OK lets start off with the car. Its an 85 IROC that originally came with the L69 HO 305 and 5 speed. Someone swapped a 350 in and cut all the computer out of the car so it has a mechanical carb and old school vacuum advance distributor.

The problem is the stock tach reads 4500rpm at idle and goes up from there. It seems like the tach is doing its job, its not sporadic in operation or anything, its just getting too many pulses or something. Is there something in the wiring between the stock tach and the old electronic advance dist that used to be in the car that would cause this issue with a mechanical dist?

Thanks in advance.

 

Shameless bump....

Anyone able to help with this?

 

Your post brings up an interesting question! And that is... What exactly is an RPM in an engine?

Lots of possibilities with an 8 cylinder 4 stroke engine! As it turns out, this is when the crank turns one turn.

Then next is the question of how does a tachometer work?

The answer to your question may be found in the following...
http://en.wikipedia.org/wiki/Tachometer

Specifically... ["In older vehicles, the tachometer is driven by the RMS voltage waves from the low tension (LT) side of the ignition coil, while on others (and nearly all diesel engines, which have no ignition system) engine speed is determined by the frequency from the alternator tachometer output. This is a special circuit inside the alternator to convert from rectified sine wave to square wave, and the electrical potential difference is directly proportional to engine speed. Tachometers driven by a rotating cable from a drive unit fitted to the engine (usually on the camshaft) also exist...
...On recent engine management systems found on modern vehicles, the signal for the tachometer is usually generated from an engine ECU which derives the information from either the crankshaft or camshaft speed sensor."]

 

So basically whenever the "Switching deivice" (points or ICM) grounds the ignition circuit it sends power to the tach causing it to spike/jump up. Depending on how many times or how fast this happens determines the tachometer reading. Does this sound about right?

 

Basically when you get one revolution of the crank, there would be an on/off "pulse" of electricity.

So if you were to place a ground wire on a light bulb, then briefly touch the + wire to the other contact on the bulb, the bulb would briefly flash with that pulse of electricity.

And there would be 100 pulses of electricity for 100 RPM.

On, off, on, off, on, off... 100 times.

A pulse would look like this on an oscilloscope...

 

This doesnt sound like my problem. Its not spiking or jumping. The tach shows the car idling at 4500ish RPMs when its obviously idling at about 700. If you rev the car the tach shows an increase in rpm very fluidly, without any spiking or jumping, until the tach is pegged at 7000 RPMs. I would guess the engine is only running at about 1200 when the tach shows 7000.

Its kinda like when you put an aftermarket tach on a v8 and forget flip the little switch on the back of the tach from 4cyl to 8 cyl. It shows WAY too many rpms. Thats exactly what the stock tach is showing in my car.

I know the car came with a L69 HO 305 with an electronic advance distributor. It now has a 350 with a vacuum advance distributor. Whats the difference in these 2 distributors? Its makin me crazy.

 

I seem to recall tach connections on coils. So I searched google and found some interesting stuff on all this. Here is the search...

(coil tach connection)
http://www.google.com/search?hl=en&q...n&aq=f&oq=&aqi=

 

Its an HEI distributor, so there isnt an external coil.

Billybob, I am not completely ignorant as to the wiring of a standard aftermarket tach to an HEI distributor. What i am in need of help with is the third gen dashboard and the stock tachometer wiring. I was hoping that someone with more experience than me in this field could offer a remedy without me having to rip apart my dash and try to make sense of the wiring, since this is the board that experts in the realm of third-gen call home.

Hopefully someone has run into this problem and has a simple answer. I cant wait to hear it. Thanks.

 

My 84 TA the tach is showing twice the RPM the engine is running. I checked it with another tack hooked up. I want my dash tack working. I bet we both have the same problem. Any suggestions!!!!

 

sorry larry but i dont have an answer for you. Mine is still jacked up.

as of right now im just dealing with it, ill get it figured out when the new motor goes in.

 

has the needle ever dropped below 4500? maybe the needle just isn't set properly or something. the tach may work fine but it bottoms out at 4500 as opposed to 0. or maybe something is sending it a little extra power and it reads higher. also, if it idles and reads 4500 and then shows 7k when you think its at 1200, assuming you're idling at about 700, you gained 500rpms but the tach shows you gained 2500? that means the needle is rising at a rate 5x that of your engine. weird.

 

I have the same issue with my 90 iroc. My tach is way off & high, but smooth in operation. My speedo as well is messed up, but it is on the low side when its not buried wide open with the odometer clicking away & me barely moving.
I have an external tach mounted in car that reads fine. Would be nice to get ALL gauges working though.

 

I have kinda given up on mine for now since im planning on doing a full dash plate/gauge conversion similar to sinister's when i pull the car apart over the winter. My T5 isnt in good enough shape to justify needing the tach to be accurate at this point so ill just finish the summer with 4500-5k shifts and worry about it when the white stuff falls. (long Chicago winters give me lots of time to make her right)

If i happen to come across the problem though ill be sure to post it.

 

Have you guys tried this yet

About 2, maybe 3 years ago there was quite a discussion on tachs reading high and how to fix the problem. You just need to search the Electronic board on this site.

Good hunting and maybe you will find a solution to your problems.


Try the keywords tach fix. You should get about 33 threads. read through several of them and you may find a possible solution to your problem.

 

I cut the pins off of the factory board and soldered on two 100k 1/2w resistors in a series this afternoon to fix my tach per the instructions from other members. I had to move the needle on the tach motor shaft to calibrate and make it read the same as the autometer tach i've put in the car. The factory tach worked great for the 15 miles or so home with one shutdown and restart on the way. I get back in the car tonite and the tach is dead altogether. Damn.
What gives man!!??