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  1. #1
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    Default Pcm Voltage Issue

    This one I pray someone has dealt with before.

    Car has big 3 battery cables, new alt and battery. Pcm voltage on logs shows 12.5-12.7, rapidly fluctuates as low as 12.3-13.1 on a good day.

    Checked pin d on c100, 14.2v running (same as bat). ok move on to the wires at the pcm, same thing, 14.2v car running backprobed 2" off black pcm connector (pin 15 and 31 as shbox says). pcm still reads low. Jumper battery positive straight to pin 15, pcm STILL reads 12.4-12.5v

    where do i go from here? @Injuneer got any ideas?

    Sent from my SM-N975U using Tapatalk
    2015 Camaro 2SS 1LE
    2017 Fiesta ST -_- Its a go-cart

    1995 Firebird Formula - Le2 H/C, SBE, CPT 4l60E, boost?
    1984 Camaro Z28 - Wifes' project. Stock LT1 with Hotcam - her cruiser

    The wrench/driver for LTConvert's 94 Z28- Ellwein 383/LE Trickflows/ P600B/ CPT Pro Race 4L60E/Pro Torque 3200
    10.78 @ 125mph. Shooting for 9.99 in 2020 for Frank Cahall!

  2. #2
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    Default

    That range of PCM volts look typical of what I have seen in the many data logs I’ve reviewed for people. But typically they don’t jump around all the time. I’ve never compared them with actual volts at the PCM pins because I'm just looking at files thar people send me. If I see volts drop below 12.0 I in the log I will call it to the attention of the owner.

    Have you tried varying RPM while measuring the volts at the PCM pins? Erratic voltage regulator???

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  4. #3
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    I've asked GaryDoug if there is any voltage regulation internal to the PCM.

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  6. #4
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    Quote Originally Posted by Injuneer View Post
    That range of PCM volts look typical of what I have seen in the many data logs I’ve reviewed for people. But typically they don’t jump around all the time. I’ve never compared them with actual volts at the PCM pins because I'm just looking at files thar people send me. If I see volts drop below 12.0 I in the log I will call it to the attention of the owner.

    Have you tried varying RPM while measuring the volts at the PCM pins? Erratic voltage regulator???
    Voltage tapped into the wire on my dvm is showing atleast a volt higher at all times. its like this with both a failing and a new alt, just once the old alt gets hot the voltage on my meter starts going down at the alt/battery/pcm 15/31, hence why i changed the alt to a known good one.

    With the new alt dvm reads 14.1 at back of alt, at battery, at terminal block, and thru the c100 to pcm pins 15/31 (i stripped the insulation back to probe the wires while they are plugged into the pcm and the car is running). BUT the scan94/95 logs show it at 12.3-12.8 erratically moving.

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    Last edited by SSlowBoat; 11-11-2020 at 08:41 AM.
    2015 Camaro 2SS 1LE
    2017 Fiesta ST -_- Its a go-cart

    1995 Firebird Formula - Le2 H/C, SBE, CPT 4l60E, boost?
    1984 Camaro Z28 - Wifes' project. Stock LT1 with Hotcam - her cruiser

    The wrench/driver for LTConvert's 94 Z28- Ellwein 383/LE Trickflows/ P600B/ CPT Pro Race 4L60E/Pro Torque 3200
    10.78 @ 125mph. Shooting for 9.99 in 2020 for Frank Cahall!

  7. #5
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    My big concern is the car starts to get pig rich as the voltage goes down. I know 2 tuners over the years moe included were concerned about the voltage being low, and even on the procharged car the alt would start dying going down track, once voltage got to 12.3-12.4 volts on a pass it would start misfiring.

    i plan on spraying this car short term and i dont want to melt somethinf down because the pcm voltage its reading is off causing the pcm to act wonky.

    I wonder if it could be a ground issue.

    Sent from my SM-N975U using Tapatalk
    2015 Camaro 2SS 1LE
    2017 Fiesta ST -_- Its a go-cart

    1995 Firebird Formula - Le2 H/C, SBE, CPT 4l60E, boost?
    1984 Camaro Z28 - Wifes' project. Stock LT1 with Hotcam - her cruiser

    The wrench/driver for LTConvert's 94 Z28- Ellwein 383/LE Trickflows/ P600B/ CPT Pro Race 4L60E/Pro Torque 3200
    10.78 @ 125mph. Shooting for 9.99 in 2020 for Frank Cahall!

