Wierd Engine problem
Thread Starter
Heel & Toe
Joined: Jul 2020
Posts: 17
Likes: 0
From: Ohio
Wierd Engine problem
1987,
Engine runs in 3 different modes. The average mileage display shows 9.9, 12.8, or correctly for current mileage. The timing changes from 10 in the 9.9 mode, runs bad. to 26 in the 12.8 mode, runs kind of okay, to 36 in the correct average mileage mode, runs great. It may change at almost any time. Once it gets to the correct mileage display it will stay there no matter how far you go, but MAY change on restart. I think I have changed every sensor at least once, computer rebuilt, new distributor, new oxygen sensor, new anti-knock sensor, no cats, smoke test no air leaks. The screen display blinks on and off when it changes modes and engine my stumble a few seconds. I have NO MAS but has been running fine 3 years without it. Any Help? Anyone in the Akron Ohio area that can fix it?
Thanks
Engine runs in 3 different modes. The average mileage display shows 9.9, 12.8, or correctly for current mileage. The timing changes from 10 in the 9.9 mode, runs bad. to 26 in the 12.8 mode, runs kind of okay, to 36 in the correct average mileage mode, runs great. It may change at almost any time. Once it gets to the correct mileage display it will stay there no matter how far you go, but MAY change on restart. I think I have changed every sensor at least once, computer rebuilt, new distributor, new oxygen sensor, new anti-knock sensor, no cats, smoke test no air leaks. The screen display blinks on and off when it changes modes and engine my stumble a few seconds. I have NO MAS but has been running fine 3 years without it. Any Help? Anyone in the Akron Ohio area that can fix it?
Thanks
Melting Slicks
Joined: Aug 2003
Posts: 3,389
Likes: 391
From: Lakeville MI
You have something going on with the ALDL Pin B voltage causing the ecm to vary between normal mode (open), ALDL mode (10 k), diagnostic mode or field service mode (shorted) and possibly factory test mode/backup fuel mode (3.9 k).
Check the voltage between ALDL Pin B and Pin A (gnd). Alternatively, check the resistance between ALDL Pin A and Pin B with the ign off. Check the wiring between ALDL Pin B and the ECM (Pin A9).
Important Pin B voltage levels:
< 800 mv
< 2.0 v
< 3.04 v
> 3.05 v
LEA28: LDAB L0035 ;
ANDB #$CF ; Clr b4 & b5 (DIAG MD & ALDL MODE) ;
;
READ A/D AND CK DIAGNOSTIC PIN B VOLTAGE....
;
LDAA #$70 ; A/D Ch 7, (Start/Run Pwr)
JSR LF1BE ; To A/D subroutine
CMPA #40 ; 800 Mv
BCS
LEA45 ; BR IF A/D LT 800 Mv, (GROUND'ED TST)
; ... else
CMPA #100 ; 2.00 VDV
BCS LEA47 ; BR IF A/D LT 2.00 VDC
; ... else
CMPA #152 ; 3.04 VDV
BCC
LEA47 ; BR IF A/D GT 3.05 VDC, (ROAD TST MODE)
; ... else
BRSET L0046,$08,
LEA47 ; BR If 8192 Mode in locked Mode
; ... else
ORAB #$20 ;
SET b5
BRA LEA47 ;
LEA45:ORAB #$10 ;
SET b4, (DIAG SW IN DIAG MODE)
; (GROUND'ED DIAG PIN)
LEA47:STAB L0035 ;
Check the voltage between ALDL Pin B and Pin A (gnd). Alternatively, check the resistance between ALDL Pin A and Pin B with the ign off. Check the wiring between ALDL Pin B and the ECM (Pin A9).
Important Pin B voltage levels:
< 800 mv
< 2.0 v
< 3.04 v
> 3.05 v
LEA28: LDAB L0035 ;
ANDB #$CF ; Clr b4 & b5 (DIAG MD & ALDL MODE) ;
;
READ A/D AND CK DIAGNOSTIC PIN B VOLTAGE....
;
LDAA #$70 ; A/D Ch 7, (Start/Run Pwr)
JSR LF1BE ; To A/D subroutine
CMPA #40 ; 800 Mv
BCS
LEA45 ; BR IF A/D LT 800 Mv, (GROUND'ED TST)
; ... else
CMPA #100 ; 2.00 VDV
BCS LEA47 ; BR IF A/D LT 2.00 VDC
; ... else
CMPA #152 ; 3.04 VDV
BCC
LEA47 ; BR IF A/D GT 3.05 VDC, (ROAD TST MODE)
; ... else
BRSET L0046,$08,
LEA47 ; BR If 8192 Mode in locked Mode
; ... else
ORAB #$20 ;
SET b5
BRA LEA47 ;
LEA45:ORAB #$10 ;
SET b4, (DIAG SW IN DIAG MODE)
; (GROUND'ED DIAG PIN)
LEA47:STAB L0035 ;
Last edited by tequilaboy; Aug 15, 2024 at 04:30 PM.
Thread Starter
Heel & Toe
Joined: Jul 2020
Posts: 17
Likes: 0
From: Ohio
Thanks a ton. One problem. I am really good at mechanical, a little good at electrical. no good at electronic. How do I fix it? Is there a way to make it run in just the good mode? do you know of someone near Akron Ohio I can take it to? I should have said it ran perfect for 3 years. I parked on day, running well, and the next day it ran terrible and led to here. Nothing I've done made much difference.
