86 reading issues
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Joined: Aug 2021
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86 reading issues
Hello there
I just bought a 1986 C4 and i love the car!
It has been quite well restored around 10 years ago, it's hard to believe it's 35 years old by looking at it.
Engine and electronics is almost original, the only relevant modification is the hypertech power ignition coil, replacing the original one, and I believe the digital dash had been rebuilt as well.
I would like to ask for some help investigating some readings issues. I'm noticing some weird acting of the digital dash oil temperature and avg/instant consumption:
The only ocde i get from the ALDL interface is 42, but the engine is running fine and i have no "service engine soon" light so i suspect it's caused by the non-original hypertech power ignition coil not sending all the data as expected by ECU.
I would like to kindly ask you what could be the cause of the readings malfunctions, to understand where should i start to investigate (cables, dash, ECU, temperature sensor, etc)
Thank you!
I just bought a 1986 C4 and i love the car!
It has been quite well restored around 10 years ago, it's hard to believe it's 35 years old by looking at it.
Engine and electronics is almost original, the only relevant modification is the hypertech power ignition coil, replacing the original one, and I believe the digital dash had been rebuilt as well.
I would like to ask for some help investigating some readings issues. I'm noticing some weird acting of the digital dash oil temperature and avg/instant consumption:
- Oil temperature reading some times drops quickly from 195ish to LO, and coolaint with coolant remaining around 190F° (and updating). Sometimes oil temp goes back to 190-200 and to LO again, and sometimes it work.I have no evident engine issues, so i suppose it's just a minor misreading problem.
- Avg/instant consumption reading is almost never working, staying 0 most of the trip, with some brief correct reading phases while driving. Range indicator is working reporting remaining miles and updating, i dont know if it's telling the truth (i suspect it's overestimating somewhat)
The only ocde i get from the ALDL interface is 42, but the engine is running fine and i have no "service engine soon" light so i suspect it's caused by the non-original hypertech power ignition coil not sending all the data as expected by ECU.
I would like to kindly ask you what could be the cause of the readings malfunctions, to understand where should i start to investigate (cables, dash, ECU, temperature sensor, etc)
Thank you!
Drifting
Joined: Jan 2021
Posts: 1,311
Likes: 383
From: Northern California
The oil temperature reading is provided by a sensor near the top of your oil filter / oil cooler housing. You'll see it there as a 1-wire connector into the sensor. Try cleaning / reconnecting that sensor and connector so that you get good contact. Those 1-wire connectors can get brittle or weak from the heat cycles over the years.
When my average / instant consumption mileage readings started acting like yours, it turned out to be a faulty ECM. Sometimes the averaging calculation would be valid, and other times the display would go to 99mpg average gas mileage. Others here can chime in with their thoughts on that.
The code 42 is electronic spark control related if I remember correctly, so I would suspect the ignition module or ESC module itself in that case. I doubt that it's your coil. Others here with more experience with that specific code can advise.
When my average / instant consumption mileage readings started acting like yours, it turned out to be a faulty ECM. Sometimes the averaging calculation would be valid, and other times the display would go to 99mpg average gas mileage. Others here can chime in with their thoughts on that.
The code 42 is electronic spark control related if I remember correctly, so I would suspect the ignition module or ESC module itself in that case. I doubt that it's your coil. Others here with more experience with that specific code can advise.
Thread Starter
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Joined: Aug 2021
Posts: 62
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The oil temperature reading is provided by a sensor near the top of your oil filter / oil cooler housing. You'll see it there as a 1-wire connector into the sensor. Try cleaning / reconnecting that sensor and connector so that you get good contact. Those 1-wire connectors can get brittle or weak from the heat cycles over the years.
When my average / instant consumption mileage readings started acting like yours, it turned out to be a faulty ECM. Sometimes the averaging calculation would be valid, and other times the display would go to 99mpg average gas mileage. Others here can chime in with their thoughts on that.
The code 42 is electronic spark control related if I remember correctly, so I would suspect the ignition module or ESC module itself in that case. I doubt that it's your coil. Others here with more experience with that specific code can advise.
When my average / instant consumption mileage readings started acting like yours, it turned out to be a faulty ECM. Sometimes the averaging calculation would be valid, and other times the display would go to 99mpg average gas mileage. Others here can chime in with their thoughts on that.
