[Marc Morgan]

Okay, my tach works but seems to be way off...shows 4000 rpms when I'm running about 2000. Is this a problem with the tach itself, circuit board or can it be fixed by using a resistor. If the latter, then can someone explain what resisitor I use and where do I put it!.

Thanks
marc

 

[hosspowerinc]

Hmm my dads 79 has the same problem. We replaced the tach board and resistor to no avail. Maybe someone will chime in. :cheers:

 

[1 MN MCN]

there is a filter with two capacitors and 2 resistors, that filter out the noise that causes this type of problem. The can type capacitor is an electrolytic type, and these can fail after about 10 - 20 years. That Resistors and other disk type capacitor are more robust, and usually won't fail. Pull the gauge cluster and replace these componants and that should fix the problem. to find the componants you need to trace the tach wire to the board and then see where the trace it plugs into goes. These componants will have one leg "in" the trace, and the other "in" a ground trace on the board, as the trace travels to the tach.

 

[mrvette]

Actually it's not a resistor, tech talk it's called a PI network

fancy way of saying in this case, 2 resistors in series, and two caps to ground on the load side (tach side) of the network....

I dunno how to get the schematic on here for it, I have it printed from years ago off the internet....

the tach out off the dizzy goes to a 5.6k resistor, then a .1 mfd cap to ground on other end of resistor, from that r/c junction take a 10k resistor to a 100pf cap to ground, and from that 10k.100pf junction take a wire to tach input....the wattage rating on the resistors should be 1/2 watt, caps around 100 vdc or better....

tech speak, i'ts a pulse integrator, smooths out the tach signal for the tach input, it was done under the hood, I SUSPECT as an aid for AM radio reception....AM radio was a hot lick back then...

:laughing: :devil: :cheers:

 

[6880 Mike]

Check back issues of Corvette Enthusiast Magazine. They recently did an article on tach board replacement and stated the tach had to be "zeroed" in order for it to read correctly.

:thumbsup:

 

[hosspowerinc]

Yeah the problem is the tach swing just isnt accurate. Its like a v8 tach on a 4 cyl. When it swings from 2-4k rpm the engine is only reving from 1000-1500 or so. We replaced the part that bolt to the manifold and is right after the tach output on the dizzy. I thought this was the resistor/filter and that there were no more in the circuit. Where are the other resistors located?

 

[mrvette]

Yeah the problem is the tach swing just isnt accurate. Its like a v8 tach on a 4 cyl. When it swings from 2-4k rpm the engine is only reving from 1000-1500 or so. We replaced the part that bolt to the manifold and is right after the tach output on the dizzy. I thought this was the resistor/filter and that there were no more in the circuit. Where are the other resistors located?

OK, when you pull the plug, what does the tach say?? it should go to zero, or whatever it said at idle....you maybe want to just turn on engine without starting....tach should say ZERO, if not, you need pull needle, and very carefully put it back on the tiney little fragile stem with it down at the same stop as the internal deflection coil...one or the other is not happy as they don't see the same zero...

when the mechanicals are zeroed in, then we go for the tach filter I described best I could....but IF you have that ~3/4 inch diameter ~2" long black can looking thing, with the clip on the side mounted to the intake and the two white wires....one to the harness, the other to the tach out on the HEI....i'ts wired....if the caps did fail though...it's bad and may as well not be there, the tach will go crazy with all the coil ringing from the dizzy....indicate all sorts of crap over the map....

 

[hosspowerinc]

.but IF you have that ~3/4 inch diameter ~2" long black can looking thing, with the clip on the side mounted to the intake and the two white wires...

This is what we replaced along with the tach board. The needle was reset to 0 with the key on engine off but the tach just isnt accurate. Its a smooth sweep, but the readings are really funky. From 1k-2k on the tach is really 300rpm the engine changes. Then from 2-3k on the tach the engine will rev 600rpm. These arent the acttual numbers but you get my drift. Its not a uniform sweep. Maybe a bad tach board we got?

 

[NHvette]

This is what we replaced along with the tach board. The needle was reset to 0 with the key on engine off but the tach just isnt accurate. Its a smooth sweep, but the readings are really funky. From 1k-2k on the tach is really 300rpm the engine changes. Then from 2-3k on the tach the engine will rev 600rpm. These arent the acttual numbers but you get my drift. Its not a uniform sweep. Maybe a bad tach board we got?

Sounds like it must be the tach circuit board itself.
The part on the intake manifold (looks like an old-school condenser)
is the tach filter. A bad filter "could" cause high readings due to ringing of the pulses.
You can build a replacement circuit to test out the filter.
If filter is not issue ... board.

Here is a fairly technical study I did of the tach filter circuit.
It compares the stock filter to a suggested alternate filter.
My analysis shows: use the stock values.
Filter analysis paper:
http://webpages.charter.net/davekimtysam/images/NHvette_filter_comp.pdf
:buhbye:

 

[mrvette]

This is what we replaced along with the tach board. The needle was reset to 0 with the key on engine off but the tach just isnt accurate. Its a smooth sweep, but the readings are really funky. From 1k-2k on the tach is really 300rpm the engine changes. Then from 2-3k on the tach the engine will rev 600rpm. These arent the acttual numbers but you get my drift. Its not a uniform sweep. Maybe a bad tach board we got?

first off, do what Dave/NH vette said on the filters there use the top schematic he has there it's what I have...ignore the bottom one (out2)....

I am referring to taking the physical pointer off the tach output needle shaft, and I mean it's really small needle size....

making sure the internals are not energised at all meaning all the way relaxed...now put the needle on very carefully....

another way to make sure the coil is reading zero, is to plug it up to +- and leave the input alone, then put the indicator on the shaft....

then hook up the tach through what Dave has there, give that a shot...

lettice know...

 

[NHvette]

Yup, as Gene points out ... the top schematic is the stock filter.

Just for a data point, I run my HEI without a tach filter ... never a problem.

:buhbye:

 

[hosspowerinc]

Yeah we bypassed the filter to see if anything changed and it didnt. Going to swap out the board and see if that fixes it.