KingKash420

my 89 c4 vette is having problems.. and the service engine light comes on and off very frequently.. so i have a friend who is good with corvettes who tried to figure out the problem.. he has an obd1 scan tool that he tried to hook up to the ecm but the problem is even after he hooks it up the scan tool says "waiting for connection" which basicially means that the scan tool is not able to communicate with the ecm.. my buddy tested the serial connection for continuity and he says it is fine.. so he told me he thinks the computer is shot.. what do you guys think is the problem.. if the computer was shot wouldnt the car not work at all?

i am hoping that maybe it is something small that can be fixed.. because i know if he could get this scan tool to work that he would be able to adjust all my parameters and get the engine running good again..

any help or input is much appreciated.. it is killing me that the vette is not running well..

Morley

read through this thread, especially the last post or 2 http://forums.corvetteforum.com/show...48&forum_id=82

KingKash420

yea i checked that thread out.. so basicially you do think my ecm is shot then?

Red Tornado

it certainly seems to me thats what he/they are saying.

KingKash420

how much do replacement ecm's usually go for?

and how do i know which one to buy.. my vette is a 89' c4 and its automatic

Morley

Your symptoms seem almost exactly like his. Before shelling ot the $$ for an ECM...see if youcan borrow one from a friend...any Corvette or F body ECM from 86-89 will work. Just remember to put your MEMCAL in the ECM when you swap it...especially if using one from an F body.

Bluevette85

Have you tried jumping pins A+B to see if the check engine light flashes with trouble codes? They are the upper right pins.

You might need a 10k ohm resistor between pins A+B to communicate with the scan tool.

KingKash420

so by jumping pins A + B instead of just saying "service engine" it will actually display me a code telling me what is wrong?

also how would i know if i need the 10k ohm resistor or not? would it help if i told you the specific name of the scan tool?

Bluevette85

I have a '165' ecm in my car. This ecm was used from 86-89. I converted from the '870' ecm.

Put a paper clip between pins A+B and turn the ignition switch on but do not start the car. The engine light will flash once, then twice (which means code 12). It will do this 3 times then it will flash any stored codes then end with three code 12's.

Post the stored codes and we can tell you what they mean.

I don't know what your scan tool needs to work. The 165 ecm needs a 10k ohm resistor to send data to scanning software (Winaldl, Tunerpro, Datamaster). Check with the maker of the tool.

To pull the codes, you don't need a scanning tool other than a paper clip.

hebbe

Do all codes disapeare after disconnecting the car-battery or are there old codes in under to pic up with more sophisticated tools ?
Thanks ! Hebbe / Glad finding this thread ! Been wondering about how to pic up the codes for a long time.

Bluevette85

The codes clear after disconnecting the battery unless the problem remains.

msslp04

I just went thru this same thing with my 89 with auto. It was a bad ECM. The Chevy dealer charged $150 for the ECM plus labor. I had no codes either and the exhaust smelled like it was running really rich when the SES was on.

Kevin

KingKash420

it seems like i could save some cash by buying a new ecm off ebay.. but how do i know im not just buying another dead ecm for someone else?

if i absolutley have to i can fork out the $150 for the ecm from the chevy dealer but damn my money is tight right now and i would really rather not..

any other suggestions guys?

Bluevette85

Have you tried jumping pins A+B?

gbody5

Should be around $100 from NAPA etc. I would not buy anything electrical off EBAY, or anywhere else where I could not return it and/or get at least a 1 year warranty; which is what I believe you get from NAPA for an ECM.

mrmuzic

Just an FYI

Electrical items that you buy and you install are gauaranteed until yoiu walk out the door. This is the policy of almost every parts dealer. Electrical items atre guaranteed when they atre installed by a repair shop. This policy is usually posted in all the parts houses that I have ever been to.

mrmuzic

gbody5

Not the Carquest I patron. As long as I have the receipt they honor the warranty implied on the receipt. My ECM had a 1 yr warranty.

Bwbob

I just went through a similar thing with my 86. it turned out to be the PROM. I could pull the codes through the SES light with the pins jumped but my scan tool would not connect. I could look at the sensor list and see goofy data. I bought an ECM and nothing changed. I swapped my computer and Prom into a buddy's 86 and had the same issue. put his computer in mine and it worked. swapped proms and the problem followed the Prom so don't rule that out.

Cliff Harris

Putting a 10K resistor across pins A & B will put the ECM in "10K mode". You do NOT want this. It sets the idle RPM to 1000, fixes the timing and a few other things that interfere with reading the data.

On some ECMs (not Corvettes), they start out sending data at 160 baud on pin D. When a 10K resistor is applied across A & B they send 8192 baud data on pin M. Corvettes with 1227165 ECMs don't have a pin in cavity M, so this does not work.

I just checked the date, and the original thread is 10 YEARS OLD!

Bwbob

OK, there is somethings wonky about 86. I have one that the scan tool has to read for an 87 in order to connect to the ECM. production date 04/86