Dead tach, RPM signal to megasquirt ok

[mikesarge]

Hi all-

I've just about got my megasquirt EFI/ITB system installed. Still waiting on a temp sensor, but otherwise we're close.

I did start it once, and while cranking the tach pops up around 1200-1500 RPM like normal, but once the car is running, the tach is down at 0. However, megasquirt (MS2, V3.57) shows an accurate RPM signal, and a blip of the throttle shows an accurate reading on tunerstudio just fine.

Could tapping into the black/violet wire in the back by the CDI connector introduce too much signal noise for the tach? Did I kill my tach somehow? any other ideas? The fact that it bounces while cranking has me stumped, because it doesn't seem totally dead. How should I go about diagnosing this one?

[timmy2]

How did you tap into the purple/black wire?
Did you somehow cut the wire that goes through the 14 pin connector and only feed the MS ECU?
Tach likely jumping on start when it sees gauge power applied.

[mikesarge]

I cut the wire between the CDI and the 14-pin, stripped it back, then put in a heat-shrink crimp connector with the MS RPM signal wire on one side. I had tried to use a razor blade and a pick tool to make a small hole in the wire without severing it, but I wasn’t satisfied with the connection so I went with the cut and crimp method.

The wires seem firmly in place in the crimp connector on both sides and the heat shrink has not been… heat shrunk.

[timmy2]

Do a continuity test on the purple/black wire from the 6 pin connector to the 14 pin female.
May be a bad splice.

[mikesarge]

We have continuity between the CDI and connector, so the splice appears good. Is it possible something I did with the wiring might have otherwise killed the tach? Would be a bummer if so, because I just paid north Hollywood to rebuild it not even 2k miles ago..

What’s my next best bet here? Is there a good way to test the tach itself?

EDIT: I see other posts noting I can buy and try a 12v square wave generator. Guess it’s one more thing to put on the shopping list.

[Jonny H]

The CDI TD output pulls the tacho input down to ground to trigger the tacho. The tacho has a pull up resistor.

There is probably a pull up resistor in the MS input circuit too. By adding the splice, this resistor is effectively now in parallel with the pull up in the tacho, reducing the value so could now be too 'strong' so not pulling it low enough for the tacho to 'see'.

Is there anything in the MS docs about removing or changing the pull up resistor?

[spuggy]

My money is on Jonny's explanation. You might be able to figure out values for additional resistors to add so everything is in spec/works. Not a good approach.

Mike,"megasquirt" is pretty non-specific. MS2? MS3? PCB version? Specifically, does your MS have a tach output, can you jumper it to provide that, or does it require soldering flying leads and additional components on?

MS2-Extra Hardware Manual

A Pelican driving his SC tach from the MS tach output here Megasquirt Tach Wiring

As the Megasquirt 12V square wave tach output is exactly what a 911 tach from late-middie up to 993 wants for input, seems the obvious thing to do - drive the tach directly from the MS?

[mikesarge]

Spuggy and Johnny- thanks for the additional help here.

MS2, V3.57. I’ve seen the diagram in the megasquirt tach wiring thread, and to be honest, I can’t make heads or tails of it.

My harness is from DIYautotune. Harness itself does not have a tach out indicated, and the instructions I received (including after asking the clarifying question) just say “tap the blk/purp wire on the CDI to tach, and wire it to pin 24,” so my understanding is that no resistor should be required here, and simply tapping the wire shouldn’t introduce any signal but instead should just accept the input.

The manual indicates I can use the wires I’m currently using for an IACV for a tach out- I’d like to keep idle air control, so unless absolutely necessary, I’d like to avoid that.

One note: the instructions and car-specific diagram I have from the vendor do not have me grounding the black wire to pin 1. This was also a clarifying question I asked, and was told not to use the twisted pair, which is confusing since pin 24 is in fact the white wire in that twisted pair.

Maybe I should take this all to my vendor?

