From: "Lawrence M. Cooper"
Subject: Tachometer Hookup & Jacobs Ignition
Date: Fri, 13 Aug 1999 09:42:42 -0500

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.