CJ Tach Wiring

[jay79cj7]

I just put a painless kit in my 79 CJ-7 and haven't got my tach working yet. The kit has a tach wire that goes to the negative side of the coil and leads back under the dash to hook to the tach.

I think the tach I have is original, and if not, it is a replacement because it looks factory. Besides the wires for the lightbulb, there are 2 heavy gauge (like 8 or 10) red wires coming off the back of the tach. I figured one must be hot, the other must be for that tach wire on the new harness, since that's how all the other gauges work. I was wrong, connecting it that way kills the engine.

I looked at the way the old harness was connected, and one of the tach wires comes straight off the ignition connector, and the other goes to the fuse block which leads through to the main 12V wire coming from the battery terminal of the starter solenoid.

So, it looks to me that the tach is hooked in series with everything on that main circuit. Does this make sense? Is this right? I do remember noticing that wire was pretty warm when I was under there one time!

Can someone with a CJ check to see if this is how theirs is wired? I want to be sure before I start hacking into my new harness.

Thanks!

[Inspector-Gadget]

Hi-Ya jay79cj7,

Very savvy troubleshooting!!!

You are correct. The CJ OEM tachs of that era counted ignition system current pulses. Hence, the tach was connected in series with the primary side of the ignition system and carried the electrical load of that system. That is the reason for the heavy gauge wiring of the original CJ tach.

The vast majority of modern OEM and aftermarket tachs count ignition system voltage pulses and carry very light current loads.

Modern aftermarket tachs are indeed usually wired in parallel with the primary ignition system.

Modern OEM tachs might not actually be connected directly to the igntion system at all; rather, these tachs count voltage pulses generated by a computer.

Regards,

Gadget

[jay79cj7]

Thanks! Now I guess I get to make yet another modification to the "painless" harness. Modifications required so far on this universal harness:
1) Relocating ignition control module b/c the wires were on the passenger side of the engine compartment.
2) Re-wiring it to work with my stock neutral safety switch.
3) Re-wiring it to work with my stock reverse lights switch.

It was so nice and clean, and now I am hacking it up.

[redrooster]

I thought Painless made a CJ specific replacement harness.
I generally don't like a Haynes manual, but the wiring diagram in the back of the CJ manual is pretty good. If I remember correctly the original tach wire is purple with white...or was that green with white? :?

[jay79cj7]

This is the 76-86 CJ7 harness that I have. I don't know if part of its made wrong, or my auto CJ is just an oddball or what. I did get an email reply to my tech support question after a few days and they told me that neutral safety switch thing was because so few CJs were auto.

My tach, which looks stock, but may be replacement look-alike, has two red wires, one with a white stripe.

I have checked out the Haynes manual but either I didn't see it or it didn't have good info on the tach. I'll check it out again, but I think Gadget's response solves my riddle.

Thanks again, Gadget!

[redrooster]

Jay,
I read your other post. You will need to wire in the kickdown for the TH400 unless you want to manually downshift. It will start out in first after coming to a complete stop but will not downshift accelerating uphill without the wire connected.
You do not have to keep the neutral safety switch. You can pull that circuit out altogether and it will allow the Jeep to start in any gear. Mine will do this and start in gear like a manual. If you are doing a resto then you may want to put it back in.
I would say that you do have a rare setup. Usually the Th400 came behind a V8 and the 727/904/999's came behind the six cylinder. You must be sporting a quadratrac as well.

[jay79cj7]

Quadratrac indeed! The previous owner put in a part time kit.

I already figured out how to wire the neutral safety and reverse lights, and they are working fine. That wasn't too tough, more frustrating than anything having to cut into the new clean harness. They aren't cheap.

Thanks for letting me know about the kick-down. I was probably going to do it, but wasn't sure. Now I will make sure I get around to it.

Thanks for the help guys.

[Inspector-Gadget]

jay79cj7 wrote:

My tach, which looks stock, but may be replacement look-alike, has two red wires, one with a white stripe.

I have checked out the Haynes manual but either I didn't see it or it didn't have good info on the tach. I'll check it out again, but I think Gadget's response solves my riddle.

Thanks again, Gadget!

jay, the color-coding of the tach wires of your Jeep is identical to that of the Bubbacon looking at you from my sig line.

and...

You are welcome.

Best regards,

Gadget

PS: When I did the GM HEI mod to the Bubbacon, I also obtained and installed a replacement tach on the Bubba's steering column. I always found the original dash-mounted OEM tach difficult to see.

The hard part of the tach mod was finding a 6-cylinder tach with a low max RPM indication. The typical 12,000 max RPM aftermarket tach will not show enough needle swing to be usable when connected to a 4,000 (very approx) max RPM engine.

For example: a tach that shows 0 to 12,000 RPM, using a 270-degree gauge face, will swing the tach needle through only 90-degrees when monitoring a 4.2/4.0 liter I-6 engine running at 4,000 RPM. The tach needle swing range is too compressed to be usable because roughly 180-degrees of the tach gauge face will never be used.

I solved the problem by using the tach from a later model Jeep Cherokee. It was a 6-cylinder, max 7,000 RPM, tach originally mounted in the dash instrument cluster of a Cherokee.



This big round electronic tach (that I can actually read) needs only a key-on source of +12 Volts and a wire run out to the "tach" connector of a GM HEI distributor. (There are two additional tach wires for lighting.)

The large needle swing of the Cherokee tach provides much finer resolution of engine RPM, and that bad-boy is instantly readable.

That is important when I have to smoke those little Turbo S-Minis and ricers off the line!

Anyway...worked for me.

[jay79cj7]

Well, I spliced it in the ignition wire and it works like a charm. Darn "painless" kit. Were all of the 76-86 CJ tachs wired like mine? I thought they were all interchangeable... If so, I don't see any reason the kit would be wired like this.

[Inspector-Gadget]

Jay,

As I understand the CJ wiring setup...

The tach and clock were an extra-cost option or included as part of a "trim package" -- possibly the "Laredo" package. (Uncertain on this.)

I suspect (total speculation) that "Painless" used the same reasoning as with the auto trans option; to wit: there were so few of those made that we are not going to bother integrating this feature into our so-called "universal" wiring harness.

CJs without the tach-clock option had a connector or jumper included in the vehicle wiring that was used to complete the ignition system wiring in the absence of a tach. (Red and red-white wires connected directly together.)

Nice job on your troubleshooting-upgrade.

Regards,

Gadget

[redrooster]

Jay,
Don't feel bad about hacking up your new harness. I bought a Painful harness for my GM Throttle Body and HEI upgrade and ended up removing and cutting half the wires right when I pulled it out of the box. Many more had to be shortened. Just be patient and do a lot of routing and planning before you take the final step of taping and running wire conduit.