C4boy

'86 Corvette, OBD-I. I have a few questions relating to the ABS system and hand held diagnostic scan tools. If anyone can answer any of them, I'd appreciate it.

1. For anyone who has a scanner, can you tell me what info the scanner checks and displays when checking the ABS circuit?

2. I've seen mention of an "ABS tester" in the service manual. I'm assuming this is a specific electronic test tool. Anyone know what that sucker does, tests and if it hooks up through the ALDL port?

3. Anybody know of any advanced, early model C4 ABS info available on the web? TIA!

Canam

I would like to know also. I'm pretty sure my ABS isn't working. I've never seen a light even under very hard braking and have never had pedal pulse.

IRAraid

Our cars' years are different so these comments may be off the mark. But here goes.
The ABS system can be read thru the ALDL port but the scanner (depending on type) usually requires an ABS 'plug-in' which is year dependent. AutoZone and Advance Auto may have a suitable scanner and if they do, they don't charge for the scan. The local Advance Auto could read my 95 Corvette (ODB I) but could not read nor reset my 96 Blazer with an ABS code set. It cost me $50 at a local garage to read and reset.
If you want to check out your ABS, find a curbless road and get one side of the car off onto loose gravel or grass and stomp on the brake. If it skids with no pulse, your ABS isn't working (but there should be a light to tell you that unless the bulb is burned out). If it's working, you should feel the pulse and have minimal skidding. It feels really strange.
Hope this helps.

C4boy

Thanks Ira. I checked Autozone this afternoon, but all they had was an OBDII scanner for rental. I was curious if anyone here ever used one with ABS testing before. I think the scanner can report back on valve & sensor status, but I obviously don't know first hand. I'll have to try my mechanic on that one.

My Anti-lock light comes on now (since I replaced the freekin bulb!) and my pedal doesn't pulsate. So I know there's a problem, it's just a matter of finding out where it is.

pcolt94

That is a good test for testing the ABS on dirt or gravel. In order to feel the pulsing of the pedal, you have to hold down the pedal firmly or you might not feel it. You can’t let up when the wheel locks. You only have to do 10 mph or so. My ABS works great on gravel, the pump runs and pedal pulses. But my brakes are bad so it won’t activate on payment. Maybe I can get closer to my brake problem this weekend.

Does your self test work OK which is the pump running with the ratcheting sound for a second when you reach 4 mph. If this works and there are no error ABS lights, I’m not sure how much you would gain from a scanner. The scanner will only look for error codes.

A TECH 1 or ABS test tool can activate the pump and valves and probably some other functions.

If your ABS is truly defective, I would start out following the diagnostic charts in the Helms books for your model.

AGENT 86

I was under the impression that the 86 ABS is not linked to the ALDL, so a hand held scanner is useless.
Here is some info for those having a problem with ABS (86)
http://members.shaw.ca/agent86/ABS.pdf
http://members.shaw.ca/agent86/ABS%20Electrical.pdf

pcolt94

Thanks for the correction and help 86. I am more familar with the 92-94 vintage types. You should take here, you are more familar with these models. Just trying to give all I got.

C4boy

My ABS light comes ON after driving, as mentioned above. I've already gone through a few of the service manual test procedures and found a couple problems. Yes, I hear the pump cycle when starting up the car. No, I don't feel the typical ABS pedal pulsation when braking.

Agent, I found your links a few days ago (from another post) and printed them, as well as my service info. Yes, I believe you are right in stating the ALDL isn't used for ABS troubleshooting. I think the way it works is: With some scan tools, you can connect an optional harness to them (or whatever J35890-20 is) and read data from the ABS control module. Such as diode, speed sensors, modulator valves, lat. accel switch, etc. I was hoping someone had done this before as it would be a quicker, more detailed way to check several components at once than what the shop manual provides.

