240Z Tach with 280Z ign.

[JEFF]

A couple of weeks ago some one posted a wiring diagram of both - The stock 240 Tach diagram & a ZX diagram for ZX electronic ignition swap. The diagram for the ZX showed how to hook up wires to make the 240Z Tach work again with the ZX ign.

However... I installed a '78 280Z electronic ignition in my '72 240Z.

Does anyone know how to hook the Tach up for that swap??

- Jeff

[JEFF]

Anyone??

- Jeff

[AxtellZ]

Have you looked here: http://www.geocities.com/zgarage2001/engine.html

I'll cut & paste what i found:

HOW TO HOOK UP THE E12-80 MODULE and D2 DISTRIBUTOR INTO A 240Z?

(If you have a 280Z or 280ZX don't do this mod)

The module must be mounted on a 1979-83 280ZX distributor (won't work on points or D1).

Connect a key-off +12V supply to the "B" terminal on the distributor IC module and coil +. Then connect the module's "C" terminal to the coil's negative terminal.

Make sure the metal rear of the module is clean so it can ground itself to the side of the distributor.

Remove the triangular hold-down plate on the base of the distributor. Burnish it, as well as the spot where it contacts the distributor and replace. This helps everything ground well to the block.

Use quality spade lugs and wire with TIGHT crimps, and solder the wire/spade connections if possible. Don't solder or crimp onto the module though, just make sure the spades fit on tight.

A 240 tach will not work with this setup since it's loop wiring can no longer connect to the coil. Replace it with a 75-78 280Z tach and connect the tach sensor lug to the - terminal on the coil. If the tach jumps around, solder a 7500-10,000 ohm resistor inline with the sensor wire to reduce the signal voltage. If it still won't work buy a Autometer #3990 black face tach.

WARNING: if you have a MSD or Crane, DON'T hookup the module or tach to the coil. The 450 volts will fry it. Follow ignition's hookup guide instead.

280Z TACH INTO A 240Z

I would recommend getting rid of your old 240Z tach. Vintage, but not designed well, they're inaccurate and start jumping around as they age: Replace it with a 1975-78 280Z tach.

Put the new tach in the 240 metal housing, and swap the faceplates to keep the redline accurate. The needle pops off, don't damage it. A drop of rubber cement might be needed to put it back on. Don't use superglue or you'll never get it off again.

Connect the tach's sensor lug to the negative terminal on the coil. NOTE: Make sure you reverse the power leads on the back of the tach, compare them before you swap for a reference.

The little "loop plug" from the 240 tach should be plugged back into the wiring harness in the dash, otherwise the car won't start.

If you have trouble getting the tach to work, a simpler solution is to replace it with a 5" aftermarket model like Autometer makes

[AxtellZ]

And from www.zhome.com:

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.

Coop

[z-ya]

I swapped the guts from a 75' tach into my 72' housing, and it works much better than the 240Z tach ever did. I always had problems with the needle jumping around when the car interior got hot.

Anyway, it's and easy swap, and well worth the effort.

One note, if you 240Z tach is a current sensing type (probably, if it is a 72' or earlier), remeber that the ignition coil gets powered through the 240Z tach. If you unplug the tach from the harness, you will need to make sure that you are still getting 12v at the + terminal of ignition coil when the ignition switch is on.I think I just made a jumper to direct power to the positive terminal on the coil. You also may need to run a new wire from the negative terminal on the tach, back to the input terminal on te 280Z tach.

Pete