Thanks again
Thanks again
Melting Slicks
Joined: Nov 2016
Posts: 2,212
Likes: 3,045
From: So Cal
2023 C4 of the Year Finalist - Unmodified
This is why I read tequillaboy responses. I'm going to learn something.
Measure the voltage at cavity B using Cavity A as the "ground". But also measure the voltage at Cavity B using a jumper to a good ground on the engine block itself. If there is a difference in the two measurements, check the ground connections on the block. The ECM uses Circuit 450 for 'data level' grounds. 450 is a black/white wire. I'm not sure where 450 grounds on the block in 87. It's either adjacent to the thermostat, a bellhousing bolt on the driver side, or a stud above the oil filter. Vetts Vet has an 87, he should know for sure.
Measure the voltage at cavity B using Cavity A as the "ground". But also measure the voltage at Cavity B using a jumper to a good ground on the engine block itself. If there is a difference in the two measurements, check the ground connections on the block. The ECM uses Circuit 450 for 'data level' grounds. 450 is a black/white wire. I'm not sure where 450 grounds on the block in 87. It's either adjacent to the thermostat, a bellhousing bolt on the driver side, or a stud above the oil filter. Vetts Vet has an 87, he should know for sure.
Melting Slicks
Joined: Aug 2003
Posts: 3,389
Likes: 391
From: Lakeville MI
If the short is external to the ecm, you could simply remove ECM Pin A9 from the connector and leave the wire and pin hanging, but you would lose diagnostic capability. If the short is internal to the ecm, a new ecm may be in order. Another alternative would be to reprogram the eprom to ignore the diagnostic pin voltage or modify the voltage thresholds which is something I might do (if I didn't care about scan data).
Thread Starter
Heel & Toe
Joined: Jul 2020
Posts: 17
Likes: 0
From: Ohio
If the short is external to the ecm, you could simply remove ECM Pin A9 from the connector and leave the wire and pin hanging, but you would lose diagnostic capability. If the short is internal to the ecm, a new ecm may be in order. Another alternative would be to reprogram the eprom to ignore the diagnostic pin voltage or modify the voltage thresholds which is something I might do (if I didn't care about scan data).
Safety Car
Joined: Apr 2016
Posts: 4,648
Likes: 1,059
From: N.E. Ohio OH
Finalist 2020 C4 of the Year - Modified
If you don't get it worked out, then:
Van Devere on Vernon Odom has an older mechanic that is really into Corvettes. I talked to him a bit. You could go over there and ask to have him come to the desk and talk to him a moment and see if he thinks he could fix it for you.
over on Waterloo, close to the Airdock (well they used to be there) is a company you could call. You might want to start with them over Van Devere.
Wire Wizards Inc
1766 E Waterloo Rd, Akron, OH, 44306
(330) 784-2259
Automobile Electrical Systems Service and Repair
Van Devere on Vernon Odom has an older mechanic that is really into Corvettes. I talked to him a bit. You could go over there and ask to have him come to the desk and talk to him a moment and see if he thinks he could fix it for you.
over on Waterloo, close to the Airdock (well they used to be there) is a company you could call. You might want to start with them over Van Devere.
Wire Wizards Inc
1766 E Waterloo Rd, Akron, OH, 44306
(330) 784-2259
Automobile Electrical Systems Service and Repair
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Thread Starter
Heel & Toe
Joined: Jul 2020
Posts: 17
Likes: 0
From: Ohio
If you don't get it worked out, then:
Van Devere on Vernon Odom has an older mechanic that is really into Corvettes. I talked to him a bit. You could go over there and ask to have him come to the desk and talk to him a moment and see if he thinks he could fix it for you.
over on Waterloo, close to the Airdock (well they used to be there) is a company you could call. You might want to start with them over Van Devere.
Wire Wizards Inc
1766 E Waterloo Rd, Akron, OH, 44306
(330) 784-2259
Automobile Electrical Systems Service and Repair
Van Devere on Vernon Odom has an older mechanic that is really into Corvettes. I talked to him a bit. You could go over there and ask to have him come to the desk and talk to him a moment and see if he thinks he could fix it for you.
over on Waterloo, close to the Airdock (well they used to be there) is a company you could call. You might want to start with them over Van Devere.
Wire Wizards Inc
1766 E Waterloo Rd, Akron, OH, 44306
(330) 784-2259
Automobile Electrical Systems Service and Repair
Thread Starter
Heel & Toe
Joined: Jul 2020
Posts: 17
Likes: 0
From: Ohio
New Info
Disconnected CTS, single wire from Distributor to ECM, and ESC. Set the timing to 30 degrees. Ran great for 3 days. Came out of church, no start. Cranking fine, and spark. Nothing not even a cough. Towed home. Cranked with timing light. Timing on 30 degrees. Put plug against block, got spark. Hooked up fuel pressure gauge. 42 lbs pressure. Shot in ether. Nothing. Took Distibutor cap off, checked coil. module, and pickup coil. All checked okay. Took off both battery cables all night. Started in morning, runs fine. Any ideas?
Thread Starter
Heel & Toe
Joined: Jul 2020
Posts: 17
Likes: 0
From: Ohio
Sorry for late reply, thanks for asking. I'm just hoping to understand, how it ran fine, shut it off and it would not restart and I could not figure out why. Then it starts the next day, with nothing having been repaired.
Melting Slicks
Joined: Aug 2003
Posts: 3,389
Likes: 391
From: Lakeville MI
Did you get a CTS error (Code 14 or 15)? If so, the ecm uses a default value for the coolant temperature (142 °F) which may lead to some strange fueling behavior. Not sure why you unplugged the CTS. I wouldn't recommend this.