How to modify a 280z tach to work with MSD for free

[calpoly-z]

I recently converted my car to megasquirt and decided to an old MSD 6A I had laying around from a truck I used to have and encountered a problem that seems to be pretty common with MSD units; a non working tach. Now everyone knows you can go out and buy an adapter for $30-$50 that usually fixes the problem, but I'm cheap so I decided to keep looking around and see if anyone had found another solution. I found a lot of mixed responses; some said the 280z tachs worked fine without doing anything and others said they couldn't even get the adapters to work.Then I came across this:

 

Well, I changed my ignition system while upgrading everything else so I have no real idea. I know I am getting huge sparkes though. As far as the tach, here's an idea, from the 240z.org mail list. I am going to do it soon... well probably later, anyway here's the info!

 

Here's a post from 240Z.org e-mail list, it's not the digital MSD but it may work the same way!

 

After missing a functional tach after my MSD 6A install, I found a post on

utilizing a modified 280z tach.

 

I didn't swap the face (kept 280z tach face), I simply performed the

following steps:

 

Remove 280z tach from tach housing (280z units are attached to dash

differently).

 

While you have the 280z unit exposed, find the medium sized resistor under

the tach signal/input feed and jump it w/a small wire using a solder gun

(believe resistor was brown w/some stripes...but don't quote me).

 

Install 280z unit into 240z housing.

 

I am sorry for not documenting this more clearly, the resister was pretty

easy to identify once I opened up the 280 unit.

 

You could test your newly modified tach before completing the dash

installation by wiring it up under the hood (near the ignition coil).

 

The modified 280z Tach will now get it's input signal directly from the TACH

output on the MSD box.

 

After you verified that everything works, simply return the tach to the

original dash location and you are smiling again.

 

Don't grin too hard,

 

manny 72DD

 

This seemed like a much better solution to me but I couldn't find a reference from anyone else saying that it worked or not. Since the tach is useless now anyway I figured I might as well pull it out and give this little mod a shot.

 

I was skeptical at first, but it worked brilliantly. Here's how I did it along with pics to help you through.

 

First pull the tach out of your dash. This can accomplished without pulling the dash by unscrewing the one bolt in back and the one in front and carefully pushing/pulling it out the front. I had to pull my steering wheel and the steering column cover to get it out, but I have an after market small diameter wheel. You may not have to.

 

Heres what it looks like from the back with the harness removed.

 

 

The red circles indicate the screws and nuts to remove in order to get the guts out of the housing. Don't unscrew the other two as they hold the tach assembly together. Before you do this take the four screws out on the sides and remove the black plastic housing and lens.

 

 

Once you have removed the screws holding the tach assembly into the housing you will probably find that it doesn't want to come out of its shell. I had to apply a pretty good amount of force to break the seal, but please do this CAREFULLY. The needle is VERY sensitive and will break or bend easily.

 

Here is the unit out of it's shell

 

 

Next you need to locate the "medium sized resistor" mentioned in the post I found this was the the trickiest part for me as I was not sure which one exactly he was talking about. The next pic shows the correct one circled.

 

 

All that has to be done to get a working tach is bypass this resistor with a jumper. I just stripped a short piece of wire of its insulation, tinned it, and then soldered it into the joints the resistor was soldered into.

 

 

I then reassembled the whole unit and plugged the harness back in to test. Before I test to see if it worked though, I needed to remove yet another resistor. This one is much easier as it is simply plugged into the wiring harness in the passenger footwell and taped in place.

 

Here is the resistor

 

 

and here is my ghetto short term connection to see if it worked.

 

 

on my car the blue/white wire was the one that went to the tach, so I just connected that directly to the TACH output on the side of the MSD box. I'll make a much nicer more permanent connection later when I have some more time.

 

I hope this helps ease some of the confusion I faced when trying figure out to get my tach working without having to spend any more money.

[AK-Z]

Sweet. think this should be a sticky