DK83

Hiya everyone. Trying to diagnose and hopefully resolve some of the issues I'm having recently.

Still fairly new to track days, but been somewhat consistent this year. I noticed that the car pulls/wiggles under heavy braking zones. At first, I thought it was me being too hard or being bad at modulating the brake pedal. But I've been told by many that the car shouldn't pull to either side even when I slam on the brakes. It doesn't happen all the time, but happens frequently enough for me to be concerned about it.

Obviously at the track, I don't have the capacity to look down on the gauges to check for any signs when braking hard. Had a friend do some hard braking from 50mph-80mph+. I noticed the ABS lights flashing almost every time under hard braking. For what it's worth, the friend has much more experience and is a much better driver than me. He was surprised at how easily the ABS kicked in, even at such low speeds.

On the track, I always ran with the TC on. And almost every single time, usually on the first session, I'd have the "service abs" lights flash on the first few laps. I would pit in and restart the car, the lights would go off and it would be fine. Sometimes, the light would flash up again, but a restart of the car would resolve that.

Starting in April this year, I started running with TC off and the "service abs" light hasn't come on since. However, on my last track day in June, the "service abs" light came on while running with the TC off. I pit in and restarted the car, which turned off the lights. I ran the codes and the following popped up.

C1221H Left front speed sensor?
C1222H Right front speed sensor?
C1243H ABS/AH
C1248H Brake fluid low?

It was my first time getting c1221, c1222, and c1248. Upon inspecting the car last night, I noticed the LF speed sensor a bit loose from the zip ties. I'm running spindle ducts so I lost the OEM clip that it used to have. I tied it back up securely again. Also inspected the speed sensor plugs from chassis and on the hub, cleaned them and reinstalled. The brake fluid was right in the middle of the "low" and "full" marker. One thing to note is that I've had about 30% meat left on the front pads and it was my first time running the pads that low as I never had a full season done on the car. It was a two day event, so I just replaced the front pads to fresh ones for the next day.

I went out to test drive last night in "competition mode" just because and after my second hard braking "service abs" light came on and c1243h code popped again.

I tested the car last night and here were my observations.
-The ABS does seem overly sensitive even when I'm not slamming on the brakes at any speeds, low or high
-But some times, the car does stop without abs kicking and without wiggling/pulling to either side.
-When the car pulls/wiggles, it's usually to the right side.
-The car pulls/wiggles sometimes even when the ABS light doesnt flash.

Here's the car setup right now
-NT01 tires square
-ap racing 4 pots front and rear
-ds1.11 or ds 2500 pads
-brake fluid was fresh with ap600 in February, just flushed to SRF yesterday
-delrin bushings were replaced last year
-ball joints and end links were replaced last year
-already had absfixer.com repair done
-aleady tried the jack up abs "exercise" last year with no difference.
-all four hubs were replaced to skf racing last year

Spoke to Essex Parts regarding this and they suspect a suspension component. I usually run 1.11 pads and swapped to 2500s last night to see if there's any difference and there was non. Well, the brakes were ALOT quieter than running 1.11s. *lol*

I plan on doing some more test driving tonight when the traffic is very low. I'd appreciate any suggestions and thanks for reading my long post!

davidfarmer

the 1243 is a BPMV Pump Motor issue, which I've never seen. A quick search tells me it's bad though, and it has to be replaced.

With balanced brake pads, your ABS should kick in regularly, but smoothly and you should generally not even notice it. Perhaps this motor has been slowly failing, and has been causing your problems.

Again, I haven't seen this particular issue, just sharing what I could find quickly.

DK83

From what I've read on the forums and online, c1243 is the abs pump stalling due to lack of use. This code typically comes up when I drive on track with tc and ah turned on. It randomly pops the code when street driving.

sccaGT1racer

The 1243 code means the pump motor stalled/isn't running. On a c5 corvette the front to rear bias control is in the abs module. When there are active codes there is no bias control.

kdm123

I don't have any ABS failure lights like you, but I also get the squirlyness under braking---see video link.

My understanding is that on our cars there is no proportioning valve, and that the computer reduces brake pressure to both rears when it detects near lock up. I think what's happening is that with this type of system, as opposed to a traditional proportioning valve, you do get just a little bit of lock up initially, and that cause the squirlyness. It doesn't feel unstable when I drive, just a little squirly at first.

But perhaps there is another explanation?

Bill Dearborn

The two front wheel speed sensor codes should have also given you a service active handling message and service traction control messages as well.