  8. #6
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    Or, if the pcm actually gets its voltage reading from different pins than b15/b31. I wonder if it reads voltage from the pcm ignition + pins. i didnt check those yet

    Sent from my SM-N975U using Tapatalk
    2015 Camaro 2SS 1LE
    2017 Fiesta ST -_- Its a go-cart

    1995 Firebird Formula - Le2 H/C, SBE, CPT 4l60E, boost?
    1984 Camaro Z28 - Wifes' project. Stock LT1 with Hotcam - her cruiser

    The wrench/driver for LTConvert's 94 Z28- Ellwein 383/LE Trickflows/ P600B/ CPT Pro Race 4L60E/Pro Torque 3200
    10.78 @ 125mph. Shooting for 9.99 in 2020 for Frank Cahall!

  9. #7
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    Gary indicated there is internal voltage regulation, since the PCM has to provide an accurate 5 volt reference to many of the sensors.

    Also suggested you measure the volts at the PCM by accessing both the hot and ground terminals at the PCM. Not a random chassis ground. SHOEBOX’S PCM wiring diagrams show the terminals for positive and ground.


    Finally, are you sure your meter is accurate?

    Random thoughts:

    How does your dash voltage gauge reading compare to the PCM voltage on the data log? I've been asked several times why the dash gauge typically shows about a volt higher than the PCM data. I’VE always assumed it the result of the dash taking the measurement at the alternator and the PCM having circuitry between the alternator and the connector pins. Never gave it much more thought than that.

    Another question.... pins B15 and B31 are “PCM BAT” (always hot) power, to maintain the memory. Have you tried terminals B30 and D3 (ignition switched)? That circuit also supplies the fuel pump and MAF sensor. Losing voltage to the fuel pump is probably going to be as big a problem as losing voltage at the PCM.

    I'll poke around i) some old data logs.

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  11. #8
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    Quote Originally Posted by Injuneer View Post
    Gary indicated there is internal voltage regulation, since the PCM has to provide an accurate 5 volt reference to many of the sensors.

    Also suggested you measure the volts at the PCM by accessing both the hot and ground terminals at the PCM. Not a random chassis ground. SHOEBOX’S PCM wiring diagrams show the terminals for positive and ground.


    Finally, are you sure your meter is accurate?

    Random thoughts:

    How does your dash voltage gauge reading compare to the PCM voltage on the data log? I've been asked several times why the dash gauge typically shows about a volt higher than the PCM data. I’VE always assumed it the result of the dash taking the measurement at the alternator and the PCM having circuitry between the alternator and the connector pins. Never gave it much more thought than that.

    Another question.... pins B15 and B31 are “PCM BAT” (always hot) power, to maintain the memory. Have you tried terminals B30 and D3 (ignition switched)? That circuit also supplies the fuel pump and MAF sensor. Losing voltage to the fuel pump is probably going to be as big a problem as losing voltage at the PCM.

    I'll poke around i) some old data logs.
    dash is reading above 13 volts. reason i went with pins 15 and 31 is per shbox's post on a ls1tech thread about someone having the same issue.

    Thinking about it this morning i was thinking of trying the pcm ignition + pins like you/garydoug are suggesting. I also am going to get the car in the air and check the resistance for the pcm ground that goes above the knock sensor.

    I didnt put it in the post but i did backprobe between pins 15/31 (individually respectively) to pin B6 (sensor ground) because my fiance was jiggling the black pcm connector in its entirety while i watched the log and suddenly got a tps code (that was cleared and didnt return)

    Yes my meter is accurate. Fluke 375 FC that was calibrated recently (I work in facilities maintenance, primarily hvac)

    Thank you and @GaryDoug i have a more definite direction to start on this evening. I also have a spare pcm, maybe this one is wonky (i did find some questionable wiring like knock sensor wired to trans performance switch input on pcm and vice versa when i bought the car)

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    Last edited by SSlowBoat; 11-11-2020 at 11:28 AM.
    2015 Camaro 2SS 1LE
    2017 Fiesta ST -_- Its a go-cart

    1995 Firebird Formula - Le2 H/C, SBE, CPT 4l60E, boost?
    1984 Camaro Z28 - Wifes' project. Stock LT1 with Hotcam - her cruiser

    The wrench/driver for LTConvert's 94 Z28- Ellwein 383/LE Trickflows/ P600B/ CPT Pro Race 4L60E/Pro Torque 3200
    10.78 @ 125mph. Shooting for 9.99 in 2020 for Frank Cahall!