SunCr

Only place I know to troubleshoot it is through a Dealer - but we've had plenty of posts where the Dealer no longer has the Tester. I'm pretty sure ABS was unique to your Vette and no other model used it, so this equipment is really hard to come by. I believe the wheel sensors can be checked with a DVM, but if you've ruled them out, you're better bet may be to replace the Module. Unfortunately, I don't remember that being real easy to come by either.

edcmat-l1

You hit on it in your first sentance. If it comes on while driving its most likely a speed sensor. Id check them right at the wheels. Specs are 900-1500 Ohms. You'll probably find one "open". That is, no continuity at all. If you dont find any thing there repost for more help.
PS You may also want to make sur the tone rings are clear of any grease, grime, etc. And the magnetic end of the sensors are clear of any metal dust. Both of these can skew the speed readings the sensors send to the computer. Neither will have an effect on the resistance of the sensor itself

C4boy

Ed, thanks for the info. I finished the entire test process for the modulator, harness & 3 relays. So far everything looks good, though there is a little fluctuation in the modulator continuity.

I disconnected all 4 wheel sensors at their connection plugs ( I didn't pull them from the hub). Resistance is within spec from the test point to the hub. I can't test the wheel sensors at the control module since I don't have the pin-out box, and the connector is neither numbered nor are the wires visible to each pin. I haven't pulled the lateral acceleration switch yet - will do that Monday.

Sun, thanks as well. I believe the Bosch unit in the early C4's was also used across lines for different makes. Since the connector used with a test module is $169, I think I'm gonna pass on it right now. lol I've never taken a car to the dealer before, but I'm pretty sure they'd get me good for diagnosing this one!

Mike_88Z51

AGENT 86 is correct, the 86-89 ABS systems DO NOT interface with the ECM and do not set codes like the 90+ Vettes. They use a simple Fault/NoFault condition to decide if the ABS is to stay active or not. There was only one controller tester made for the 86-89 ABS controller and that was the a Kent-Moore J35890 ABS Tester. It will test the controller circuitry and the relays at the same time.

If your system passes the initial low speed check, and comes on while driving, I agree with edcmat-l1 that it's most likely a speed sensor. Drawing on my own experience and info from a Corvette Fever article by Gordon Killebrew there are several checks that can be done.

1. Pull each sensor and clean with WD40. The sensors are essentially powerful magnets and they will accumulate a lot of road dust and debris which affects the sensor readings. The sensors are screwed into each wheel knuckle with a 10mm screw. After you clean them, use a little anitseize on the sides to keep the steel sensor from welding itself to the aluminum knuckle.

Static Test #1
2. Using a digital volt/ohm meter (DVOM) with a 10 megohm reisitance, disconnect and check the resistance across the two pins on each of the wheel sensors. They should read 900 to 1500 Ohms. They should also all be within 100 ohms of each other. If not in range, clean with WD40. If still not in range, replace the sensor. You can pull the sensor connector where it connects to the main wire for each sensor going into the main harness, or you can disconnect the 8 pin connector in the ABS compartment on the drivers side under the plastic tray. If you use the connector behind the seat you need to get the correct pair of wires for each sensor. Check the ABS wiring diagram for you year car in the manual. For my 88 the wire colors are
LF - yellow & blue/white stripe
RF - Green/white & brown/white
LR - red & black
RR - brown & white.

Dynamic Test #1
VERY IMPORTANT: Disconnect the main harness that connects directly to the ABS Controller first.
Note: this test is essentially the same as what the Kent-Moore ABS tester tests in the controller, we aren't testing the relays at the same time like the K-M tool.

1. Next set the DVOM to AC volts and you will now have to go to the ABS 8-pin connector in the ABS compartment on the drivers side under the plastic tray (unless you have or can borrow a Kent-Moore J35592 Pinout Box). Again, for my 88 the wire colors are
LF - yellow & blue/white stripe, RF - Green/white & brown/white,
LR - red & black, RR - brown & white.