[al lkosmal]

Mike,
Feel free to email me...so we can clarify

"I cut the wire between the CDI and the 14-pin, stripped it back, then put in a heat-shrink crimp connector with the MS RPM signal wire on one side". . Did you tap into the blk/purp for the incoming ECU input signal and also keep the bal/purp connected to the tach....or did you only connect it to one side....i.E. the ECU? Have you checked continuity of the blk/purp at the tach?

The MS/DIY wiring diagram is a generic diagram, showing the most common features/wiring etc and I provide it for reference. The sensors that it shows for speed input are VR (2-wire and Hall (3-wire sensors) that use both the black and white wires in the twisted wire/shielded cable........I did provide you with a Wiring diagram, specific to your application and we are using the stock tach input (blk/purp) signal as the speed signal into the M'squirt and, as you can see, it is driving the ECU just fine. It aslo will continue to drive the Porsche tach....this is a common way to provide the input to the M'squirt and I've used this method many times and not seen it fail yet with the stock tach..Why yours is not working properly is a mystery, but we'll figure it out....please check continuity at the tach itself to verify that there is no open circuit between the 14 pin connector and the tach. Just a thought, but what work did Hollywood Speedo do for you? Any chance they changed the tach circuitry?

Anyway, I'm just an email away and glad to help..



regards,
al

[mikesarge]

Originally Posted by al lkosmal

Mike,
Feel free to email me...so we can clarify

"I cut the wire between the CDI and the 14-pin, stripped it back, then put in a heat-shrink crimp connector with the MS RPM signal wire on one side". . Did you tap into the blk/purp for the incoming ECU input signal and also keep the bal/purp connected to the tach....or did you only connect it to one side....i.E. the ECU? Have you checked continuity of the blk/purp at the tach?

The MS/DIY wiring diagram is a generic diagram, showing the most common features/wiring etc and I provide it for reference. The sensors that it shows for speed input are VR (2-wire and Hall (3-wire sensors) that use both the black and white wires in the twisted wire/shielded cable........I did provide you with a Wiring diagram, specific to your application and we are using the stock tach input (blk/purp) signal as the speed signal into the M'squirt and, as you can see, it is driving the ECU just fine. It aslo will continue to drive the Porsche tach....this is a common way to provide the input to the M'squirt and I've used this method many times and not seen it fail yet with the stock tach..Why yours is not working properly is a mystery, but we'll figure it out....please check continuity at the tach itself to verify that there is no open circuit between the 14 pin connector and the tach. Just a thought, but what work did Hollywood Speedo do for you? Any chance they changed the tach circuitry?

Anyway, I'm just an email away and glad to help..



regards,
al

Hey Al- will follow-up via email. Continuity is confirmed on both sides of the splice, as seen in the photo above. You can actually see the white wire and the crimp connector in that photo too, to the left of my hand. I’ll see what I can do it confirm continuity all the way to the tach. Not sure how I could have interrupted the signal with the work I did so far, but stranger things have happened.

I had N. Hollywood add a damper, and go through the tach, since it was bouncy. It worked fine before, for the last couple thousand miles.

I appreciate your help here, and via email. Between this, and the other issues we’re debugging, it’s a lot to juggle but I’m hopeful we’ll be on the road shortly.

Thanks
Mike

[spuggy]

Originally Posted by mikesarge

I had N. Hollywood add a damper, and go through the tach, since it was bouncy. It worked fine before, for the last couple thousand miles.

"worked fine before" - until you added the Megasquirt, right?

My spidey senses tingle/the mystery deepens... What's this "damper" NHS added?

As both Al and DIYautotune seem to expect that the wire you're splicing into can drive the stock tach and the MS without issue - and you've got both issues AND a modified tach.... Well...

(connecting the dots left as an exercise for the reader, LOL...

If it were me, I'd be very inclined to swap in a standard/unmolested tach quickly and see if that changes anything?

[mikesarge]

Originally Posted by spuggy

"worked fine before" - until you added the Megasquirt, right?

My spidey senses tingle/the mystery deepens... What's this "damper" NHS added?

As both Al and DIYautotune seem to expect that the wire you're splicing into can drive the stock tach and the MS without issue - and you've got both issues AND a modified tach.... Well...

