ecm bad ?

[89blackvette]

1989 6 speed. Has been running great, today I started it and idle went high to 1000 rpm and car ran like crap. I have a code reader that has always worked great on the car (it is OBD1 & OBD2 reader). Now I can not get codes with the reader and code 12 is only code with shorting connections method. I also noticed that instant mileage display was showing 0 even when driving the car and avarage mileage disply was showing 99. FSM leads me to bad ecm, wht do you guys think. I am planning to sell this car as it seems there is always sonething to fix on it, I just can not depend on it. Black 1989, 6 speed convertable with removable hardtop, black interior and black convertable top, 2009 ZR1 style 18 inch wheels with Nexon 3000 tires and Corsa cat back exhaust 51,000 miles. Thanks
Mike

 

[tmkassin]

Clean all your grounds. I cleaned the fusable links on my 92 also. Might be an easy fix.

 

[jamison]

My 1990 did something similar to this. it drove fine all day and then i went to bowling league which is a mile from my house with no troubles. after league i get ready to leave so i start my car and it starts idling like crap. it kind of sounded like i put a mean cam in it except it would just burn gas like crazy just shooting black from the exhaust. so i drove it home and i could barley touch the gas pedal otherwise it would die right away. after checking the spark plugs and fuel injectors i could not figure out why it was running so rough. i talked a friend that owns his own garage and he told me after so many years of turning on and off the car eventually the ECM will just give out to age. so i bought a rebuilt ECM from autozone for around $100 with exchange of mine and that fixed the problem.

 

[89blackvette]

Could you possibly direct me to the grond connections for the ecm? Thanks guys, I ordered a rebuilt ECM today. I see now why they call cars her and she, guess it is just her time of the month. Again thanks for your help.
Mike

 

[TedC]

89blackvette, Nice job on pinpointing the issue using the FSM. The ECM provides serial data to diagnosic port (ALDL) which you confirmed is not working using your external code reader. Next, the ECM provides serial data to the cluster so it can calculate things like MPG, which you confirmed is not working right and points to the ECM. Just because the serial data component has failed, you can still pull codes using the jumper. Heck, the car still runs. I had the same issue on my 1989. Even the upshift light would come on, and mine is an automatic. I purchased a rebuilt ECM from Advanced Auto for under $90. I opened the case to view how and if it was really rebuilt. All of the electronics were properly cleaned and recoated with electronic sealer. Don't forget you need to transfer your prom chip, so swap first, then return the core. There is a very small chance that the wire used to transfer the serial data is broken before it spilts up to the ALDL and cluster. You could trace this wire once you have the ECM removed. I did not waste my time. ECMs are static sensitive so be careful, even when plugging in your reader to the port. ECM part number should be 1227165. Good luck, and keep us posted.

 

[Hib Halverson]

If you only get code 12, there there is no problems that the ECM's OBD can detect or its possible the ecm is bad, but before I use that new controller, as suggested elsewhere, check all the grounds.

The engine controls ground on an L98 is a giant bolt, with a lot of black wires going to it and is located above and near the oil filter.

Remove the bolt, clean the wire connectors, clean the block where the connectors seat and clean the underneath of the bolt head then reinstall and tighten the bolt.

Next look for ignition problems.

Also, check for a vacuum leak

 

[TedC]

Hib, I can't agree with you on this one. He lost serial output data which the ECM generates. This is a known (common) issue with these ECMs. Odd MPG readings are the first sign. No serial data at ALDL is a good way to confirm. He pulled a code 12 because the ECM thinks everything is fine, and other than the serial data output, it is.

 

[TedC]

What I can't answer is why did the ECM go bad? My suggestion is to start with the known facts which point to no serial data output. Yes, there could be more than one issue, but having the ability to monitor the ECM will help in the diagnostics.

 

[boomdriver]

He may also want to look at the power post and fusable links behind the battery. when/if a wire gets broken down to the last strand of wire or the connection is corroded, it breaks power to ECM or the display. This will not permit any level of control but it will sometimes show a complete circuit or path. I had a battery support break and crush one of the wires underneath, that supplied power to the ECM. It would NOT show a code because that path was broken. Most of the other lites would come on but no ECM control was there.
That power distribution post behind the battery is subject to corrosion in the wires (batt acid fumes, spills) and breakage hidden by the insulation.

A code 12 is the ECM "thumps up" signal. But there is nothing that says a 12 is'nt possible with certain ECM failures. Code 12 with no other displayed calculations as evidence of ECM life is suspecious.

