Once the screws were removed, I removed the cap. I did not remove the black rubber gasket below the cap with several pins protruding out of it. There is no need to. Leave that part alone. Just remove the cap, although it may be stick to the rubber part below it. Be careful here.

Once the cap was off, I could see the printed circuit board underneath. Your goal now, is to get to the bottom of the board, where most of the solder connections are made with the electronic components on that board. At that point, I could see there were two pieces of spring steel plates that hold the electrical plugs firmly against the inside of the cap. Those plates are inserted so they fit into grooves in the cap and are pressed in. Those plastic electrical plugs are also sealed with some type of adhesive sealant, like silicone, or something similar, to prevent water from entering the cap. See the photos for the plate locations and the pry locations.

The spring steel plates need to be removed. In my case, I used a large flat head screwdriver to gently and slowly, but firmly, pry up on the top end of one side of the plates by wedging the screwdriver under the lip of the plate and using the part of the cap where the plates slide into the groove as the fulcrum. I pried up one side of one plate at a time and just slightly, then moved to the other side of the same plate, doing the same thing. Do this a little bit at a time and don't force it so hard that you break the aluminum cap where the plates slide into the cap. Do this slowly and take your time. They will come up, but be patient. Pull them completely up and out of their respective grooves and set them aside.

There is also a sort of figure 8 piece of spring steel forcing two semiconductor chips against the aluminum cap to provide for a heat sink for the chips. I used a larger size pair of needle nosed pliers and inserted the ends into the outer most holes, than spread the pliers apart. Be very careful here too. This takes some work to get this piece off. It has to slide upward and off completely. You can also use two pairs of needle nosed pliers and twist them. Looking at the PCB (printed circuit board) with the electrical plug ends to your left, the right side pliers would be twisted clock-wise and the left side pliers would be twisted counter-clockwise, in order to relieve the spring compression and get the spring to move upward and off of the chips.

Once all of the spring steel plates and clip have been removed, there are two Phillips Head screws (see the photo) that hold the PCB to the aluminum cap. Take those out and set them aside. Your next step will be in removing the adhesive sealant used between the plugs and the aluminum cap for moisture prevention. A scalpel type blade works well for this. It is time-consuming, but it needs to be done in order to remove the PCB from the cap. Work slowly and carefully until the plugs free themselves from the cap. The plugs can be bent slightly at the electrode wires you can see coming out of the plugs. Don't bend them too much to where they may break or become in contact with each other. Be careful not to slip and damage any part of the PCB or the wire connection electrodes.

Once the plugs are free, the PCB can be removed from the cap. This is an electronics board and it has static sensitive components mounted to it. If you are not familiar with grounding and static prevention techniques, this would be a good time to learn about them before moving forward.

So having the codes giving me a probable electronics issue rather than a sensor issue, my first thoughts were to check the underside of the PCB for "cold" solder joints. Cold joints are often caused by the constant heating and cooling of the joint through normal use. What happens is that the solder joint develops a small, almost hardly visible crack in the solder. It is usually round and runs the circumference of the chip or component electrode where it protrudes through the PCB from the other side. I have attached photos of what broken solder joints look like. Unfortunately, I thought I had a photo of the entire soldered bottom portion of the PCB, but when I went to look for it for this write-up, I didn't have one. So I only have a couple pictures of the solder connections.

I then took a magnifying glass and inspected each solder joint on the bottom of the PCB. The most probable sections where cold solder joints occur are at electrical plug connections. The two plugs had several solder joints that had been cracked. I went ahead and re-flowed (melted the connection and used flux and new solder) on each of both of the plug pins that were soldered to the bottom of the PCB. I did the same thing on each semiconductor (larger transistors or power chips) that had a heat sink, and especially the two chips that had the figure 8 spring clip on them. During the inspection of the top side of the board, I also noticed that a ground solder near one of the Phillips head screws appeared to have cracked as well. I re-flowed that solder too. See that pic. The soldered portions were then cleaned of any flux residue using alcohol and q-tips.

Once I was confident that all of the solder connections were back in order, I then cleaned the Aluminum Cap of all of the old sealant. The two semiconductor chips that were behind the figure 8 spring clip had heat sink compound on the back sides of the chips and an insulating pad. If you have disrupted that heat sink compound, you will have to reapply more. It is crucial to ensuring heat dissipation.

Once everything was cleaned. I went about re-assembling the PCB back into the Aluminum cap. You have to add a heat resistant sealant between the electrical plugs and the Aluminum Cap on the inside of the cap. I placed a small and very light bead on the electrical plugs before replacing the PCB back into the cap. Be careful here, as you don't want to ruin the integrity of the sealant around the entire plug. Once the PCB is back in, replace the two Phillips head screws.

Now it is time to replace the two spring steel plates and the figure 8 spring. The plates fit in grooves. The lip end is on top. I placed one in at a time and then tapped them down with a small hammer as far as I could go, and then used a screwdriver head to finish the pounding until they seated. Be very careful here, as you don't want to miss and accidentally strike the circuit board with the screwdriver. The figure 8 piece was done with two needle nosed pliers. Be careful here too and take your time.

Once the PCB has been replaced, you can go ahead and replace the module back onto the rest of the system. I replaced the two multi pin plugs first, and then the plug that looks like it is the power wire. Note that the best practice is to remove the negative battery cable before starting electrical work.

