GM hei Tach issues

[crazyoctopus]

So I have been googleing and searching around multiple forums and I have yet to actually find a response to my query.

I recently installed a GM HEI module with an MSD Blaster II coil hooked up to a 280z distributor into my l24 with triple webers. When I had SUs and points the tach would work its way up to 6-7k rpm with no issues, but now that the GM HEI unit is in there the tach doesn't like to go past 5k. The engine sound does change, and the engine keeps pulling all the way through the apparent power band, but the Tach does not show a change past 5k.

Is this normal? Do I need to add a diode/resistor somewhere. Other than not having the visual reference, it doesn't seem to impact performance but curious if I am the only person that has dealt with this.

[yetterben]

I am not gonna say anything i all ready have not told you

[crazyoctopus]

yeah yeah, but the GM HEI unit will "work" until I find a 280zx dizzy in a junkyard. For now though, I am interested if I was the only one that is dealing with this.

[Walter Moore]

The tachometer not working correctly with electronic ignition seems to be a common issue with the early cars. Mine reads about 10% high all across the range since I installed the ZX distributor. There are some threads on the subject, but there doesn't seem to be a common denominator to the problems.

The tach stopping at a certain point seems to be common on after-market EI installations as well.

Edited April 25, 2010 by Walter Moore
Top of head mistake, tach reads high, not low.

[Jetaway]

I recently installed a GM HEI module with an MSD Blaster II coil hooked up to a 280z distributor into my l24 with triple webers. When I had SUs and points the tach would work its way up to 6-7k rpm with no issues, but now that the GM HEI unit is in there the tach doesn't like to go past 5k. The engine sound does change, and the engine keeps pulling all the way through the apparent power band, but the Tach does not show a change past 5k.

Is this normal? Do I need to add a diode/resistor somewhere. Other than not having the visual reference, it doesn't seem to impact performance but curious if I am the only person that has dealt with this.

If there is a solution, I'd like to hear it. I have a similar set-up except my coils have been a stock Nissan and now, an Accel coil (Actually, no triple Webers but they don't enter the equation). I used an E12-80 module until it began to spit it out and switched over to a GM HEI module. The stock 240Z tach has never been satisfactory and always begins to hang near 5,000. I also replaced the nonfunctional and redundant stock clock with a Sun Super Tach II. True, not an ideal location, but I rarely take my 200,000+ miles and counting L24 out past 5,500, but I have on occasion and the Sun keeps going out to 7,000.

So, with two different coils (stock for the EI and aftermarket) and two different ignitors (stock for the EI and kludge), I've had four combinations, and with none of them has the stock tach been usefully accurate. You wrote that your tach worked with points. My Z came with the EI installed, so I don't know if the tach would have been accurate with points, but it really seems to me that the root of the problem is electronic ignition, not the GM HEI and not the particular brand of coil installed.

Chris

[alhbln]

Which tach do you have installed, the four wire (current driven) or the later three wire (kickback drive) type? Do you use a ballast resistor with the Blaster coil?

A 5K limit sounds like the four wire type which is not getting a strong signal due to the lower coil current in higher rpms.

A possible fix could be to recalibrate the tach using the trimmer which is accessible through the taped hole the back (use an insulated or plastic screwdriver). The trimmer changes the sensivity of the input stage.

Regarding the precision of the old 240Z tach, i added an additional driver board to my four wire tach to connect to a mapped ignition, and after a recalibration the tach is now quite on the point, with a maximum error rate of +3-4% between 3K-6K. Not too bad for an 38 year old tach.