L98 spark retard mode questions
10-09-2004, 01:22 PM
#1
Instructor
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L98 spark retard mode questions
I'm debugging a knocking problem in my '86 (no SES codes thrown) and followed the Helms procedure for checking the spark retard mode. The knock sensor checked good (> 80mV at 2K RPM, or something) and the timing light shows timing backed off (along with knock counts getting logged on the WinALDL scanner) when knocking on the block. So I get an indication that timing is being retarded via the timing light but no apparent RPM drop per the Helms troubleshooting procedure. I believe the lack of perceptible RPM drop, and perhaps my knocking problem, is due to a short length of time timing is being retarded and/or the degrees of spark retard.
1) How long is spark retard mode, is this a constant when knock is first detected? Helms ignition system description states that retard mode is "held for approximately 20 seconds" where my retard seems to persist for a second or two. I also measured a low-voltage pulse from the ESC module to the ECM when inducing knocking -- I'm assuming this is normal and the retard mode length is coded into the ECM. I read somewhere on this forum about a recovery factor that dictates how fast timing is put back in after knock retard. But what is the normal time, 20 second as described in Helms? I'm trying to determine if my ECM is bad. This happens with the Stock and a Hypertech chip. Pulling the connector on the ESC lights up the SES light and the RPM does change (It doesn't go back to the base 6-degree timing however. It does go back to 6-deg if the blk/tan wire is subsequently disconnected. Not sure if I understand this and would expect the 6-deg base timing as a result of pulling the ESC connector alone).
2) How many degrees of timing is retarded? I'm measuring *a couple* degrees or so (can't quantify since the timing mark is off the numbered tab) initially, and seems to give a few more if the counts stack-up before the initial retard times out (time out after 2 seconds or so as described above). I thought I remember reading about 10 degrees of initial retard and another 10 degrees if knock persists but this may have been for another EFI motor (but makes my couple of degrees of retard seem low by comparison). Again, I'm trying to determine if my ECM is bad. I'm assuming none of these are affected by the HEI ignition module (?).
Any insights into the these would be most appreciated.
1) How long is spark retard mode, is this a constant when knock is first detected? Helms ignition system description states that retard mode is "held for approximately 20 seconds" where my retard seems to persist for a second or two. I also measured a low-voltage pulse from the ESC module to the ECM when inducing knocking -- I'm assuming this is normal and the retard mode length is coded into the ECM. I read somewhere on this forum about a recovery factor that dictates how fast timing is put back in after knock retard. But what is the normal time, 20 second as described in Helms? I'm trying to determine if my ECM is bad. This happens with the Stock and a Hypertech chip. Pulling the connector on the ESC lights up the SES light and the RPM does change (It doesn't go back to the base 6-degree timing however. It does go back to 6-deg if the blk/tan wire is subsequently disconnected. Not sure if I understand this and would expect the 6-deg base timing as a result of pulling the ESC connector alone).
2) How many degrees of timing is retarded? I'm measuring *a couple* degrees or so (can't quantify since the timing mark is off the numbered tab) initially, and seems to give a few more if the counts stack-up before the initial retard times out (time out after 2 seconds or so as described above). I thought I remember reading about 10 degrees of initial retard and another 10 degrees if knock persists but this may have been for another EFI motor (but makes my couple of degrees of retard seem low by comparison). Again, I'm trying to determine if my ECM is bad. I'm assuming none of these are affected by the HEI ignition module (?).
Any insights into the these would be most appreciated.
10-11-2004, 01:11 PM
#2
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The Knock Sensor (KS) is simply a transducer designed to 'hear' engine vibrations of
a specific freq and amplitude, in the audible range. When the KS triggers it sends a
voltage signal to the ECM.
If your KS passes the electrical tests, and the ECM receives the signal (retards timing
when you tap on the block) then the system is working.
Re your other questions:
The ECM program contains code to modify the (commanded) spark advance when it gets
a knock signal. Typically, the code retards spark according to the engine RPM and the
duration of the KS signal; this retard in tabular form reads something like:
>> Quantity of spark retard to add per usec (that KS signal is present) in an RPM range
Whether that spark change results in an observable engine speed change depends on
other factors.
Once the KS signal ends, the ECM removes the 'retard' slowly - as in so many degrees
of spark retard to remove per msec, until the spark advance is back to the proper
commanded amount per the ECM instructions. The quantity of retard added or subtracted
per unit time is usually called the knock 'attack' or 'decay' rate. The total timing change
is limited to some max amount, usually by RPM ranges. All these control features are
spelled out in the ECM program code, so the amount & time of spark changes depend on
what is written in the program.
I'm not familiar with your particular ECM, but that's how the code works in later
models - and I imagine yours has some similar features. Hence there is no 'single'
amount of spark retard to expect (or duration of that retard) - it will vary by engine
speed and duration of the KS signal, at least as I read my program.
As to the HEI module, if you are getting normal spark advance when you rev the engine
then it's working. The HEI per se has nothing at all to do with the KS control loop in the
ECM code.
a specific freq and amplitude, in the audible range. When the KS triggers it sends a
voltage signal to the ECM.
If your KS passes the electrical tests, and the ECM receives the signal (retards timing
when you tap on the block) then the system is working.
Re your other questions:
The ECM program contains code to modify the (commanded) spark advance when it gets
a knock signal. Typically, the code retards spark according to the engine RPM and the
duration of the KS signal; this retard in tabular form reads something like:
>> Quantity of spark retard to add per usec (that KS signal is present) in an RPM range
Whether that spark change results in an observable engine speed change depends on
other factors.
Once the KS signal ends, the ECM removes the 'retard' slowly - as in so many degrees
of spark retard to remove per msec, until the spark advance is back to the proper
commanded amount per the ECM instructions. The quantity of retard added or subtracted
per unit time is usually called the knock 'attack' or 'decay' rate. The total timing change
is limited to some max amount, usually by RPM ranges. All these control features are
spelled out in the ECM program code, so the amount & time of spark changes depend on
what is written in the program.
I'm not familiar with your particular ECM, but that's how the code works in later
models - and I imagine yours has some similar features. Hence there is no 'single'
amount of spark retard to expect (or duration of that retard) - it will vary by engine
speed and duration of the KS signal, at least as I read my program.
As to the HEI module, if you are getting normal spark advance when you rev the engine
then it's working. The HEI per se has nothing at all to do with the KS control loop in the
ECM code.