From the Factory Service Manual here is some information about the test generates these codes when it fails:

DTC C1221-C1235
Circuit Description
As the wheel spins, the wheel speed sensor produces an AC signal. The electronic brake control module (EBCM) uses the frequency of the AC signal to calculate the wheel speed.

Conditions for Running the DTC
C1221 through C1228
• DTCs C1232 through C1235 are not set.
• The brake pedal is not pressed.
• The ABS is not active.

C1232 through C1235
The ignition is ON.

Conditions for Setting the DTC
C1221 through C1224
All of the following conditions exists for 2.5 seconds:
• The suspect wheel speed equals zero.
• The other wheel speeds are greater than 8 km/h (5 mph).
• The other wheel speeds are within 11 km/h (7 mph) of each other.

Action Taken When the DTC Sets
If equipped, the following actions occur:
• The EBCM disables the ABS/TCS/VSES for the duration of the ignition cycle.
• The DRP does not function optimally.
• The ABS indicator turns ON.
• The Traction Control and Active Handling indicator turns ON.
• The DIC displays the following messages:
- Service ABS
- Service Traction System
- Service Active Handling

This is usually an indication of a loose wheel hub which contains the wheel speed sensor. Check both of your front wheel hubs to see if they are loose or have some roughness to them as you turn them by hand (to turn by hand remove brake pads and maybe the rotor so you can feel just the hub itself. It is better to remove the hub and turn it in your hands but that is more work).

From the FSM:
DTC C1242 or C1243
Circuit Description
The system relay is energized when the ignition is ON. The system relay supplies voltage to the solenoid valves and the pump motor. This voltage is referred to as the system voltage.

The electronic brake control module (EBCM) controls each solenoid valve by grounding the solenoid.
The EBCM controls the pump motor by grounding the control circuit. The pump serves 2 purposes:
• Transfers brake fluid from the brake calipers to the master cylinder reservoir during pressure decrease events.
• Transfers brake fluid from the master cylinder reservoir to the brake calipers during pressure increase events.

Conditions for Running the DTC
C1242
• The system voltage is greater than 8.0 volts.
• The system relay is ON.
• The pump motor is commanded OFF.

C1243
• The pump motor is ON for at least 0.3 seconds.
• The system relay is ON.

Conditions for Setting the DTC
C1242
The voltage across the pump motor is between 1.7 - 10.2 volts for 2 seconds.

C1243
The pump motor is stalled or turning slowly.

Action Taken When the DTC Sets
If equipped, the following actions occur:
• The EBCM disables the ABS/TCS/VSES for the duration of the ignition cycle.
• The DRP does not function optimally.
• The ABS indicator turns ON.
• The Traction Control and Active Handling indicator turns ON.
• The DIC displays the following messages:
- Service ABS
- Service Traction System
- Service Active Handling

If the pump motor is stalled or turning slowly that means the motor is pulling excessive current through the relay in the EBCM. At some point the relay will fail and you will get a C1214 code telling you to replace the EBCM. There are some shops that can repair C1214 by replacing the relay. That is the only feasible repair on the EBCM. There is no repair of the BPMV. It has to be replaced.

One of the reasons for the motor running slowly or being stalled could be because the pump motor windings are shorting to ground. The resistance between the windings and ground has to be infinite so if you can read any resistance that is less than infinity the motor is shot. It could also be due to some loose/corroded connections to the pump motor through the EBCM or to ground.

Figure you may be in for new EBCM and BPMV along with two hubs.

DTC C1248
Circuit Description
The dynamic rear proportioning (DRP) is a control system that replaces the hydraulic proportioning function of the mechanical proportioning valve in the base brake system. The DRP control system is part of the operating software in the EBCM. The DRP uses active control with the existing ABS in order to regulate the vehicle's rear brake pressure.

Conditions for Running the DTC
One or more faults have been detected by the EBCM in the ABS/TCS systems.

Conditions for Setting the DTC
One of the following conditions exits:
• DTC C1236 sets and ignition voltage is less than 8.5 volts.
• DTC C1254 or C1255 sets.
• DTCs C1265 and C1267 set.
• Two wheel speed sensor DTCs on the same axle set.

Action Taken When the DTC Sets
• The EBCM disables the DRP for the duration of the ignition cycle.
• The red Brake warning indicator turns ON.

It looks like you have this code because you have the two wheel speed sensor codes.

