Travis93

What year shorts pin H and A? in 92 and 93 shouldent you short A and G to get into diagnostics?

Aardwolf

A and B are used on OBD-1 to get into the engine diagnostics, those are the two top right pins in the top row.

He mentions this works on OBD-1 car, I can't remember the exate year for OBD-2 going into the car, 1992 perhaps? If I remember correct the ALDL has more pins so you could just check that. H is the brake sense speed input pin. My OBD-1 connector has 12 pins of witch three do not have a wire in them. Hope this helps!

CentralCoaster

OBD1 connector has 12 pins.

Some OBD1 corvettes have OBD2 connectors though, same functionality, but more pins, and more confusion.

Wildride

I can't say specifically what Pins a 1992 or 1993 ALDL use because I don't have the Service Manuals for those years, but A to H should work from 1991 and older. I believe that B is the ECM diagnostic pin and G is the CCM diagnostic pin. Maybe someone with the Service Manuals for those years can confirm that.

As was mentioned earlier the C-4 has three computers; engine-HVAC (PCM/ECM or CCM and the brakes (EBCM). Each has their own diagnostic display procedure. The engine and brake (EBCM) computers use the ALDL while the HVAC uses the HVAC control panel on the dash.

To access most of the Diagnostic Codes for the brake system, with the vehicle stopped ground the H pin to the A pin (using the bent paper clip trick). Turn the ignition on but don't start the engine. In about 3 seconds after the pins are connected the computer will start flashing the code sequence beginning with 12. Each stored code will flash 3 times and after all codes have been displayed 12 will flash again.

12 Diagnostic System Operational

21 RF Wheel Speed Sensor Fault
22 RF Toothed Wheel Frequency Error
25 RF Wheel Speed Sensor Fault
26 RF Toothed Wheel Frequency Error

31 RF Wheel Speed Sensor Fault
32 RF Toothed Wheel Frequency Error
35 RF Wheel Speed Sensor Fault
36 RF Toothed Wheel Frequency Error

71 EBCM Fault
72 Serial Data Link Fault

75 Lateral Accelerometer Fault (Short to B+ or Ground or Open Circuit
76 Lateral Accelerometer Fault (Signal out of range or Incorrect)

Codes 41 through and including 63 can be read only using the Tech-1 or Tech-2.

CentralCoaster

After reading that I'm going to have to go rejet my carb.

Kinkajou

This info seems to good to be true.
I have a 94 and having intermitent brake trouble.
I lost brake pressure on Saturday at a Track day. I t has happened before, I just have to pump a little and get some braking action again.
Then I change a little of brake fluid and the car will work perfectly next day.
I thought that it was boiling fluid, but maybe the computer has something to do with it.
Maybe it is scared of Track days!

Mike_88Z51

You are correct. No ABS in 85. 86 was the first year for ABS in the C4.

The 86-89 ABS system does not store codes. If your ABS controller sees a fault condition, it sets the ABS light in the dash telltale and quits operating. It is an All-Or-Nothing type of setup. The only tool that can scan the early (86-89) C4 ABS is the Kent-Moore J35890 ABS Test Tool, or the use of a 10 MegOhm multimeter and the Kent-Moore C4 ABS pinout box. Things started changing for the better in the 90 C4 ABS.

The early ABS system only releases pressure in the brake lines to counteract wheel skid. It doesn't apply pressure. If the rears are lasting forever (under utilized), I'd guess that there is a problem with sufficient fluid pressure being applied by the seperate master cylinder/booster circuit for the rears, or the ABS valves are jammed in such a way as to be releasing pressure when it shouldn't be doing so.

This is inaccurate info concerning early OBD-1 C4 ABS systems (86-89). There are no codes available via the ALDL for the 86-89 C4 ABS systems. There is no connection between the ALDL and the ABS on the early OBD-1 C4 ABS. The ABS controller was able to store codes starting in 1990-91. NO such capability prior to that. The early ABS system was very simple compared to the later versions. ABS changed significantly starting in 90. All 84-93 and most all 94/95 Vettes are OBD-1, that has nothing to do with the ABS system in use.


Wildride's post was misleading. What he posted does not work for ABS systems on early OBD-1 (86-89) Vettes. Nearly all 84-95 Vettes were OBD-1. The OBD1/2 version isn't significant for determining the capability of ABS used in a given year C4 prior to 96.

By federal law, all cars sold in the US were required to be OBD-2 in 1996. It was ok to start earlier, and some 95 Vettes were reported to have been released with OBD-2. Most 95s and some 94s have the 16-pin OBD-2 type ALDL connector, but most all are OBD-1 systems. The (required) emmissions sticker that states the actual OBD version in use, should be found on the radiator.

Aardwolf

OK, thanks Mike!

Kinkajou, have you checked the booster rod at all? Perhaps it is to long witch leaves pressure in the system, creating extra heat. Having to pump the pedal could be pad taper to. Just some ideas.

Kinkajou

Thanks Aardwolf, I will check that also. It has been a lot of time and problem finds its way out.

C4boy

Sounds like we have twin cars with similar problems at the same time. I bought the Kent Moore tool and it's pointing to a hydraulic problem on my '86. I'm wondering if the rear solenoid is restricting flow. That would also explain the low pressure to the rears when I flushed the fluid a while back.