The code 42 is electronic spark control related if I remember correctly, so I would suspect the ignition module or ESC module itself in that case. I doubt that it's your coil. Others here with more experience with that specific code can advise.
did your ECM had other issues? i'm wondering if i can just ignore it or if it will lead to other issues.
Drifting
Joined: Jan 2021
Posts: 1,311
Likes: 383
From: Northern California
My ECM had no other issues at the time. It was replaced under warranty since the car was less than 2 years old back then. I have a feeling that the mother board just had a problem with that particular logic circuit or maybe the internal pin connections went bad somehow, but the replacement ECM has been in the car since then.
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just a quick update for the first point, it turned out the oil temperature sensor cable was literally hanging from the bottom of the car. i plug it in and now it works flawlessly.
About Avg/instant consumption instead it's still not working, i've inspected the main ground points but they looks ok. in the next days i'm going to remove the dash front cover as i want to change the stereo, so i was wondering if it makes sense to inspect the cables around, i read the MPGs are calculated based on information from the ECU which are sent to the dash through some cables. Since this value is sometimes displayed after i turn the car and keeps working for a variable amount of time (seconds or minutes) i suspect it could be another loosened cable.
So my question is, which cables connection would you suggest to look for to check MPG values?
thank you!
About Avg/instant consumption instead it's still not working, i've inspected the main ground points but they looks ok. in the next days i'm going to remove the dash front cover as i want to change the stereo, so i was wondering if it makes sense to inspect the cables around, i read the MPGs are calculated based on information from the ECU which are sent to the dash through some cables. Since this value is sometimes displayed after i turn the car and keeps working for a variable amount of time (seconds or minutes) i suspect it could be another loosened cable.
So my question is, which cables connection would you suggest to look for to check MPG values?
thank you!
Drifting
Joined: Jan 2021
Posts: 1,311
Likes: 383
From: Northern California
I don't think that's the right path. If your MPG display was intermittently going dark or displaying weird gibberish, then maybe you could research an issue internal to the dashboard itself (you mentioned it was previously rebuilt), or cable connectors, etc. But if the display is good and it's just that the numbers are wrong for MPG, that is most likely the ECM itself. The logic for the mileage calculation is all internally done within the ECM. Maybe it would help to disconnect the battery and then re-seat the cable connections at the ECM itself. But that's a longshot. Still worth a try I suppose since you'll be in there anyway.
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Melting Slicks
Joined: Aug 2003
Posts: 3,378
Likes: 388
From: Lakeville MI
Here are the (5) bytes which are sent to the dash at 160 baud by the ecm (MW2 + the 4 byte string shown below in the transmit table): Note: this is applicable for 86, 87 and 88. In 89 MW2 Bit 0 instead represents CARS ACTIVE (or cags).
; Bit 0 = (1= OVERDRIVE ON; 0= OVERDRIVE OFF) or (1989: 1=CARS ACTIVE)
; 1 = ERR 14 or 15 THIS STARTUP
; 2 = REF PULSES OCCURRED (6.25 MSEC CHECK)
; 3 = 1= ALDL MODE, 8192 LOCKED IN, & MODE 4 ;
; 4 = DIAGNOSTIC SWITCH IN DIAGNOSTIC POSITION
; 5 = DIAGNOSTIC SWITCH IN ALDL POSITION
; 6 = HIGH BAT. VOLT. , DISABLE MCU SOLENOID DISCRTS
; 7 = SHIFT LIGHT (1= ON, 0= OFF)
;-------------------------------------------------
; >>> Serial Data Tables & Params <<<
;--------------------------------------------------
LC703 FCB 4; Dis String length, (Bytes)
******************************** * DIS ADDRESS'S , XMIT ********************************
LC704 FCB $C009; Num O cyl Addr
LC706 FCB $011A; Running total of fuel supplied Addr
LC708 FCB $011E ; Running total of Dist. Traveled Addr
LC70A FCB $C712 ; Gal's/Sec Scale Factor Addr
LC70C FCB $0000 ;
LC70E FCB $0000 ;
LC710 FCB $0000 ;
LC712 FCB 122 ; Gals/Hr Inj flow rate ; Num = Arg * 32, (3.8125 G/Hr)
The average/instantaneous mpg calculation is done in the dash with the raw data supplied by the ecm as shown above.
The OP's issue is in the dash or the serial data line connection to the dash.
