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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
I've got an issue w/my LT1 that I'm trying to diagnose. To help me with my diagnosis, I'm using a "period correct" OTC Monitor 4000 scan tool. I took a recording while the car was doing the symptom and what I don't "get" is what the numbers represent, that are displayed under the "HiRes" and LoRes" displays? During the symptom, the Low Res signal only shows "002"....while the symptom is NOT happening, the Low Res shows a variety of numbers, constantly jumping all over the place.
Here is a vid of me doing a frame by frame play back of the recording. You can see the:
RPM...HiRes...LoRes....TPS%
Codes.MAP....Left 02....Frame #
When I get to ^5000 RPM, the low res signal displays only "002" for the duration....
Scan tool data can be a crapshoot because it depends on GM giving the scan tool manufacturer the right data. HiRes and LoRes are meaningless to me and I both tune and am familiar with typical live data on vintage and modern FI systems.
*edit* Apparently it's got something to do with the Optispark triggering. Which is probably why I haven't seen it. I know the Opti can be troublesome though so your issue might be related.
GD
Last edited by GeneralDisorder; 11-20-2019 at 11:12 PM.
Car acts like it's hitting a "soft" limiter above 5k RPM...AND above ~3/4 throttle. Below EITHER, it runs great.
Originally Posted by GeneralDisorder
*edit* Apparently it's got something to do with the Optispark triggering.
I agree. It throws a Code 16 (low res lost) so I'm sure that's what it is, but knowing that that low res count number represents would likely PROVE it.
Well my guess is it's related to the trigger patterns being output by the optispark module. Low res and high res probably being something like cam angle vs. crank angle on modern setups.... I've seen optical distributors that have trigger wheels with dual patterns - a simple 8 slot pattern lets say for TDC and another 360 slot pattern for finer control..... the count probably being the trigger count or the error count..... just a WAG on my part.
I would be on the hunt for known good waveforms so I could scope the output from the optispark.
GD
Last edited by GeneralDisorder; 11-22-2019 at 11:56 AM.
Yeah, you nailed it; The opti has an 8 slot pattern AND a 360 slot pattern too. The 8 slots are differing lengths so that the ECM can "know" which cylinder is at TDC.
I should mention that I also borrowed a Snap On Solus Ultra. What did that show me? The same exact thing. The only diff was that the Solus graphed that low res number....rather than simply showing the number as it was happening, (it did that too), it also graphed the number over time. It started off high at idle....ended up at "2" when the car was languishing at over 5000 RPM and WOT.
You're also right that if I had a scope, I could watch the actual low res signal for each cylinder and actually see when (if) it drops out. I don't have a scope though...
It's weird that no techs on here are familiar with this data.
Last edited by Tom 400 CFI; 11-22-2019 at 10:26 AM.
Here is the scan of a pull, w/the Solus. As I said...same numbers the 30 year old OTC displayed...just nicely graphed. I wasn't able to understand how to get the Solus to scope the wave pattern. Anyway, you can see that I start out from idle and the low res number on the graph is midway up the scale. I floor it and as RPM increases, low res signal "number" goes down until it is at "2"...at over 5k/WOT...
I don't think you have a choice but to scope the waveform from the optispark. Time to invest in a USB scope maybe.
Also could identify the ROM that this ECM is using and try to find a hac of the code you could go through and see if you can determine what exactly that piece of live data means.
Sounds to me, based on the symptom and the code, that the optispark is going out or you have signal noise, etc. A scope trace verified against a known good waveform should be enough to prove the failure.
Noise in the signal is another possibility - check your grounding, etc. High RPM is where I would expect to see noise problems if they were going to show up.
The Low Res and Hi Res are displayed as counters. It's the same on the GM Tech 2 scan tool. They start at zero and count up to IIRC 255 then start over at zero. Staying at zero means the signal has dropped out. It looks like you're losing the Low res signal as the engine revs. I can't see much in the image you posted but reflections on part of a Solus screen. The PID titles are cut off at the left side of the image. GD is correct that to verify this issue, you need a labscope. Carefully check the connector terminals to the Optispark. I have often found them corroded. Honestly though, I've spent alot of time chasing issues on the Optispark system. You can be pretty sure it just needs a new optispark assembly. Make sure it has a good vent connection to the intake air boot. They do like to take on water.
10-19-2019, 08:17 PM