Mike, actually I came here looking for your screen name. I'm going to drop you another e-mail if you don't mind.

Danspeed1

I've been out of automotive school for a few years now and I feel like all this ABS stuff is a hazy. Goes to show how little attention OBDI got when I was in school.

As far as the above statement and what I know about OBDII, I would agree... Resetting the codes on most systems as far as I know does nothing more than erase stored codes. The problem is not solved the only thing that changes is the computer forgets that it has the problem. Normally after driving the vehicle a few miles or so, the computer will rethrow the code anyway, but all the while the MIL light was off the problem still existed.

For the record, the brakes on my 89 SUCK! I've driven box trucks that stop faster and give you more confidence. Between my 96 Impala and the vette, I think its safe to say the Impala stops like an Indy car next to the vette. Something needs to be done.

DG

Mike_88Z51

The early ABS system only releases pressure at the wheels, it doesn't increase it. I'm guessing that the valve or solenoid is allowing brake fluid to be redirected away fom the rear circuit to an extent that insufficient pressure is applied by the rear brakes at all times. Could be the ABS relay for the rear circuit or the ABS solenoid/valve itself. Most probably the valve. It might also be a problem with the master cylinder rear circuit.

C4boy

Mike, to release pressure to a caliper (and prevent a wheel from locking up) doesn't the solenoid in the module restrict fluid flow as needed? Unless I'm misunderstanding the ABS system, I thought that's how it worked. One guess was, the rear solenoid is stuck in the restrict mode which is reducing the fluid pressure to the rear calipers.

Da Mail Man

...has anyone really drawn any solutions for "crappy brakes"?....geez, i hate to re-post my situation but, here it goes;

1988 vette,
new rotors,
stock pads,
multiple flushes ,
multiple bleeds,
vacuum booster check valve fine and has been replaced,
m/c replaced,
booster is holding vacuum,
no codes whatsoever,
abs initializes test sequence at +/-4mph,
all 4 flex lines at the wheels have been replaced with stainles steel,
no known or locked up or frozen calipers

***AND STILL, i have fred flintstone brakes!....

kowalski340

I agree my 89 brakes eat sh*t and I have the J55 big brake option!!!!!!!! the pedal inspires no confidence and I replaced everything you can replace!!

Da Mail Man

...upgrading for me is NOT an option as i simply don't want to! the design of the brake system SHOULD be able to abruptly stop my car. however, it doesn't as compared to every other car i have driven in the past 10 years including my totalled 81 vette which, if you applied the brakes, would send you through the windshield (had no abs).....

kowalski340

it's wierd isn't it?

engle1147

I hear you.....I thought my brakes sucked from day #1 on my '89.....then one day the booster went out.....then my brakes sucked for real. I've tried new OEM everything & the brakes still suck even with a strong engine vacuum. Anybody run an extra vacuum reservoir?

Da Mail Man

..i gave some advice about a year ago about making one to another forum member...you take a length of schedual 40 (or 80) pvc pipe, maybe 3'' diameter or better....get a couple of end caps for each and some pvc glue and some good epoxy....drill a hole into the pipe or one of the ends and force-fit and screw in a fitting that you can connect a large vacuum line to. add some epoxy (like jb weld) to the hole and screw in the fitting and let harden...then, glue on/fasten the two end caps to the pipe...now you have a cylinder with a vacuum port on it....get a vacuum "t" and "t" it into your brake booster vacuum line.

...in some cars running radical cams, vacuum is low and requires additional "storage" for vacuum and this may work.....the guy i gave this too had fred flintstone brakes like me and so many others however, it made little to no difference at all......this is for test purposes only and if it were permanent, caution should be exercised to shield the pvc from heat for obvious reasons....

Mike_88Z51

You may be correct. I don't have any training or experience working with the ABS modulator valve internals. My 88 FSM says that the Modulator valve consists of 3 solenoid valves, 2 accumulator chambers, and the return pump. There is one solenoid valve for each front wheel brake line circuit and one for the rear circuit pair. It is my understanding that the modulator valve lets fluid into the accumulator chambers in order to lessen the pressure in the brake line and reduce brake line pressure. In this way it can only LESSEN pressure and not increase it. (1988 FSM Page 5E-2) It might be possible that the ABS modulator valve in some way is able to restrict brake fluid to the rear brakes, but my understanding is that it does not do so by cutting off fluid directly. It just redirects fluid away via the solenoid valves which allow flow into the accumulator chambers, lessening pressure in the circuit overall. If the rear brake line solenoid valve or its relay is mal-functioning, it might be stuck open, allowing fluid into the accumulator chamber(s) and directly decreasing pressure from the master cylinder for the rear brakes. The same is also true for the fronts. This could be the cause of the 'Fred Flinstone Syndrome' some have reported. The FSM says the entire modulator valve assembly must be replaced as a unit if any part malfunctions.

The 88 FSM has a chart on page 5-3 outlining the possible causes for braking problems. There is no listing for "Fred Flinstone braking' but there are listings for 'Excessive Brake Pedal Effort'. The 2 primary causes listed are Leaking Vacuum System and Glazed Brake Lining. Also listed are: Restricted Brake Fluid Passage, Brake Pedal Linkage Interference or Binding, Restricted Air Passage in Power Head, Damaged Power Head.