 

[Hib Halverson]

Hib, I can't agree with you on this one. He lost serial output data which the ECM generates. This is a known (common) issue with these ECMs. Odd MPG readings are the first sign. No serial data at ALDL is a good way to confirm. He pulled a code 12 because the ECM thinks everything is fine, and other than the serial data output, it is.

Without a scan tester how do we know the ECM is not sending serial data?

If I missed something about that in an ealier post, I'm sorry.

 

[Schrade]

It's been running great, but there's always something needing fixing? :confused

Someone said to not plug in the new ECM. This was good advice, so you can return it after you test the affected circuits, as I've shown in the following pics, and find the ECM to be good.

(you WILL need FSM pinout ID's for harness connectors to do this; they're at the beginning of Book 1)

I was testing 2 circuits at the same time. Some text explanation is here, starting post 68

This is a bypass test of a circuit from the ECM to fuse block.

Test for continuity between the paper clip, and the socket connector, before doing the test. Insert paper clip carefully yup.

I think your code 12 means nothing more than ECM - to - ALDL interface coms established. You do not know just by this that ECM is good - there could be circuits on the logic board that need butter (since they could be toast). You can confirm this only by doing the circuit test as shown.

Buying a new ECM won't fix a failed circuit OUTside the ECM NOPE.

Questions about my grammer/semantics? Seems to be a problem here sometimes, so ask away for clarification

 

[TedC]

Schrade, This is an L98. ECM is behind the passenger side dash.

Hib, I interpreted "... a code reader that has always worked great on the car (it is OBD1 & OBD2 reader). Now I can not get codes with the reader ..." to mean that at one time his reader could pull serial data codes from the ALDL port. Doesn't a code reader use serial data to pull codes from the ECM? Shorting the ALDL port is using the ECM's internal code generator that flashes the lights and does not depend on serial data. Having a confused cluster seems to reinforce the lack of serial data.

 

[Hib Halverson]

Schrade, This is an L98. ECM is behind the passenger side dash.

Hib, I interpreted "... a code reader that has always worked great on the car (it is OBD1 & OBD2 reader). Now I can not get codes with the reader ..." to mean that at one time his reader could pull serial data codes from the ALDL port. Doesn't a code reader use serial data to pull codes from the ECM? Shorting the ALDL port is using the ECM's internal code generator that flashes the lights and does not depend on serial data. Having a confused cluster seems to reinforce the lack of serial data.

By definition a "code reader" only reads DTCs and, in the cases of some more expensive code readers which support OBDII, can also read the I/M readiness flags. If the device discussed by the OP was, in fact a "code reader", then it would not display any serial data.

By definition a "scan tester", not only reads fault codes, but can scan the serial data being transmitted by the ECM.

When the OP used the term "code reader" I assumed that the device being used could only command the ECM to flash codes.

Obviously, this is a key issue with this thread as if what he has is a tester and he's getting DTC 12 and no serial data, there's a problem. But, if he using a code reader and getting DTC12 but can verify serial data transmission, then there might or might not be a problem.

 

[boomdriver]

The quick way to tell which instrument someone is using is its cost...

A "code reader" will cost about $45.

a "scan tool" starts at around $299.

 

[Schrade]

Schrade, This is an L98. ECM is behind the passenger side dash.

Oh.

:confused

Soooo you can't backprobe L98 behind-the-seat ECM? Lemme' guess - they use smoke signals?

I put $$ on FSM has diags flow chart to test voltage AT ECM, sooner or later, in any faulty circuit, and ain't no better way than in the pics YUP!

 

[89blackvette]

Thanks for the input everyone. Put new ECM in her and runs great again and scanner will read it again. I also cleaned the grounds, they looked like they had just come from the factory but I wnet ahead and cleaned them. One reason I purchased this car is that the wiring looks new, all connectors and wire itself looks new. That original owner had keep the car in a climate controlled garage and I do the same. Again thanks for your help.
Mike

 

[TedC]

Great news!!! Keep the ALDL covered when not in use. If you don't have the stock black plastic cover you can find them in the corvette catalogs. These ECMs are static sensitive and exposed at this port. Not the best design being so close to the carpet.

 

[Hib Halverson]

Glad you got it fixed, "89blackvette".

BTW...what scan tester are you using on that car?

 

[TedC]

Here is a link to the ALDL cover...
C4 Corvette ALDL Cover 1984-1993E - Mid America Motorworks

 

[89blackvette]

Scanner is:
ACTRON
Super AutoScanner
Model CP9145

It works on the Vette. Works on OBDII also. Comes with connectors for most domestic and some foreign cars. I have had the scanner for a couple of years and it has worked on every car I have tried it on.