no tach signal when first cranking

[wayneandcathy]

MS II, v3 board, MS tune 2.9. I have a 1984 Avanti with a SBC 305 with cross fire injection. Yes that is stock.
I built the Magasquirt kit and stimulator with no problems. It all worked well on the stimulator. But I originally had it set up to use the 7 pin hall sensor module that was standard in the distributer. Never could get a tach signal. So I converted it to just the VR sensor. Wow did it sound good when it started right up. It runs well too. But it has a quirk. When first cranking there is no tach signal, but if I stop cranking but leave the ignition on then start cranking again, it has the tach signal and starts right up. I have been hesitant to mess with the pot adjustment since it runs well otherwise. I have logged the engine running and see nothing obvious though I don't really know what it should look like.
Also, another problem, maybe related: I installed the USB card in stead of the DB9 as per instructed. All works well on the stimulator and with the ignition on when running the tuning soft ware.
But if I start the car Tuner Studio will monitor every thing just fin for maybe 10 or 15 minutes then it gives an error warning and disconnects. I can unplug the USB cable and plug it back in and it will reconnect again for a while. At first it would only stay on for a few seconds. I tried a noise filter, no help. I had several things added to the proto area. I thought maybe I had a ground loop, so I removed them. All that is there now is the circuit for the knock sensor. Once I did that it stayed on for a while. The knock sensor might have a problem. The engine idles smother when the sensor module is plugged in even if it disabled.
I have had a great time working on this DYI project and I hope someone can point me in the right direction. Thanks

[Matt Cramer]

USB communication is a lot more vulnerable to noise than RS232; you'll want to shield the cable. A lot.

Do you have a data log of the startup issue you could post, along with your MSQ?

[wayneandcathy]

thanks Matt for responding. Here are the data logs the best I could get them with the shut downs. I don't Know what an MSQ is? Meanwhile I see if I can get a better USB cable. Any shielding I can do inside the case?

Thanks Wayne

[DonTZ125]

The tune file read by TunerStudio has a .msq file extension - can you post it up here?

The cable itself is what's absorbing the noise, acting like an antenna. Shielding in the box won't help, you need to shield along the run of the cable.

[wayneandcathy]

Here you go - thanks

[Matt Cramer]

Battery voltage is a bit unstable - where exactly is the MS grounded? Low impedance injectors are pretty unforgiving if the grounds are even slightly marginal.

Do you have a data log capturing the problem of a lack of tach signal?

[wayneandcathy]

Good morning Matt. From DB37 #s 8,9,10,11,& 18 are each grounded separately to the engine block. All sensors including the knock sensor go back through #19. The injectors are low impedance, 3 ohms each. I will try to get a better log while cranking but until I solve the communication problem its hard. Thanks Wayne.

[wayneandcathy]

Hi Matt I got some more data. I disconnected the injectors so I could get a longer cranking log. With the injectors disconnected I did not get a communications shut down.
I checked batt. voltage with my muitimeter: At the battery 12.18 volts, at the battery cranking 10.24 volts. At the ecu 11.70 volts and at the ecu cranking 9.14 volts. New cranking log attached.

[Matt Cramer]

I would try this mod to reduce the noise from low impedance injectors.

1. Cut the center leg on Q9 and Q12 or desolder it so it does not reach the PCB.
2. Run a length of jumper wire from the center leg of Q9 and Q12 to an unused pin on the DB37 connector.
3. Wire this pin to a 12 volt source in the wiring harness, separate from the Megasquirt's power supply wire.

[wayneandcathy]

Thanks Matt. This sounds easy enough. I assume this source would be hot in run and start. Is there any advantage to low impedance injectors? They are a lot more trouble than high impedance injectors. I remember in the v3 board assembly instructions it said you could add resistors to low impedance injectors to change them to high impedance. Is this a good idea and if so how many ohms and watts would you use? Thanks Wayne

[Matt Cramer]

Correct, the source needs to be hot in Run and Start.

The main advantage of low impedance injectors is that they are (or were) cheaper in high flow rates. If you want to use resistors, try a 3 to 5 ohm, 25 watt or higher resistor.

[wayneandcathy]

I think I will go with the fix to Q9 and Q12. Somewhere in the literature I saw a fix for noise for low impedance injectors with a resistor and capacitor in series across the injector terminals. Would this help as well? What parts would I use? Wayne

[wayneandcathy]

Hi Matt. I got the modification done to Q9 and Q12 on the MS-II. It helped steady the tach signal. It maybe wanders around 30 RPS now, and the USB communications with Tuner Studio is much improved. No change to the lack of tach signal when first cranking though. I checked the voltage at the VR while cranking and got 3 volts AC. I put a new shielded wire from VR to ECU. Still no change. I wonder if I have the ignition sittings right for an MSD? Do I need to jump to another forum for that issue? I have it set up like the manual says, but I see some discussion about if that is correct. Thanks Wayne

[Matt Cramer]

Try increasing the skip pulses and see if this avoids RPM noise while cranking.

[wayneandcathy]

Hi Matt. It has been a while since I have been able to work on this problem. Had to fix a gas leak in the tank. I tried increasing the skip pulses from 3 to 5. Did not seem of have much affect. I can better describe what is happening. If I get in the car and just start crankin,g the engine it will not start because there is no tach signal. If I turn the ignition on and wait 5 to 10 seconds, then crank it will immediately have a tach signal and start. If I hear the second injector prime shot, I know its good to go. Its like a transistor needs to warm up or a capacitor charge up or something. Is that possible with solid state? Other wise the car is running very well. Thanks, Wayne

[Matt Cramer]

Does this delay also happen on a Stim?

[wayneandcathy]

No, everything works fine on stim. I immediately have tach signal when I plug it in. Can I imitate cranking somehow on stim? If the engine dies and I crank without turning the ignition off, it starts immediately. If I turn the key off first, I have the delay. Engine hot or cold - the same.

[Matt Cramer]

It sounds like the problem may be with the sensor.

[wayneandcathy]

I have wondered. When I could get no signal through the HEI 7 pin, I checked the voltage of the VR sensor and got 3 volts AC. But I did not notice if it was slow to come up. They are not too expensive. I guess I will try changing it. Thanks

[wayneandcathy]

Bingo! I finally got around to replacing the pickup sensor. Problem solved.
Thanks for the help.
Wayne