Of course the first thing you should do is to go over all voltage and ground connections for both the EBCM and the BPMV. The EBCM circuit ground is at G108 which is located on the frame below the battery near G104. The BPMV ground is on the left front frame rail right near the alternator. Also check battery cable ground connection to the frame and battery. Check the positive battery cable for good connections to the battery and the underhood electrical center. Then make sure the battery is in good shape and providing a proper voltage.

Bill

DK83

Nice video!

I believe starting with 02 z06's, some sort of brake proportioning valves were added. I see how much movement there is with your steering wheel when you're braking hard. I don't think mine is that bad.

DK83

Thank you for such a detailed reply.

When i have the flashing lights of "service abs", "service traction control, and "service active handling" I pit in and restart the car, and all the lights go off. I honestly stopped checking for the codes right then and there because I already know it'll be c1243. When I did run the codes, it was 2 weeks after the track days. That's when I saw c1221, c1222, and c1248.

I could only check 2 out of the 3 bolts for the wheel hub and they were all tight and torqued. I marked them just in case. They did not appear loose and spun smoothly while I had the pads off.

I've checked and cleaned the ground just this past winter and they weren't too bad. I will check on the voltages next.

Bill Dearborn

A little bit of miscommunication. What I meant by loose was the bearing inside the hub. I do realize the hub bolts do come loose at times and should have indicated checking both items. If the bearing is loose the sensor may not work properly either. The two codes on one axle indicating varying sensor readings when you aren't on the brakes is an indicator that maybe both front hubs may have loose bearings. If you have stock hubs in the car that can be a dangerous situation since the stock hubs have been known to come apart letting the attached wheel and brake rotor assembly go where it wants to go.



Bill

DK83

Thanks for clarifying what you meant Bill. I replaced all four hub bearings to SKF racing last year and have about 7 track events on them now.

I was at WGI this past weekend for my first 3 day event. Weird thing is that I did not get a single C1243 code pop during the 3 days I ran with TC and AH on. Being there for the first time, I left everything on. Now as the days progressed and I picked up a little bit of speed, I've had some moments where the car wiggled again during hard breaking and at times pulled me off line while braking for turn 1. Luckily, my instructor also drives a corvette(c6) and noticed the wiggle too. I will go through my video files and see if I can find the laps that I had no braking issues.

I'm not complaining, but no codes came up during my 3 days at the track. This is the first time it's done this. LOL

kdm123

I wonder if other people have noticed the same problem. As I've said, I have the same problem with my car. I have new SKF hubs as well.
I do SLAM on my brakes, and whether that's right or wrong, I'm surprised that the rears are locking before the fronts and making the car squirly. I've had a pro driver in my car, and he noticed the same thing.

dclafleur

The rear on my '01 Z06 squirms under hard braking. When I was last on the track another C5 driver suggested I check my alignment and perhaps run a little more toe-in.

DK83

It certainly feels a lot more worse than it looks. Here's a video clip of what I'm experiencing.

sccaGT1racer

Does it do it with the ah/tc turned off? I was dealing with a customers c5 that had ap brakes and hoosiers pretty much had the same issues with no codes. Turned off the nannies and bingo the car is perfect.
If it is still has issues check the rear toe and bump steer. I measured .300 bump steer in 3 inches of travel on a c5 that I recently setup. That was only 1 wheel.

Bill Dearborn

I never had any issues with brake pull going into that corner with my C5s. What tires were you running? The 01-04 cars have DRP which proportions brake pressure to the rear wheels based several factors including how close to lock up they are. I always thought within limits that DRP might actually apply more rear brake pressure if the car is running sticky tires which with higher weight transfer might make the rear end lift a little thus changing rear toe and making the car a little darty under braking.

Bill

DK83

Yes, it happens even with tc/ah off. It doesn't happen all the time though, but often enough for me to be concerned with. I'm leaning more towards rear toe at this point(need to find my alignment paper) and I'm not too sure about the bump steer.

DK83

I'm running nt-01s right now. And it's not just this corner, this happens randomly at at corner under hard breaking.

Olitho

This is a likely cause. If you have poorly set toe, especially toe-out and a poor corner balance your car will feel very unsettled.

DK83

Found my alignment sheet finally!
The rear toe settings are .09 on both left and right.

mgarfias

Those things don't stay fixed. They can, and do move. Pull the ABS fuses. See if the car does the dance under braking. Do not slam on the brakes, squeeze them until you're braking hard. A C5 with no ABS has a lot of rear brake and will lock up the rears and do fun things if you slam on the brakes.

If the bad behavior changes, then you have a definite ABS issue. If it doesn't change, check the rear toe, and change out the ABS pump. Good luck finding one though, GM wasn't making them a few years ago.