Greetings,
I just bought an 89 C4 convertible to restore with my son. It's got quite a lot of work to do, and I'm working though most, but wanted to ask for advice before diving in deeper to the ABS issue.
When I bought it, I discovered the ABS fuse was removed (assumed there was some problem). I put it back in, and expected to see ABS light, but when starting car, neither of the ABS lights come on (which they should for a few seconds to indicate it's functioning...or stay on if it's an issue...but I got nothing). I guess it's possible those bulbs are out or have been removed, but all the other lights work except the check engine spot..which may have been removed too. I'll pull the front panel and check the bulbs today. I also checked the ABS unit behind drivers seat (unique position in convertible I guess), and it looks in good shape, all connected.
Any ideas what else to check or experience with potential root causes? Thanks!

 

First thing, I don't just "not trust" the Previous Owner. I assume they either lie or are stupid and have done something stupid on the car. I ALWAYS suggest a PDI before money changes hands. No ands, buts or ifs. Either that or I am familiar enough to tech out the car and ensure you haven't done anything. I bring my scanners and compression tester. We don't agree to those terms, I'm not talking any further. Too many "OMG!! I didn't know." to make me believe anyone is telling the truth. I really suggest you get it teched out by a mechanic so you know what you are into.

I suggest you buy a scanner and get the compression and fuel pressure tested out to begin with. Scanner will tell you what the ECM sees and not what the bulb blinks. For all we know, he might have pulled the ABS unit.

https://www.corvetteactioncenter.com...abs/index.html

 

If you don't have a FSM, get one as it will make your life a whole lot easier.

ABS on this era takes a special tester which is next to impossible to find. If it was mine, I'd check for the bulb being present then start in the well and insure that you're getting power to the unit. Next is the relays (which was my problem). If you have a relay problem, at least the light will come on.

The fact you're not getting any light at all leads me first to these 2 items (power and bulb). Without having the right tester it will not be straightforward to diagnose and test but some things can be tested or checked without it.

Remember though AFAIK even if the ABS is out, you will still have at least your normal brakes baring having the pump itself plugged or stuck.

 

I have a 1988 Coupe with the BOSCH ABS system in my car. There are so many ways for it to fail it is scary. Our 1988 was sketchy for a while and then I found my Lateral Motion Sensor was swinging around on it's harness.

How it is "Supposed" to work is explained in the FSM but like a previous poster pointed out the testing devices are no longer available.
This is strange because they were made for virtually every Corvette dealer but seem to have vanished completely.

If you decide to drive the car permanently without the ABS I would switch it back to thew old fashioned Power Brakes mode. Classic Tubing sells them either way in steel or Stainless. I am retaining the ABS system and trying to get it to work properly.

My Master Cylinder (1988) has two outlets, Both go to the ABS box behind the drivers seat. From there it comes out three ways, Left Front, Right Front and Rear Brakes. From what I have read there is No Pump in the ABS system as we have it, It simply uses two solenoids and monitors the individual wheel speeds. It is a very crude version of ABS at it's best. When working it functions well and is totally in the background.

On my car I found a place where the brake tubing coming out of the ABS unit looked like "ate" an acorn. Apparently somebody did not flush the brakes out properly. I also popped a brake like while pushing down the Brake pedal hard. The car sat for a couple years and the ABS lines coming out of the ABS unit box is right where they rusted out. I ordered a whole set out of steel and am currently replacing them, every inch. Thoroughly check every inch of the brake lines of your car and Flush the Fluid at least every 5 years at minimum.

The individual Wheel Speed Sensors are a very common cause of ABS light issues. If one stops then you get a light. I am fortunate that mine all work (as of now). Get the manual and read the "Brake" section.

Good Luck with your Corvette! The ABS issue is likely something simple and easy to fix.

By the Way, Welcome to the Corvette Forum!! This is the "Best Place" on the Internet regarding Corvettes! Post your Questions and Post your Solutions so We All Learn!

 

Not strange to me. You are driving a car older than some of the mechanics. It is an ever shrinking pool of customers, some of whom are too cheap to fix it which reduces the number of actual people wanting that job. Shops might assign C4 jobs to one or two "specialists", aka guys who drew the short straw to fix these cars. In light of that, I suspect that some shops don't want to fix it so "no tools" is a good way to decline it. Also some employees might have stolen it or misplaced it so it is an incomplete set.

I wouldn't drive it without ABS since I prefer to keep one standard set of braking techniques and not have to remember in an emergency stop situation whether to pulse the brakes or stand on them but that's me.

