External Tachometer connection

Richard Hartley · Post by **Richard Hartley** » Mon Aug 12, 2002 6:01 pm

Hi folks,

New to the list. Normally I'd lurk a while, get a sense of things, before
diving in, but I got a bit of a pending question (or two) that I'd like to
get resolved before I do something stupid (i.e. expensive).

After many a year with my head under the hood of quite a few computers I
figured it was time to wrap my head around the joys of infernal combustion
with my 1990 Eunos Roadster. (I was told that it was a limited edition
model, but I don't know how much I can trust the guy who sold it to me.)

Well, when I got the car, the guy threw in a copy of "Ron's Miata
Enthusiast's Shop Manual" (Veloce Press, 1998). Was reading through on how
to check the timing and was told to get an external tachometer. So picked
one up with a xenon timing light. Ron's says to connect the "tachometer
probe to the IG terminal (in the Diagnostic Connector). If the tachometer
requires an external power feed, connect it to the blue single-pin power
connector." It goes on to talk about jumping the TEN and GND terminals to
put the car into self-test mode. Similar instructions are there for
connecting the timing light.

Now, I've read a couple of good options for the timing light placement, but
I am really worried about putting the Positive lead of my Tach on a 12V
output and then plug the Negative lead into the Diagnostic Connector which I
can only assume has a feed to the ECU.

Everything I've learned about computers is making my skin crawl at the
thought. Is Ron right? Or am I gonna fry something important if I do this?

Richard

Nigel · Post by **Nigel** » Mon Aug 12, 2002 6:12 pm

Trust the book.

There is also a good article in the garage section on miata.net - under the
ignition section from memory.

JST4FN

Nig.

Richard Hartley · Post by **Richard Hartley** » Mon Aug 12, 2002 7:45 pm

Nigel,

Thanks for that. Unfortunately, though it mentioned an alternative
placement for the timing light, it didn't go into any specifics about
connecting the tach. Also, I found a few very good threads in the archives
here. Either way, next stop DickSmiths for a couple of probes and jumper
leads.

Richard

Fletch · Post by **Fletch** » Mon Aug 12, 2002 10:32 pm

Richard,

Connect the positive connection of the timing light or tacho to the blue
connector behind the left head-light...

Connect the negative connection to any suitable ground point (I often use
the un-painted nuts near the suspension towers).

Connect the input signal of the tach to the IG pin in the diagnostic
connector...

Good luck,

Fletch.

mcheevy · Post by **mcheevy** » Tue Aug 13, 2002 7:39 am

Fletcher,

Unfortunately, my tach doesn't have a probe, just positive and negative clips (well, their red and black). Something tells me I should take it back where I got it from and get a better one.

Richard

[...]

Fletch · Post by **Fletch** » Tue Aug 13, 2002 7:45 am

Not neccesarily...

You just need to read the instructions, and figure out whether it expects a
constant +12V and a RPM-switched ground... or a constant ground and RPM
switched positive signal...

Fletch.

lou Girardin · Post by **lou Girardin** » Tue Aug 13, 2002 8:07 am

I know it's not the done thing, but I use the car's tach.
Lou

jeff · Post by **jeff** » Tue Aug 13, 2002 10:34 am

Richard,

I'm a bit puzzled about the tacho thing - if you want to check the idle/timing, then jumper-ing TEN+GND will lock the idle at 850 rpm. If you want to check the normal idle speed, a good trick is to alternately jumper/unjumper the two terminals and listen for any change in engine speed, then adjust the idle screw until there's no difference.

I set my timing w/o a timing light, advancing it for power, then backing off for knock. Just "Use the force Luke!"

I only adjusted the crank angle sensor ~1mm. (not quite kosher tho', and no #8 wire involved)

mcheevy@rhpositive.co.nz wrote: Fletcher,

Unfortunately, my tach doesn't have a probe, just positive and negative clips (well, their red and black). Something tells me I should take it back where I got it from and get a better one.

Richard

[...]

Gary Morrison · Post by **Gary Morrison** » Tue Aug 13, 2002 11:06 am

When I do timing I just use the tach in the dash. You need to go around and
look at it and then back, but it does the job for me. You're just shooting
for 850rpm at idle.

Gary

lou Girardin · Post by **lou Girardin** » Tue Aug 13, 2002 3:34 pm

AFAIK, it does not lock the idle speed. at least on my 1.6.
Jumping TEN/GRD isolates the various sensors and ISC controls so that the base
idle can be adjusted.
Lou

Fletch · Post by **Fletch** » Tue Aug 13, 2002 7:54 pm

I'm with Lou... jumpering Ten to GND doesn't lock the idle-speed to 850...
it turns off the ECU's idle compensation circuitry, and lets you adjust the
"base idle" speed... which will only be 850 if you've adjusted it
correctly...

Fletch.

Richard Hartley · Post by **Richard Hartley** » Wed Aug 14, 2002 6:05 pm

Well folks... here's the skinny on the Tach.

Ron's (the manual) was right. IG diagnostic plug worked like a charm. The
El Cheapo tachometer I got (Tisco) only had markings for 6 and 8 cylinders.
So just read the 8 cyls count and divided by 2. Good thing to! Damn thing
was idling at 400rpm! Got it up to 850-900 range, turned off the engine,
removed the jumper (GND/TEN), restarted and was delighted to heard the
engine sound almost exactly the same. Too bad Tisco's timing light stank
like a dead crab.

Richard

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