I just bought an automotive DMM, it has a tach settings for 3,4,5,6,8 cylinder engines. I want to set the idle etc, etc and would like to avoid running back an looking at the dash to do it

QUESTION: would any of these settings give me the true rpm on my '91 N/A?

Also, where would you connect the beast to take the reading?

 

Somebody will correct me if I am wrong, but it seems to me that the rotary is equivalent to a v6. So I am assuming you would use 6.

 

I think it would depend where you hook it up. If you hook it up to the trailing you have to selected 4. I hink if you hook it up to the leading is six because of the wastespark. Use the trailing coil, is more stable.

C

 

what about 3 because the rotor fires 3 times per plug per revolution...

 

Trailing - 3

 

The test connector at the trailing coil (the furthest front coil set) will give you the reading of a 6 cyl. This is what I have used for remote starts that monitor RPM, and it is generally very accurate.

 

I'm at work, so I can't look at them, but are you referring to the single
green colored test connector that you normaly ground or the ones near by?

 

On both my old 87 and my 88, its a single small round connector, with a Yellow/blue or yellow/black wire. The connector was clear when new, but now with 130K as I recall they look rather dark or even black.

 

OK, here is the theory on this. For every revolution of the eccentric shaft, each rotor will fire once. This is a total of 2 firings/revolution. On a 4-cylinder boinger, 2 of the cylinders will fire once for each revolution (otto cycle, each cylinder fires every other time it reaches TDC). Therefore, 2 firings/rev = 4 cylinder, all day and forever, even! On the 89+ cars, both the leading and trailing coils have a tach wire on them. I believe that both of them output at exactly the same frequency, since the trailing and leading trigger signals are the same, with the exception that the trailing coil has an additional select signal, to pick the coil. I believe that the tach is derived from the timing signal, not the select, and they are therefore the same, that is 4-cylinder equivalent.

Well, that's my story and I'm sticking to it!!!

Irv, Keith's dad

Oh yeah, if you are going to measure tach right off of the coil, you must use leading (front) coil, since each trailing coil fires only 1/revolution, the equivalent of a 2-cylinder!

 

4 cyl is the setting you need

 

Soo, which is it? 4 or 6cyl? The DMM I'm using doesn't say anything about rotary (of course) but has 3,4,5,6,8cyl options and it says to multiply the reading by the number of cylinders selected.....
**I'm using the induction coil/signal pick-up clamp ("clamps" loosely around spark plug wire to detect the current) provided with the DMM

 

lol 2001

 

Hey, I used the search function. The function EVERYONE complains about people NOT using. I want answers, was thinking I might get some from this thread. I DGAF if its 12yrs old. Why should I start a new thread?

 

Post #6..
Try it on 6 cylinder setting.
Induction clamps are tricky too,so you gotta set them "just right".

 

Set it to "4 cylinder", that's what will read correctly.
The Miata tach in my cluster (from a 4 cylinder, obvs) reads perfectly using the stock RX tach signal.

 

Rotary Tech Tips: Tachometer & Spark Plugs

 

a two rotor has a spark event every 180 degrees of crankshaft rotation, just like a 4cylinder. a 3 rotor has a spark event every 120 degrees like a 6 cylinder.

 

Right on! Thx guys! -I assume that I still multiply by 4 as well? Or? -Tryin' to verify if my starter is cranking fast enough....don't believe it is...