You will need someone to make AC voltage readings for each wheel sensor while you drive at 15 MPH. The left front should read .65 millivolts and the right front close to the same. The rear sensors should each read 1.1 millivolts. If your reading is high it means the air gap at the toothed gear is too small. A lower reading would mean too much of a gap. If the air gap does not match left to right it could be caused by worn wheel bearings. If the air gap is out of adjustment it will cause the ABS light to come on. I personally called Gordon about adjusting the air gap and he said that it is not really adjustable. To decrease the gap you may need to sand the area where the sensor attaches to the wheel knuckle. To increase the gap you need to put a layer of paper between the sensor and the knuckle. A meer fraction of an inch makes for a major change so DON'T do this unless you are sure it is not due to a loose or bad wheel bearing. You can check with Gordon at 1-800-4YOURCAR. He charges a fee of $10 + $3/minute, so be prepared to be brief or it will cost you. Gordon was a mechanic at GM who helped write the Service Manuals for the C4 vettes.

Personal note:
I have found that the front wheel bearings have a minor amount of play in the way that they mount to the car. Once they are bolted in, they are in solid, but they can be mounted a tiny bit more forward or aft of center. Since the toothed wheel that induces the voltage to the ABS front sensors are on the front wheel bearings, this will allow you to make a minor adjustment in voltage if your reading is slightly too high or too low compared to the other front sensor, by adjusting the wheel bearing. This doesn't work for the rear.


If the Static #1 and Dynamic #1 tests do not pinpoint the problem, you can do the following:

Static #2
1. Check your laterial acceleration switch, remove the right console carpet and locate the switch behind the AC/blower controls (it will have a orange and orange/black wire). Test it by holding it in your hand in the same position as it was mounted in the car and measuring the resistance with an ohmmeter. It should read zero ohms. Then turn it 90 degrees to vertical and check for an open (or infinite reading). If it fails this test replace it. Check the junk yard for one. Nobody usually buys them there because they seldom go bad. So you should be able to get one for about $20.

Dynamic #2
2. Take you car to a dealer or someone who has a Kent-Moore J35890 ABS Tester. It will test the entire system including the relays. I bought a used one for $150 on eBay. They sold new for $500. Make sure that if you take it to a dealer, that they have a working ABS tester. The Kent-Moore tester is the GM Authorized tool.

If the dealer suspects that it is the ABS Controller be sure to get in writing that if replacing the controller doesn't fix the problem that you don't pay for it. They are over $900.00 new from GM.

edcmat-l1

Mike, awsome feedback. I use a scope to test the speed sensors. But then again I use a scope for everything!! C4boy. One quick check you can make to try and narrow it down is simply check your speed when the light comes on. If its a complete open in a wheel speed circuit the light will come on almost immediately after the car hits about 8 MPH. If its delayed quite a bit longer, or comes on at a higher speed, its more likely to be a variance in the speed being reported. Most likely caused by crap in the tone wheel, crap on the magnetic sensor tip, fluctuation in the air gap, etc.

C4boy

Mike, let me first say.....Outstanding info! Thank you very much. It's also ironic that when I googled the J-35592 connector, a CF post of yours for DaMailMan came up.

My Anti-Lock light comes on once I hit 8 mph, then stays on. I checked the lateral acceleration switch today (ok) and pulled all the wheel sensors. The front 2 looked fine, the rears needed cleaning. I put a little anti-seize in the knuckle and popped them back in. The right rear sensor was badly corroded in the knuckle and I broke the ^%$#@!# trying to get it out. I'll have to replace it before I can go any further. All 4 resistance readings at the plugs were between 990-1050.

I believe I've completed every ABS test so far, with everything looking ok. The exception is your AC voltage test and testing the wheel sensors through the harness. The only way I'll be able to do those is if I can find a schematic for the control module harness. I don't have an 8 pin connector at the control module. It's a molded 35 (?) pin connector where 11 connections are used on one bank and 12 are used on the other. It's impossible to tell which color wire terminates at each connector unless I break it open. I guess this is why GM opted for a $170 harness connector to use with a scan tool. If you have any info on where I can get a schematic or pin orientation for the harness, that would be fantastic.

Thanks again.

Mike_88Z51

I don't know where you can get a schematic for the 35 pin connector. I bought the Kent-Moore test unit with the J35592 Pinout Box as part of the deal.

There is a relatively small white plastic connector C242, that has 10 pins and is where the wheel sensor wires connect into the wiring harness to the controller's 35 pin connector. You should be able to test AC from those pins.