Not sure why people will argue about which oil is better for their car but it seems like brakes are a "lifetime" deal. Seems like the european cars like to advocate 2 year flushes while this side seems to try sell the car with the most corners cut on maintenance.

They are also made of Unobtanium so if you see a good one and you intend to keep this car for long, I'd get it. Some I have seen look in worse condition than mine. I talked to a shop about removing them for cleaning and they are afraid of that job because it something breaks, they won't be able to get a replacement part and have to scour junkyards for something that MIGHT work

 

Here's some info I found on relays and some other info while researching my ABS issue

ABS Relays

Update: Went back and started looking up some of the models
Module Relay 0332002171: BMW E30 ABS Anti Lock Brake Pump Relay 318is 318ic 325i 325is 325ic 325ix
Mercedes W126 W123 W107 W124 W201 relay 0332002171
0 332 002 171 | PORSCHE / AUDI / VW OEM ABS PUMP RELAY
Solenoid Relay 0332205003; BMW E30 Mercedes Corvette ABS Pump Breaking System Relay Bosch 0332205003
NEW Bosch Relay (ABS) 0332205003 BMW 34511154919
ABS SYSTEM RELAIS VOLVO 740 - 760 - 780 - 940 - 960 / BOSCH 0332205003

Possible alternative for 0332512006: 0332512007. That one has a skirt that would have to be trimmed, but first look appears it may be the same relay in a different housing.

I was just going against the Bosch numbers on them I wrote down from mine:
Module Relay: 0332512006 - Looks like there's another possible but would have to do a little trimming on it.
Pump Motor Relay: 0332002171
Solenoid Relay: 0332205003

Some info from another thread I found
Possible substitute for Kent Moore J-35890?
________________________________________
After doing some reading about the Bosch ABS 2 system it seems the basic systems are the same with the only thing different being resistance values of wheel speed sensors. I remember a older post on this forum stating that different late model Porche and Cadillac wheel sensors working on earlier C4's and a Google search brought up a gentleman who had success analyzing his Corvette ABS with a old LED 2 tool from Porsche. I decided to take a small gamble and purchace a OTC Mitusubishi ABS Tester MB991131-02. It looks identical to the Kent Moore tool down to the 35 pin connector. I figure at best it works completely or enough to test everything but the wheel sensors and if not I'm not out that much cash. Maybe this will be a lower cost alternative to the Kent Moore tool. I'll attach some pics of both
Attached Images


Additionally I found what looked like abs cables from a Porsche that looked similar to the ones on the Vette. Don't know the numbers or whether they will actually work or not. You'd have to work through whether they are correct and workable or not.

All I can pass along.

 

Thanks for the info, I ended up getting into it this weekend and found just that... bulb was removed and fuse was removed...so I assumed there was some larger problem they were masking, but it oddly enough, it seemed to all work then. I heard it do it's self check at the 3-5mph, and then stomped the brakes when wet (only on my driveway...it's long) and the abs did kick in..heard the motor run etc. I had to do it a few times, but started feeling a bit more normal. The guy I bought it from says he didn't know about this, but didn't drive much in 7 years... so I guess it had to get the fluid flowing into the module. Thanks!

 

Thanks so much for the info! Glad it was an easy fix this time... (bulb and fuse removed...but when I re-installed, it seemed to work ok). Maybe they were drifting..?

 

Thank you all for the great advice and guidance! I did acquire this car via unique circumstances and price, so I expected a project and am OK with. Another odd thing when I had the diagnostic light panel off...they had used electrical tape to block off the check engine light from showing. I removed, and did the jumping thing on the OBD connector and it actually blinked the 1, 2, or 12 which means no problem.. now that light functions correctly too. I found a spark plug wire off...so it was possibly a light for that reason...but really odd someone wouldn't check and just cover up.
Anyways, I do have a FSM on the way, and will flush/fill brake fluid as well as replace pads (and possibly discs) soon to try to make sure brakes are at their best.
Thanks again for all the help!

 

People refer to the Previous Owner as PO. I refer to them as liars and morons. So if someone is selling me a car, I bring my SCANNER and compression tester as a minimum. I probably would like to have it PROFESSIONALLY checked over, aka Pre Delivery Inspection or PDI. Why? Because the seller is motivated to lie and I don't trust him after being burned several times.

NO IT DOES NOT. It means the primitive system cannot see anything falling out of the plausible value range. I would pull the battery to clear any codes and you can check and see.

