tach drops under boost but not in TS: diypnp tach output problem? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 09-04-2011, 02:01 AM   #1
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Default tach drops under boost but not in TS: diypnp tach output problem?

so I've been trying to look up people who have had a problem with their tachometers dropping while under boost, however all I am finding are people whose megasquirts are resetting. My car works just fine, and my megasquirt doesn't reset (been logging and I don't have any RPM dips when I'm stepping on it).

The tach will drop to about 1 or 2k while I'm doing an almost WOT run. The car runs great, no problems at all. No loss in power when the tach goes wacky. Is there something wrong with my tach output on the diypnp? This ONLY happens when I'm in boost, and I've only hit about 8 or 9 psi max. besides that, I'm having a blast tuning this thing, I just wish I could tell what my rpms are going. If i'm not in boost, and I rev the engine up or shift late, the tach never does anything weird and acts normal. I'm stumped
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Old 09-04-2011, 11:29 AM   #2
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90-94?
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Old 09-04-2011, 11:41 AM   #3
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It's a 94
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Old 09-04-2011, 02:02 PM   #4
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Then the tach signal does not come from the ECU, it comes from the ignition coils themselves.

Specifically, the black / white wire at each of the two coils is commoned together, and that signal then feeds the tach, the relevant pin in the diagnostic connector, and it also goes to the ECU (as IGf) however this signal is not used by any MS.

Since the MS's "tach" signal (as displayed in the logs and on MegaTune) is derived from the CAS signal, and the tachometer in the dashboard is driven independently of that, this is why you are seeing the discrepancy. It sort of narrows down the list of suspects.

What's your plug gap set at? I'm suspecting that the coils themselves just aren't up to the task.
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Old 09-04-2011, 02:59 PM   #5
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I had had this, and I cleaned the ALT ground where my megasquirt and wbo2 grounded, and then found a loose coil bolt. After patching these up, it went away.
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Old 09-04-2011, 03:20 PM   #6
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Disable the tacho output signal in the MS and see if it goes away.
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Old 09-04-2011, 05:07 PM   #7
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right now the plug gaps are set at .30-.33 My grounds are pretty clean, especially the one my diypnp is hooked up to along with the gauge. I will have to double check the ground by the coils. I'm hoping my coils aren't just too old to handle it, but the thing is I don't notice any performance impact on the car, beside just the tach. No bucking or weird noises. Exactly what I expect from when I'm stepping on the throttle

tacho output was already off

Also, when i'm stepping on it, and the tach starts dropping, if i let go of the gas a little, and I mean, just a tiny bit, it'll stop doing it too. Haven't checked if i'm still in boost at that time though. Guess I'll double check the grounds again and replace the plugs just in case.
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Old 09-06-2011, 03:22 PM   #8
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Quote:
Originally Posted by Reverant View Post
Disable the tacho output signal in the MS and see if it goes away.
I have a hard time with this one. If the MS were connected to the tach wire, I'd expect this problem to happen all the time.




Quote:
Originally Posted by bmxfuel007 View Post
Also, when i'm stepping on it, and the tach starts dropping, if i let go of the gas a little, and I mean, just a tiny bit, it'll stop doing it too.
For what it's worth, my totally stock '90 does this sometimes. I can be cruising down the highway at 70 MPH, and the tach needle will drop down to 3,000 RPM, hold steady there for a second, and then pop right back up to 4,000. I've heard of other people having this same problem.

Never managed to figure out why, since it's so damned intermittent. Either the coils / igniter aren't producing a clean IGf or the tach itself is just crapping out.
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Old 09-06-2011, 03:36 PM   #9
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Gap down to .025 and see if it goes away. If so, you need to change the ignition system for something a bit more potent.
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Old 09-07-2011, 10:39 PM   #10
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so I've gapped down to .25 and it's still the same. I've actually ordered a replacement coilpack just in case. Going to do some cleaning soon once the car cools down. should I be looking into COP?
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Old 09-16-2011, 04:19 AM   #11
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so cleaning out the grounds, replacing my coil packs, and gapping my plugs smaller haven't helped anything. Should I redo the wiring from the coils to the gauge cluster? or is there an easier way? if I went to COPs, would it still have the potential to have this same problem?
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Old 11-23-2011, 09:26 PM   #12
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so an update: I'm not using Toyota cops. I've got two 4700uf caps wired in parallel to my GND and +12v. 1k ohm 1/4w resistor in the diag box.

I'm experiencing a weird thing in my tach, but it's different than before. If i'm around 2700-3500 rpms, and I let go of the accelerator, my rpms actually rise and bounce around before dropping. Also if I'm idling and I'm experiencing a load, like if my cooling fans come on, or if I turn on my fan inside, the tach needle will also rise and bounce around. This is all separate from what I'm seeing in TS.

No rpm deviation in TS. No driving problems or cutting out just like before. Car drives excellent. This is at least much better than before as I can see where my RPMS are when I'm stepping on the gas. This only happens when I have my foot off the accelerator. If my tach is going wacko, I step on the gas a little bit and the needle goes to exactly where it's supposed to be.

Any clue what the heck is going on? I'm going to extend my coil pack ground to reach the ground near the TB hoping it'll solve it. Other than that, I'm wondering if I am eventually going to have to use the TACHOUT off the MS
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Old 12-13-2011, 06:46 PM   #13
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ok, sooooo. I've been dealing with this for awhile, but after cleaning grounds, doing a COP conversion, testing my bro's new 94M's tach in my car, I'm still getting the same problems where the tach will be erratic either while in boost, and sometimes when I take my foot off the pedal.

If I use the TACHOUT on my DIYPNP, do I just open it up and solder in the wire for the pullup and disconnect the tach signal from the cops and it should be good? do I still need the resistor in the diag box?

I'm suspecting wiring issues in my stock harness so I'm just going to bypass that instead of trying to find the problem in the harness. Or do I actually have to run a wire from the diypnp to the tach manually since i'm a 94 with a coil driven tachometer?
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Old 12-19-2011, 01:11 AM   #14
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What is your dwell set to? Does it do it at high rpm's or only in high boost?

-Lars
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Old 12-19-2011, 01:34 AM   #15
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my dwell right now is at 3.5ms for cranking, 2.5ms for maximum dwell at 12v

The dipping of the tach only happens in higher boost. I can go to redline no problems if I keep the car even in low boost, as in 0-7 psi of boost
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