Another BII 4.0 tach question

[POINTMAN]

For those of you who have swapped edis int your broncos and kept your first gen dash, exactly how far off do they read? mine seems to read about 400 rpm too high at idle (assuming it is idling at 700- 750). Is that in line with what others have found. Does that deviation stay consistant throughout rpm range? Or does it get further or closer in higher rpm?

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[adsm08]

My tach seems to read accurately to 2000 RPM, and then tops out there. For some reason being in 1st or 2nd will get it up over 2000, but I doubt the accuracy at those times.

I have read that using an older ECM, pre-93 makes the tach read correctly.

 

[POINTMAN]

Are you saying that your tach is only accurate to 2000, or that 2000 is literally as high as it will go period? You could have something there about year model as mine is from 94 explorer. Mine will read throughout entire range but it shows to idle at 1150-1200 rpm but it sounds and feels like it idle normally?

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[adsm08]

My needle rarely goes up past 2000 RPM.

And I don't know how it could be a year/model thing. I have an 87 Ranger, with factory tach, and I'm using the wiring and computer from a 94, same as you.

 

[kunar]

When I swapped in a 4.0, I usdd my original 87 cluster with 90-91 wiring and computer. Tach reads exactly 1.5x actual. Never did figure out why, its been that way for years.

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[POINTMAN]

Well, 750 rpm x 1.5= 1125 so that leads be to believe yours is very consistant with mine. How are you measuring "actual" rpm? My dmm is supposed to have an engine tach feature and i wondered if the waste spark from the edis would throw it off just like it does the factory tach. Guess i need to get off my butt and go see.

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[kunar]

Actual rpm was taken from from the computer using a live data reader.