I wouldn't make that assumption.

Let me explain. More problems, less money at a sale. Now why would you think I wouldn't try to hide problems again?

I would also replace the rubber lines and check the steel lines for corrosion.

I have a scanner which is something you WILL need. You will find that the scanner will tell you what the ECM is seeing. The codes are kinda worthless when compared to data. Codes are only generated when a sensor falls outside the plausible value from what I see. FOR EXAMPLE if you are at 80 degrees outside but the coolant sensor says 50, it is plausible and probably would not trip a code. The OBD1 stuff is better than nothing but not too much good either.

 

Thanks for the nice words,

One item I would strongly suggest is a set of steel braided brake hoses that attach from the car to the caliper. I bought a set from Classic Tubing but they are easily found in several colors even. The rubber brake hoses are too old to trust. Zip products has them in stock.

I bought 4 Drilled/Slotted brake rotors from ROCKAUTO.com They are beautiful and inexpensive and enhance your braking ability. I got all four for less than $200. These hoses make a HUGE difference and help with safety. Rockauto is a good company (in my humble opinion) just be careful, when the price is TOO good there may be a reason. They do stock the calipers if you don't feel like rebuilding four calipers, just stick with the AC Delco parts when possible.

When I pressed the brake pedal down purposely hard and the line popped ALL my brake fluid leaked out of the master cylinder. I was under the impression that the idea of dual reservoirs was so you would still have half of the brakes left. I am one of those crazy guys who want as much stopping power as "going" power. If my Brakes are not Perfect then I don't drive the car let alone my wife or kids.

Take your time working on your project, it is great thing for a young guy to learn. I have a 19 year old daughter who has been helping restore the 1988 so SHE can drive it. She is an Aerospace Engineering student and understands cars better than some men I know do. She wants to know everything, questions are non-stop and she is learning volumes. She installed a new set of spark plugs and wires recently and did a beautiful job, very detailed. We bought all the replaceable parts for the digital dashboard and others from Bryan at BATEE.com and rebuilt the digital dashboard and the C68 AC controller together. Great experience for these young people to see how things work. Knowing how to solder properly is another MUST in my book.

When driving a car with all the fancy items like ABS, anti-roll back (so you can just stop on a hill without using the E-brake) our kids don't learn all we did in our day. I take my kids to a huge parking lot and show them how to drive a car without ABS or anything. Technology may be great but it is helping society raise a generation of people who have no idea how a car does what it does.

Good Luck with your cool Corvette project, just think of all the smiles that are coming your way cruising in the Corvette!

 

Thanks ctmccloskey!.. I'm looking forward to it all working too
ABS light came on again, so I'll need to dig a little deeper, but now that I have FSM, lot's of places to check.
Thanks for the advice on the braided brake hoses, will get when I get to the brakes (I'm not driving much yet, so tuning engine is still priority)

 

https://www.corvetteactioncenter.com/tech/c4/abs/index.html

Get the lines fixed bleed the brakes so you can see what gives.

 

Sounds like you may have a relay issue like I did. Here's how I tracked it down: open the well and take a plastic handled screwdriver and rap the 2 relays on the top of the abs pump a couple times. Now try starting the car and see if the light is out or not and the abs pump cycles or not. If it is good then it's one of the relays that's sticking at times, likely the silver one as that seems to be the one that energizes the unit from what I can figure out. Just because it works once, they will stick again later so plan on replacing them (I did both at the same time).
Understand the same relays were used in a lot of cars (see my earlier post) so that offers a better source but look by the relay number. I couldn't get them via the Chevy dealer as they're no longer stocked from what I was told so I went to Ebay. Look by the model numbers on the relays (Mine had Bosch relays for both of them), not the car. I found Vette relays for $70-$100 ea. yet the same relays I found (one for a Porsche and the other for a MB) I got for something like $7 for one and $15 for the other IIRC, same relays just widely different prices. They're plug and play - unscrew the holddown strap, unplug the old and plug in the new and put the strap back on and you're done.

I can't guarantee it's the relays, but similar to what I had. They'd work for a while then they didn't. Rapped them and they worked again.

Good luck, hope that helps.

 

AKLIM,
That is a great article you posted the link to for the ABS information. I thank you for that connection. That is some of the best information I have seen in print. I had spoken to BOSCH and they told me that the parts were used for several years in Europe on several European cars.

