From: "Lawrence M. Cooper"
I've been reading several threads recently on problems with stock
tachometers and on problems with the Jacob's system. First let me say that
on my ITS prepared 72 240Z, both work flawlessly. Now a little background.
Many years ago I found out that there are at least two types of tachs for
the 240Z and 260Z. One type is a four wire positive trigger tach and
another is a three wire negative trip tach.
The 4 wire type has one wire that sends positive voltage to the tach, one wire is for ground and the other two leads are the trigger loop, one coming and the other going. I can't say off hand exactly where these two feed to/from but they are shown in both my 72 and 73 factory manual wiring diagrams. They are also poorly pictured in these manuals. From past experience I have found that this type of tachometer is very unreliable. I have found this true in Mazdas and
Datsuns.
I have found the 3 wire type of tachometer to be much more
reliable and more adaptive to aftermarket modifications to the ignition
system. The 3 wire type has a positive lead, a ground and a negative trip
lead that runs directly from the negative terminal on the coil. Strangely,
however, I have not found a Datsun wiring diagram or picture depicting this
three wire tach. Has anyone out there seen one?
OK, here is what I have done with my 72 240Z. I removed my old 4 wire tach
and replaced it with the three wire. I ran all new wires to the tach to
assure proper circuitry. One wire from a trustworthy + supply, another to
a nice ground, and the third directly to the negative lead on the coil.
Here is where a problem crops up. Remember those two wires, on the 4 wire
tach, that formed the positive trip circuitry? Well that loop is needed to
keep your car running. Apparently the primary ignition circuitry runs
through this loop (weird). I had to just complete this loop and the car
started right up.
I now always use a 3 wire tach. You can easily hook up a shift light to it
as well as the Jacobs electronic ignition.
OK, here is where I'll get some arguments. I don't use points. I use an
Allison XR-700 (now called Crane?) optical trigger inside my stock
distributor housing(per ITS regulations). I have used this on several 240s
and 510s. It works great and I have amazingly not changed the timing for
years. I will never go back to points as they were unpredictable, poorly
manufactured and needed constant fiddling. NEVER again!!!
Soon after rebuild, my current ITS engine developed a slight misfire whose exact cause could not be isolated. I tried my backups: carb/manifold assembly,
distributor, Allison trigger, coil, wires, etc. Nothing worked as the
engine would stumble when accelerating through about 3000 rpm and would
finally hit what felt like a crisp spot only when reaching above about 5500
rpm. I don't have a dyno so my seat of the pants is about as accurate as it
gets. I decided to try the Jacobs computer, coil and wires, $500 - ouch! I
suppose I could have tried the MSD that some of my competitors use but for
some reason I didn't.
The car started right up and since I was still using my old trigger, it
didn't need re-timing. It idled smoothly. The tach(3 wire) worked
perfectly. I took it out on the road and in 4th gear at 1500 rpm it pulled
aggressively and smoothly up to 6500 rpm. Gone were all hints of
hesitation. The engine sounded crisper and felt much more powerful. I then
ran this test as well as full throttle tests starting in lower gears and was
amazed at the performance. Since this time my car has run flawlessly and
has won many races. I can honestly say that the $500 I spent is the best
money I ever spent on this car. Look at it this way. A new set of Hoosiers
will cost you just over $500 and how many sets will you run over time?
And no, I don't get any kickback from Jacobs.
Coop
Subject: Tachometer Hookup & Jacobs Ignition
Date: Fri, 13 Aug 1999 09:42:42 -0500