  12. #9
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    The B5 and B31 pins are fed DIRECTLY from the battery, through fuse #8 in the under hood box (1995 only). The only other item supplied by this circuit is the AIR pump, in parallel through fuse #7 U/H box. Without the AIR pump running, you are going to get full battery voltage, with minimal loss through the wiring.

    On the other hand, the PCM IGN pins (and the PCM data is labeled "Ignition Volts") has a much more tortuous path, through the ignition switch, shared with numerous other items on the switch, out of the switch, where it supplies the air bag fuse, and gages fuse (powers various modules), along with the PCM IGN fuse #5, then out of fuse #5 where it supplies the PCM, fuel pump relay, theft deterrent module, and the MAF sensor. There is going to be significantly more voltage loss in that circuit, compared to the PCM BAT circuit.

    The PCM will set a code if the voltage on the PCM IGN pins gets too low. That codes only sets when the PCM IGN drops to 8 volts (!!!!!). The PCM should be able to function fully with 12 volts or more. Looking through several data logs, the most common "Ignition Volts" readings are in the range of 12.1-12.8. Only once could I find a reading above 13.0 volts.

    The PCM compensates for low voltage effects on the injectors. That's what the voltage offsets are for, as far as I know. Voltage also affects the flow from the fuel pump. What pump(s) are you running? Is it supplied by stock wiring, or do you have heavier gauge wiring installed? Have you ever seen any indication fuel is falling off and the engine is leaning out?

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  14. #10
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    Quote Originally Posted by Injuneer View Post
    The B5 and B31 pins are fed DIRECTLY from the battery, through fuse #8 in the under hood box (1995 only). The only other item supplied by this circuit is the AIR pump, in parallel through fuse #7 U/H box. Without the AIR pump running, you are going to get full battery voltage, with minimal loss through the wiring.

    On the other hand, the PCM IGN pins (and the PCM data is labeled "Ignition Volts") has a much more tortuous path, through the ignition switch, shared with numerous other items on the switch, out of the switch, where it supplies the air bag fuse, and gages fuse (powers various modules), along with the PCM IGN fuse #5, then out of fuse #5 where it supplies the PCM, fuel pump relay, theft deterrent module, and the MAF sensor. There is going to be significantly more voltage loss in that circuit, compared to the PCM BAT circuit.

    The PCM will set a code if the voltage on the PCM IGN pins gets too low. That codes only sets when the PCM IGN drops to 8 volts (!!!!!). The PCM should be able to function fully with 12 volts or more. Looking through several data logs, the most common "Ignition Volts" readings are in the range of 12.1-12.8. Only once could I find a reading above 13.0 volts.

    The PCM compensates for low voltage effects on the injectors. That's what the voltage offsets are for, as far as I know. Voltage also affects the flow from the fuel pump. What pump(s) are you running? Is it supplied by stock wiring, or do you have heavier gauge wiring installed? Have you ever seen any indication fuel is falling off and the engine is leaning out?
    stock pump stock wiring. literally just got this firebird running sunday the car has 3 trips up and down the road to make sure the brakes were bled and car didnt overheat. its primary function is to be a drag car at island (as long as island stays open). Just did the basic emissions delete/full bolt ons, gear change and rear suspension thus far. I havent driven this car more than a mile total since buying it in may.

    Just trying to get it lined out enough to make a pass this sunday or black friday at island for a baseline before we yank the stock longblock (or spray it to death) and slap my le2 heads/cam motor in.

    8 volts, wow that is super low. i may be chasing something that is a non issue then. thank you @Injuneer as always for the specifics. those voltages from your logs match exactly what i am seeing.

    as for the air pump thing, i have that wiring from the air pump relay feeding my electric water pump. thats good to know that it is fed that direction.
    2015 Camaro 2SS 1LE
    2017 Fiesta ST -_- Its a go-cart

    1995 Firebird Formula - Le2 H/C, SBE, CPT 4l60E, boost?
    1984 Camaro Z28 - Wifes' project. Stock LT1 with Hotcam - her cruiser

    The wrench/driver for LTConvert's 94 Z28- Ellwein 383/LE Trickflows/ P600B/ CPT Pro Race 4L60E/Pro Torque 3200
    10.78 @ 125mph. Shooting for 9.99 in 2020 for Frank Cahall!

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