Loss of RPM Signal? TFI using VR/Optics Input

[mrselfdestruct1994]

Hey guys,

I posted in another thread recently about a strange problem I have been having with my MS2 running 2.6 Firmware in my 6 Cylinder w/Ford TFI. In my attached datalog, you can see that at entry 3379 the rpm spikes to some seemingly random number, hold for a brief moment, and then return to normal. During this period the car feels as if it has stalled and then will continue running again quite abruptly. The only thing amiss that I can see in the log is the dwell which spikes to 406.9. This appears to happen randomly when driving although I have noticed it definitely occurs more often after I have just started the car on a cold morning. It also appears to be more frequent at higher revs, but I can't be sure that's not a co-incidence.

In the course of troubleshooting this I have replaced my entire distributor (which made no difference) and tested the PIP signal at both the dizzy plug and the wire which connects the pin to the board. I am using a mono headphone jack and voltage divider to record the signal as an audio file on my laptop which probably isn't the best but you can definite see in my attached .wav that there is no obvious loss of signal, and no dramatic changes that I can see. This leads me to believe the dizzy and wiring is probably OK, but let me know if you disagree as I'm no expert.

My friend who built the Megasquirt for me told me he used the VR/Optics circuit for the ignition pickup. I have gotten a bit stuck at this point as I don't know much about electronics, making it difficult for me to test the components on the board. If anyone can offer any advice on how to solve this, please help!

[mrselfdestruct1994]

Edit: Sorry, I should have said I am using the VR input circuit, not VR/Optics. If a mod could fix the title for me that would be great. Thanks.

[Matt Cramer]

Try adjusting the VR trim pots and see if you can make the noise go away.

[mrselfdestruct1994]

Hey Matt, thanks for the quick reply.

Unfortunately I have to confess that I have a big mistake in my previous posts. I went away to try adjusting the potentiometers as you suggested and couldn't find them on the board, so I called the friend of mine who assembled the MS2 for me and it turns out I misunderstood him when I spoke to him last and we are NOT using the VR circuit, just the regular hall/optical circuit. I am extremely sorry for wasting your time so far but I am a huge confused person when it comes to electronics and I am having trouble getting a grasp on the assembly sections of the MegaManual. That said, do you have any other suggestions now that I have cleared up my mistake?

I tried tracing the signal from Pin 14 of the daughterboard on the weekend but having my homemade oscilloscope connected caused the car to stall (no rpm signal), not really sure where to go from here.

Thanks again for your time.

[Matt Cramer]

The TFI mods call for an added inline resistor, try swapping out different values within the suggested range.

[mrselfdestruct1994]

Thanks for your help mate, my friend has suggested using a small transistor to try and get a reading from the IRQ pin of the daughterboard so I am going to try that this weekend and see if I can log the issue. Hopefully there will be an obvious problem with the signal. In regards to the inline resistor, I take it you mean the one placed at D2 on the board? He says he thinks he used a 380 ohm resistor there after a bit of experimentation but that will definitely be the first thing I check.

[Matt Cramer]

Yes, that's it, depending on the directions it may replace d1 or d2.

[mrselfdestruct1994]

Cool, I tried reading the signal this morning but even with a tiny transistor it still stalls the car if I try to take a reading from the IRQ pin. I might just try swapping the resistor on D2 anyway and see what happens but on the TFI mods page it doesn't give a recommended range, it just says to use a 1k ohm resistor. Is it safe to mess around with any value below 1k ohm or do I need to stay above a certain size? Thanks.

[mrselfdestruct1994]

More updates: I tried a 680 ohm resistor and the dropouts seemed a little less harsh, so I tried a 380 and it definitely seemed to reduce the frequency of the problem. With a 180 ohm resistor it went a whole day without doing it at all which is a huge improvement and then this morning the issue occured twice on the way to work but was hardly noticeable. I was also driving much more aggressively which usually results in a lot more dropouts than driving below 2000 rpm. Should I try putting a jumper across D2 to see if it goes away entirely?

I am still a little puzzled as to why this is an issue as it has been working for more than a year with a 1k inline resistor but I think the reduced frequency and severity of the dropouts cannot be a coincidence. Thanks again.

[mrselfdestruct1994]

Well after a bit more driving it seems that the improvement must have been a co-incidence as the problem is still apparent. Very dissapointing. I have also tried a 2k resistor thinking that if a smaller resistor reduced the frequency of the problem then a bigger one should make it worse, but there was no noticeable difference. Any other ideas? Thanks.

[Matt Cramer]

It wouldn't hurt to try the VR input as they can be adjusted to filter out noise and can work with TFI.

[mrselfdestruct1994]

Well just another quick update, I tried the Hall Input Mod and used a transistor to drive the opto-isolators internal LED but didn't notice any difference so I swapped it all back, re-soldered all the components on the IRQ path and made sure all the wiring was 100% but still the issue persists. I have also tried spinning the distributor with a drill with the car "on" but wasn't able to replicate the problem. At this point I'm trying to think of anything that could cause the signal loss when the car is actually being driven. Could this be a voltage noise/spike issue? I may try running the car with the alternator disconnected for a short while and see if that makes any difference.

[Matt Cramer]

I didn't see a voltage spike issue, but it wouldn't hurt to try a run without the alternator.