DrewDude,
While you might be fairly new to Corvettes and getting their engine to idle right it is critical to follow the procedure in the FSM. Mid America sold a kit with several testing rigs for setting up or testing the Fuel Injection system. The testing rigs plug in between the actual device and the wiring connector. Using these rigs makes setting up the fuel system a breeze. Here is what I am talking about:

"Emissions Sensor Test Harness Tools Combo Kit 1985-1989
This set of six, 1985-1989 Corvette Specialty Electrical Tools include:

Throttle Position Sensor
Manifold Air Temperature (MAT) Sensor
Oxygen Sensor
Mass Air Flow (MAF) Sensor
Idle Air Control (IAC) Sensor
Coolant Temp and Knock Sensor


Use these tools with any Digital Volt/Ohm Meter. Your Corvette will idle smoother, enjoy increased horsepower, and gas mileage." (Copied from Mid America Website)

Cleanliness IS NEXT to Godliness when playing with your throttle body. Be sure to get it all cleaned and carbon free. Just be careful taking it apart, I broke the screws off the part that had coolant going through it and had to buy another throttle body. Work slow and methodically and soak any resistant screw or bolt with KROIL OIL (get it at Eastwood.com) as this oil is my all time favorite for getting parts loose.

Test your EGR system when you get a chance, it can fail and screw up your idle. Mine froze in the open mode and I found carbon everywhere inside my intake manifold. The EGR has a solenoid mounted near the thermostat housing. This solenoid tells it when to operate and if the EGR is not getting the needed vacuum then you have problems.

When I bought my 1988 (used) it had two serious vacuum leaks that kept the idle from smoothing out properly. The engine seemed like it was hunting for the right spot. One of the two leaks was at the EGR solenoid and the second was near the Cruise Control servo.

After getting the Throttle body cleaned up and the EGR working the car became itself again. Fuel regulator and Fuel pressure can require help sometimes. It could be as simple as a really dirty fuel filter....

If your motor still needs more help you might try a new HEI cap if your car uses one. But this time buy the aftermarket units like the one from DUI (Davis Unified Ignition) not the bad kind of DUI. Their coil put out a very hot spark so when you buy the cap just upgrade to the higher output coil with the cap.

If you have gone real far and it is still not idling correctly the solution might be cleaning the inside of the fuel system. Have you ever tried BG-44K Fuel Injection Cleaner? If the car is running you can get their three part cleaning system for Fuel Injection. One can goes into the fuel tank, one sprays the Throttle body and the the last one is drawn into the intake via a vacuum hose or attachment.
This BG product works GREAT, fear not, it is NOT a "Snake Oil" like some claim. It is used in car, RV and Motorcycle dealerships all over this country. You can get a great price on Amazon IF you want to try it.

I am not profiting in any way by suggesting any product, I am simply telling you of my personal REAL experience with these products.
I have been blasted for suggesting KROIL or BG-44K to others on this forum but I use them often and stand behind my endorsement of said products. Now watch 'em scream bloody murder......

Gee, I love this Forum... Have a great day and may your Corvette Idle so smoothly that you have to check the Tach to see if it is running!

 

The FSM makes a lot of assumptions. It assumes you have no air leaks, motor is performing like it shouldm timing is spot on, all your sensors are reporting good data and the tach is correct. For the most part, I agree with the FSM. In this case, get a real scanner, infrared thermometer, fuel pressure tester and compression tester (borrow from Autozone).

That is a $100 tool plus shipping. That is about all it does plus your DVOM. I got a SCANNER like THIS for $70 plus shipping. It will do all that and then some. Data is WAY MORE convenient and much more plentiful. It can read and erase codes, read way more parameters than that more expensive bunch of wires.

IF you are going to clean the TB, might as well bypass the TB for easier access next time, and there will be a next time. Only 3 screws you need concern yourself with. The rest don't matter if they all break off. I'd definitely soak them with some sort of penetrant or get it warm.

I'd cap the vacuum nipples one at a time and see if the IAC counts change upwards. IF they do, there might be the issue. If that doesn't work, I'd carefully use ether around the gaskets in a well ventilated place.

I'd be suspicious if a new cap and rotor don't fix it. Probably also change plugs and check plug wires for leaking current. Stock cars haven't shown much effect from hotter spark yet. Once you increase compression, I'd say it might work.

I'd be one of those who recommends passing on those snake oils. Just dump the injectors since it is cheap enough. I really don't recommend having injectors that force me to use gas from a very few stations. If the remans are tested, they are clean. If you are like me, I have had them tested on my own with a injector flow bench before and after.

Assuming your tach is running right, sure.