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I have been looking for some authoritative, detailed info on this subject but have come up empty handed.
I have heard from a couple of sources (accell tech support and here, read the post by "Tuner") that in limp home mode the module will supply a timing curve and a ~6000 RPM rev limit.
Does anyone know anything about this?
Thanks,
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Last edited by esc; 05-01-2008 at 09:45 AM.
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I read most of the thread you posted the link to. The distributor that Tuner is talking about with the advance curve & rev-limiter is a particular marine distributor. Which is completely different then how our car distributor/modules work.
With the EST/BYPASS connector open the distributor is in limp mode. It ignores the incoming EST signal from the ECM. This is how the distributor operates during cranking, firing at the base setting. Once the ECM sees the engine RPM above a particular value (usually 400 RPM) it changes to EST mode and takes over the timing. The ECM controls the EST/BYPASS line to do this (along the lines of us opening & closing the BYPASS connector).
With the standard car computer controlled distributor & module, while in limp mode the timing will hold steady until a particular RPM is hit. The module will then suddenly add between 12 & 16 degrees of advance.
This advance typically occurs in the 1400 - 1600 RPM range. Once the RPM falls below this threshold the advance is removed and the module is back to the base timing.
You can check this out with a timing light and running the RPM up & down (BYPASS open). When at idle the timing should hold steady at wherever it is set (0, 4, 6 10 BTDC). As the RPM's are increased the timing should hold steady. Then once the RPM threshold is reached the timing should jump advanced.
As long as the RPM is above the threshold the timing should hold steady at the advanced value. Then once the RPM falls below the threshold the timing should return to the base setting.
If the timing does strange things such as retard as the RPM is raised above idle RPM, then the distributor or module is bad. If the threshold RPM for the jump advance is over 2000 RPM then there is something wrong.
Also note that the distributor latencies are compensated for in the ECM. The latency also varies as the distributor RPM changes. Typically there is a table of latency values based on RPM.
This latency value is subtracted out after the ECM SA timing value has been converted from degrees BTDC to a time based value. Thereby advancing the timing to compensate for the latency (delay).
Can also use a timing light and a custom timing table in the ECM to check the latency. Set the entire table to say 20 degrees BTDC, zero out the compensations for CTS, LM, and so on.
Start the engine (leave the EST/BYPASS connector closed) and check the timing. Should be at 20 BTDC. Now run the RPM's up (slowly) and watch the timing. Any variance from 20 BTDC and something is not correct.
With all of this makes one wonder about aftermarket distributors and modules. Even GM distributors have different latency values among the same family of distributor (such as the small cap).
So what you are saying is that while operating in "limp-home" mode, it would not have the rev limiter but it will add in additional timing (12-16 degrees) above 1400-1600 RPM.
Actually, I'm talking about operation with no ECM present.
When I installed it (8 pin HEI) in my car I was also under the impression that, without an EMC, the timing would be static (what I need). Unfortunately, it seems that I was incorrect.
Now it seems I will have to either back off my innitial timing by 12-16 degrees, or bypass the module completeley. I think my 300+ has an option for hooking directly to the magnetic pickup.
I wonder if a 4 pin module would fit in in a small cap distributor?
OK after a little experimentation I have determined the facts about MY distributor. It originally came from an '88 Chevy pickup and I have every reason to believe that it has the original module in it.
The module provides an additional 7 degrees of advance above 2500 RPM.
That is less advance and at a higher RPM than any of the other information I have seen, but that is what mine does.
Actually, I'm talking about operation with no ECM present....
The ECM would still need to be present with an Open Circuit though, as although Bypass Mode is enabled, the Ignition Module still relies on the ECM to determine Crankshaft Position (RPM). The Module will advance the timing on it's own in this state of operation, but if you literally crack open a seven terminal ignition module, between the Bypass Coil and the Signal Converter (which in this state ignores the ECM, and connects the ignition coil to the signal converter directly, with no intervening by the ECM), you will find the Crankshaft Position (RPM) line still being connected though during Open Loop....
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Last edited by Street Lethal; 06-28-2008 at 03:21 PM.
Well the large 4 pin module probably would not, BUT the 1983-1984 S10 with the non-feedback carbed 2.8 had a very small 4 pin module that they used.
Looked through the referenced thread concerning latency. The latency figures there seem MUCH higher than what I observed on my distributor w/out an ECM. My assumption would be that it is the ECM which introduces most of the latency referred to in that thread.
Thanks for the info on the S-10 module, but I have decided that I like the module I have now. The 7 degrees of advance is enough that I can run all the advance I want and not have starting issues.
