Parallel Install - Tach signal

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Parallel Install - Tach signal

Postby rx79gez8gundam » Sat Nov 04, 2017 10:04 pm

I've been using MS-1 for about 9 years now for a fuel-only application on a SBC with large cap HEI and a Mallory Hyfire 6A digital CD ignition box in my '83 Z28. I've recently swapped the drivetrain into my '88 IROC-Z and am attempting a parallel install with the factory LB9 ECU so as to retain stock control of IAC, A/C idle up, trans lockup, and computer controlled spark. The latter is a GM 8-pin HEI in stock trim. Using the "R" pin of the HEI (purple with white stripe, stock ECU tach reference) for MS tach reference did not work, the injectors failed to fire. Instead I switched to the dash gauge tach output, coil - (white wire on coil). This triggered the injectors, but I'm left with no dash tach. I'm running a v 2.2 board, no mods, purchased assembled and stimulated from DIYAutoTune.

To make things more interesting, I have lost all connectivity with the MS unit. This happened before the drivetrain swap, and is a result of the RS-232 driver failing to allow the USB adapter to function. I have yet to find a solution, but as my current tune ran fine before the swap I hope this won't cause more than a minor nuisance with this install. I understand being able to interface would be ideal, but I'm having to forgo that luxury. If anyone can solve this incredibly handicapping problem as well, it would be GREATLY appreciated!

Most importantly, THE CAR DOES CURRENTLY START AND IDLE. I'm looking for answers to 2 issues;
1. Why does splitting the signal from the "R" pin purple/white stripe wire on the HEI not work for me as it should in MS-2 parallel setups?
2. How do I provide my stock dash tach with a signal since I've shanghaied it's input for the MS?

The MS and stock ECU have their own seperate intake and coolant temp sensors, share the TPS signal (with MS providing the REF voltage), and the stock ECU still has the knock sensor. If I could find a definitive answer as to whether the stock LB9 ECU needs a tach input to run spark or not it would solve my issue (anyone?) I'm also wondering if I need to do anything to share the TPS signal besides simply tapping into the stock ECU's lead for the MS. Does this need a filter or will the signal reach both ECUs ok?

Tech info:
1988 IROC-Z LB9 5.0L TPI factory ECU (controlling all but fuel)
MS-1 V 2.2 PCB (unmodified, controlling fuel only)
.040 overbored 327 with TPI, TH700-R4 lockup trans, factory A/C, full emissions delete
GM 8-pin HEI (stock)
.040 327 SBC roller setup, LT4 GMPP Hot Cam
rx79gez8gundam
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Re: Parallel Install - Tach signal

Postby Matt Cramer » Mon Nov 06, 2017 7:58 am

It sounds like the optoisolator circuit is drawing so much current that it interferes with the signal. Try putting a 1K resistor in line with it.
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Re: Parallel Install - Tach signal

Postby rx79gez8gundam » Sun Nov 19, 2017 1:04 pm

Ok, weather finally cleared up to try a few things;

First off, I FINALLY have connectivity with the MS again, and can tune. It was the Prolific USB driver issue, but has been solved (hopefully) permanently.

I'm still using Megatune as I haven't updated to MS Extra or had a chance to learn Tunerstudio yet. The car starts, idles, and accelerates, but there is a random loss of fueling from the Megasquirt - as if it goes suddenly dead - but only under throttle input. It's random in that it doesn't do it every time, or at the same throttle input each time. WOT does it almost everytime, but not 100% of the time. The other day almost ANY throttle input, no matter how light, would cause it, but "fluttering" the throttle would overcome it, almost like pumping the gas on a stalling carbureted car. The car would run normally at idle throttle without issue. The fueling also immediately comes back once the throttle is released back to idle. It was also intermittant on when it did it, as part of the trip it did it and the other part no problems. With the laptop connected I tested it and when cutting out still had TPS reading, just no fueling. This means MS is getting consistant power and sensor input. The tach drops to zero and goes dark, so I'm leaning towards a noisy or bad tach input (coil negative triggering).

The inline 1K resistor did nothing to give me a good tach signal from the purple/white wire in the stock ECU harness. Looking online I've only found instructions for using GM HEI 8 with MS-II, and the associated mods necessary for doing such, but always in situations for where MS is going to be controlling spark. As I'm using a MS-1 v 2.2 PCB, and doing a fuel-only parallel install, I need to know how to provide a tach input from the HEI 8 purple/white wire instead of the unacceptably noisy coil negative. This will also give me back my dash tach.

As my only other option is to remove the HEI 8 setup and revert to a 4-pin HEI non-computer controlled setup, and thereby lose all functionality of the stock ECU and negate the benefits of the parallel install I am unwilling to go that route. My MS-1 is a good unit that has never given me issues, the only change has been the ignition system I'm trying to trigger it from.
rx79gez8gundam
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Re: Parallel Install - Tach signal

Postby Matt Cramer » Mon Nov 20, 2017 8:15 am

:msq:
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