(connecting the dots left as an exercise for the reader, LOL...

If it were me, I'd be very inclined to swap in a standard/unmolested tach quickly and see if that changes anything?

Yeah- worked fine on the stock CIS system for several months/thousand miles.

The damper they put on it just makes it less bouncy- I think it’s replacing a factory/worn piece.

I don’t believe the damper does anything electrical- it shouldn’t present an issue here. I’ll add it to my mental list of possible suspects though.. interesting idea.

[mikesarge]

Alright gang. I’m back on this. I have more questions than answers at this point.

This thread has me thinking I may have fried my tach when it was bouncing wildly, due to a loose battery terminal on my first start with the new EFI system, something I didn’t think much of at the time, because I simply tightened the terminal and moved on to the other issues I was having. Link: SC Tachometer wiring - what's this black wire?

I bought a signal generator from Bob Ashlock, and my tach tests fine. Likewise I bought a tach adapt, which does not fix my problem. Megasquirt reads the RPM correctly. Tach occasionally will bump while cranking, but never while running.

I have continuity from CDI to tach along TD wire. I have 12v at the tach. I have a good ground at the tach. I have verified that the ‘damper’ installed by NHS is mechanical, and would have no impact on this issue.

Also, if I unplug the TD wire while it’s running, the car dies- presumably because the signal is no longer grounded at the tach, so megasquirt loses rpm sync.

I have much frustration.

Does anyone have any other ideas?

[al lkosmal]

Mike,
I was reviewing your last log....and I am thinking you may have a Battery/Alternator/regulator problem.......

I noticed that the battery voltage in the log was very spikey and looks to be unregulated with a range from 10.7 to 14.7 V. I often run batteries low while I'm testing/tuning....but this spikey battery input looks to be an issue that needs to be addressed.




not sure that this has anything to do with the tach operation, but it is not a good thing

Note: tapping into the blk/purp tach wire has been done many times and has worked well.....I.E. has not interfered with the Tach/gauge operation.

[timmy2]

Maybe try using the “Bitz Racing” tach adapter that runs to the positive coil terminal positive wire instead of the black/purple tach out wire of the CDI. (Adapter reduces the signal voltage to a size the ECU can use)
I have mine set up that way. (Same ECU, MS2 3.57) and it works just fine.

If the wire you connected to the black/purple is too short to reach the coil, you can just splice it all in at the white wire in the 6 pin CDI connector.

Reach out to Tony Bitz via his website and ask if there are any other changes you need to do for jumpers in the ECU.

[mikesarge]

Alright. Back at this.

I replaced the voltage regulator, the voltage signal at megasquirt is still bad. I think it’s interference from having the fuse panel right by the distributor. The voltage is fine at the battery, and elsewhere in the car. Either that or it’s the positive circuit I salvaged from the rear blower for switched power to the megasquirt and injectors.

Now, I’ve got the oscilloscope fired up and it’s showing a pretty dismal, lumpy wave form. I think this is my problem.

I can’t test it without the megasquirt rpm wire attached, because it’s required to run the car.. likewise, the Tach-Adapt isn’t working to fix the curve, probably because the wave is so ****ty. so I’ve reached out to Tony Bitz, as Dennis suggested, and I’ll try an alternate tach signal arrangement, and see if that cleans up the signal enough to get both the megasquirt and tach a workable signal.

I’ll keep this post updated as I progress here.

[mikesarge]

I think we can mark this one “SOLVED,” or at least “FIXED.”

I didn’t completely diagnose what’s going on, but I moved the megasquirt tach sensor wire to the signal-out side of the Ashlock TACH-ADAPT I picked up, and everything works just dandy now.

I’m guessing the signal coming from my CDI is either noisy, or failing, or something such that coupled with the draw from the megasquirt, the wave is too deformed by the time it gets to the tach and/or tach adapt, those parts can’t work with it. So, by conditioning the signal first, the result is a clean enough wave that both MS and my tach can work.

Anyway, thanks to everyone for the help on this one.

[timmy2]

Cool, that is a unique one.
Might want to have the CDI gone through to correct any further degradation.