RPM Output/Tach Signal issues - Miata Turbo Forum - Boost cars, acquire cats.

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Old 06-25-2013, 12:52 AM   #1
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Default RPM Output/Tach Signal issues

I'm using a Hydra 2.7, but I've converted completely to the 1ZZ COPs, sequentially driven via the Hydra. I wired the 4 tach outputs from the 4 coils together, and wired that into the yellow/blue wire that feeds the stock tach, which works great.

The Hydra (and the AEM) both had an RPM output that used to go out on the white/black wire, which I think originally went to the ignitor. I've been driving my Racelogic unit off that RPM signal for a while. However, it had always been flaky on the AEM, and when I switched to the Hydra, it stopped working all together.

When I checked out the RPM signal on the hydra, it seemed to be dropping out above ~2700 RPMs. I don't know if the AEM had a problem like that or not, but the launch control rev limits used to at least work closer to 4k, so I don't know when it cut out in the past.

So, I assumed there was noise or something on the line, tried moving it around physically, tried transferring grounds both on the ECU and on the Race Logic. No dice.

Next, I next tapped into the yellow/blue tach output line from the coils. When I threw that at the race logic, it returned no RPMs ever. Damn. That should have been the easy button.

Last, I tried just tapping into one of the coil charge outputs from the ECU. It liked that, but I had to set it up with a 4x spark multiplier (since it was only getting one cylinder's worth of sparks), which is probably not ideal. Racing the next day, it seemed to be working pretty well, but I'd rather get back up to something with a faster update rate (all 4 coils).

Is there anything funky with the Hydra's RPM output that might be causing this? I haven't put a scope on any of them yet -- need to dig it out of storage and find a few hours to find all my leads and such. Do I need a pull-down or pull-up on the tach outputs from the 1ZZ coils or something?

Just looking for any info that might help here. Thanks!
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