After everything is replaced, replace the negative battery cable.

I have driven the vehicle for approximately 500 miles since I did this repair, and I have not had the ABS light come back on, other than at start-up, like it is supposed to. Additionally, I just checked the OBD1 codes for the ABS, and there are none present. All of them are gone now. I think the repair was successful.

So hopefully, someone else may see this and do the same repair and be as successful as I was. Good luck.
 

Attachments

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H2814D

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More Photos.
 

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  • Cracked Ground Solder Joint Near Screw.jpg
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someotherguy

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Here's wishing you continued success with your repair. Those monster K-H 4WAL units in the early model GMT400 SUV's have given a lot of people trouble, but there are those of us willing to dive in with the soldering iron. I'm pretty sure all the rebuild services out there consider that particular unit non-serviceable, as they do with a few other models used in 400's and also 800's. I tore into one on a 2006 Silverado SS that was not fun as the cover is sealed to the body of the unit and I had to cut through all that sealant, then it was a real pain cleaning it from the body afterwards, because it's friggin' plastic!

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As these trucks age, keeping them on the road will require a bit more basic electronics savvy than some may be used to. The wiper motor circuit boards were our introduction to these issues, then later the cruise modules. Now it's time to get comfortable with repairing ABS units. :)

Richard
 

HotWheelsBurban

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This looks like something my cousin could (and should) get into doing. He's a mechanic and a genius with electrical/electronic stuff. He made upgrades for the police SUVs at his job, that last much longer than the ones his coworkers did.
 

H2814D

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Here's wishing you continued success with your repair. Those monster K-H 4WAL units in the early model GMT400 SUV's have given a lot of people trouble, but there are those of us willing to dive in with the soldering iron. I'm pretty sure all the rebuild services out there consider that particular unit non-serviceable, as they do with a few other models used in 400's and also 800's. I tore into one on a 2006 Silverado SS that was not fun as the cover is sealed to the body of the unit and I had to cut through all that sealant, then it was a real pain cleaning it from the body afterwards, because it's friggin' plastic!

You must be registered for see images attach


As these trucks age, keeping them on the road will require a bit more basic electronics savvy than some may be used to. The wiper motor circuit boards were our introduction to these issues, then later the cruise modules. Now it's time to get comfortable with repairing ABS units. :)

Richard
Thank you. I had seen other videos where this was done, but not one like mine. You are right, they don't want you fixing these. The attached picture is proof. Separation of the cap from the rest of the system was fairly easy, in my case, and especially when compared to what you had to go through. The spring steel plates and spring was the hard part for me. Too easy to damage something either by excessive force or a mishap.

I've already been through the windshield wiper module and the cruise control module with the similar solder joint issues as well. I've already done both of those solder repairs with success. The intermittent windshield wiper problem was encountered while the vehicle was still under warranty (I bought the truck new). The dealer only replaced the turn signal/windshield wiper lever. Because I rarely ever used the wipers, the problem was not much of a concern. That is why I was hoping the solder joints on the ABS System was the problem. It appears it was.

Anyway, hopefully this leads someone else to fix it, rather than deleting the system or trying to find a junkyard replacement that will undoubtedly develop the very same issue.
 

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Schurkey

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Re-flowing solder and replacing electrolytic capacitors is about the limit of my electronics ability.

Fortunately, that's about all it's taken to fix or upgrade (sometimes both) numerous automotive and consumer-electronics devices.

I find that a magnifying glass and lots of light helps. I didn't need that crap decades ago. I didn't need bifocals, either.

The GMT400s often need the wiper and cruise modules re-flowed; my truck has had the A/C-heater control on the dash re-flowed twice and needs it again. The display goes blank at random, and usually but not always comes back to life when I press my thumb against it in a certain location.
 

someotherguy

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Re-flowing solder and replacing electrolytic capacitors is about the limit of my electronics ability.

Fortunately, that's about all it's taken to fix or upgrade (sometimes both) numerous automotive and consumer-electronics devices.

I find that a magnifying glass and lots of light helps. I didn't need that crap decades ago. I didn't need bifocals, either.

The GMT400s often need the wiper and cruise modules re-flowed; my truck has had the A/C-heater control on the dash re-flowed twice and needs it again. The display goes blank at random, and usually but not always comes back to life when I press my thumb against it in a certain location.
I'm currently annoyed by the OEM radio in my '06 SS doing the same. Display goes dead randomly, sometimes comes back when I gently mush on it. I'll eventually get around to tearing into it...low on the priority list at the moment.

My dash illumination was going full brightness and the dimmer knob had no effect on it, until I noticed one early morning if I tapped the assembly it would sometimes dim again. Opened up the headlight switch and cleaned the contact paths on the circuit board, removed all the gummy old dielectric grease, sprayed some DeOxit on the board and switch contacts, then applied a little fresh dielectric grease. One of the contact fingers looked a little off so I gently bent it towards the board again. Dimmer's been working like a champ, and a few minutes of work saved me $50-$150 on a new switch depending on what level of "quality" I looked at - aftermarket or OEM. I had a Standard replacement in the box but sent it back because it didn't match, black face instead of dark gray, same with the knobs.

Richard
 
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