Ok i tryed searching but i couldent find any thing to realy help me, My tach jumps all over the place, at about 4500 rmps the tach will jump to about 6000 or more, but it doesnt always do this at 4,500 rmps some times is 2000rmps or 3000, it changes every time. Yet the engine doesnt feel or sound like it is reving with the tach, and it doesn't do it all the time, only when it wants to. I dunno i need some help on this one! :-)

 

my assumption is loose wire or bad fuse.

?

 

The tach is driven by a signal which goes to the ECU from the trailing coil. If you look at your trailing coil mounted half-in, half-out of the driver's side fender (open hood, and right next to the battery), you'll see this lead attached to one of the spade connectors on the side of the trailing coil.

If this connector is loose or rusted, it will send an odd signal to the computer, which will interpret it as the true tach reading - which it obviously isn't. Check the connection there, and then be sure that your engine has been running well lately other than the tach signal. If it's been idling poorly or lacks power, you may have the complete trailing ignition out due to a bad coil (very remote possibility), or a bad trailing ignitor. The engine will run without trailing ignition, but very poorly - it will not run without leading ignition.

The other possibility is that the tach has an internal mechanical problem that's preventing it from working correctly. On my car, the grease that lubes the internal tach workings slows down needle response whenever it's cold out (i.e., less than 40 degrees). On startup, the tach needle doesn't work for about 10 minutes until the interior is heated up to above 40 deg (F). Once it warms up, the lube gets thinner, and Viola! the tach starts reading correctly again. Merely an inconvenience, but it sounds like your tach is getting a signal that is errant compared to what the engine is actually doing.

Trace the trailing coil tach lead and be sure that it's not shorted against anything else and that the wire is not stripped somewhere that would cause an arc or short. This will also send a weird signal to the ECU. Good luck,

 

i wired directly from leading coil to to tach , bypassing all factory wiring, took care of my twitchy tach, and my engine will run on the secondary ign abeit rather poorly if yous wont theres something up with the trailing ign or the state of tune, also running hei modules instead of maz ignitors, maybe that helps, trev

 

*****

5 Stars.

 

I thought only the 1984 and 1985 model Rx-7 had a ECU, so how did all the other years trigger the tachometer? Just trying to follow what you are saying.


HEY I have suggestion, when posting a problem list the year and model of your car, and if you have a mods that might be related to your issue.


John

 

I had that problem a few days ago. Before anything else make sure that your battery cables are on nice and tight. I had to replace my battery terminals to fix that problem on mine.

 

Sorry about that i have a 85 gs, i recently hooked up an msd setup, Its Probly a faulty instalaion job, ill have to go back and check it out, hey ya live and ya learn, thanks for the help guys