; Bit 0 = (1= OVERDRIVE ON; 0= OVERDRIVE OFF) or (1989: 1=CARS ACTIVE)
; 1 = ERR 14 or 15 THIS STARTUP
; 2 = REF PULSES OCCURRED (6.25 MSEC CHECK)
; 3 = 1= ALDL MODE, 8192 LOCKED IN, & MODE 4 ;
; 4 = DIAGNOSTIC SWITCH IN DIAGNOSTIC POSITION
; 5 = DIAGNOSTIC SWITCH IN ALDL POSITION
; 6 = HIGH BAT. VOLT. , DISABLE MCU SOLENOID DISCRTS
; 7 = SHIFT LIGHT (1= ON, 0= OFF)
;-------------------------------------------------
; >>> Serial Data Tables & Params <<<
;--------------------------------------------------
LC703 FCB 4; Dis String length, (Bytes)
******************************** * DIS ADDRESS'S , XMIT ********************************
LC704 FCB $C009; Num O cyl Addr
LC706 FCB $011A; Running total of fuel supplied Addr
LC708 FCB $011E ; Running total of Dist. Traveled Addr
LC70A FCB $C712 ; Gal's/Sec Scale Factor Addr
LC70C FCB $0000 ;
LC70E FCB $0000 ;
LC710 FCB $0000 ;
LC712 FCB 122 ; Gals/Hr Inj flow rate ; Num = Arg * 32, (3.8125 G/Hr)
The average/instantaneous mpg calculation is done in the dash with the raw data supplied by the ecm as shown above.
The OP's issue is in the dash or the serial data line connection to the dash.
Drifting
Joined: Jan 2021
Posts: 1,311
Likes: 383
From: Northern California
Awesome breakdown of the data stream tequilaboy. Thank you for publishing it.
In the case of my ECM failure resulting in wrong MPG data back in 1991 (on an '89), your data stream chart also implies that if the ECM sends the wrong / corrupted data (via failure somewhere) that would also cause the issue. Correct?
In the case of my ECM failure resulting in wrong MPG data back in 1991 (on an '89), your data stream chart also implies that if the ECM sends the wrong / corrupted data (via failure somewhere) that would also cause the issue. Correct?
Melting Slicks
Joined: Aug 2003
Posts: 3,378
Likes: 388
From: Lakeville MI
Garbage in = garbage out, so the incoming data could be bad or intermittent causing an erroneous display. Scan data can be used to rule out signal/serial data issues with the ecm.
In this log, the running distance has been scaled into feet for display purposes/convenience. Here's what the running fuel/distance data looks like if anybody is interested:
https://datazap.me/u/tequilaboy/stri...lo=107-108-120
In this log, the running distance has been scaled into feet for display purposes/convenience. Here's what the running fuel/distance data looks like if anybody is interested:
https://datazap.me/u/tequilaboy/stri...lo=107-108-120
Thread Starter
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Joined: Aug 2021
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Thanks @tequilaboy and @ajp01 for the suggestions. The dash display looks good: speaking about instant MPG, either it shows reasonably meaningful numbers which freezes at a certain point, or just zero. The curious fact is that sometimes it starts showing number after i stiffly turn the car (last time it happened i was turning left), That's why i believe the cable related issue might be a reasonable cause. Too bad i have no scanner to make a test, and it's very difficult to find one here in the EU. I'll see if i can purchase one.
btw do you know if is there a particular cable color i should check for? I once removed the ECM but there are around 20-30 cables so it would very difficult to check all of them, plus the bad connection can be also on the dash side.
I also read there exists a dash repair manual by Mr. Gordon Killebrew, which i've been unable to get so far. May i kindly ask you if somebody have it please or know where to find it?
btw do you know if is there a particular cable color i should check for? I once removed the ECM but there are around 20-30 cables so it would very difficult to check all of them, plus the bad connection can be also on the dash side.
I also read there exists a dash repair manual by Mr. Gordon Killebrew, which i've been unable to get so far. May i kindly ask you if somebody have it please or know where to find it?
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Joined: Aug 2021
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So i found out the info of which wire should i check: it's D10 orange and should measure from 2.5 to 4.0 volts AC to ground with the engine running.
I wonder if any ground point would be ok for measuring, plus i'm a bit worried about removing the dash.
I wonder if any ground point would be ok for measuring, plus i'm a bit worried about removing the dash.