92RSStud

Ok so i used an after market coil and distributor in my 305tbi to my 350 carb swap and just need some help wiring it up... any one got some tips tricks, or some knowledge on what wire goes were for it? i took my ecm out and i dont want to go aftermarket if i dont have to.

NINÅ

Connect the lead (engine compartment end) from the tach to the primary of the coil.

92RSStud

actually your wrong, i already figured it out.

in the original harness you have a "thicker 12v+ with ignition on pink wire" and a "yellow with a quick disconnect" wire that originally connected to the stock coil you take the pink wire and put it on the positive side, and take the yellow wire and put it on the negitive side. the pink wire is hot with the iginition on so thats what you want to power your coil and then the ground switches on and off at a certain rate to "charge" the coil and emit a "spark" the gauge reads that and determines what the rpm is.


if anyone needs help on hooking a stock tach up let me know. i got very little help from any of the boards about this and figured it out on my own.

aciddrop2804

hello.I need some help with my tach. everythings stock. it will occasionally work, but thats it. ive tried a diffrent one and that one didnt work either so im pretty sure its in the electrical system. maybe its a loose wire or connection or something? im not sure. any help would be appriciated. thanks.

mShock43

Hi 92RSStud. I am also having trouble getting my tach to work on my '83 camaro. The tach worked for the 2.8 that was in it but when putting in a 350 with aftermarket HEI distributor it just reads 0. Is there a ground wire on the wiring harness for the tach and is there a hot wire from the wiring harness that will make the tach work? I was trying to do some homework and I was reading on one of the threads that the tach wire is white. I have not found too much help on here either on this issue so any answers are appreciated.

MOOKIE_EE

HELLO, I HAVE A PROBLEM WITH MY STOCK TACH. I SWITCHED OVER TO CARB AND REMOVED THE PCM. I AM RUNNING AN HEI DISTRIBUTOR. I FOUND THE WHITE WIRE LEADING TO THE TACH, BUT WHEN I CONNECT THE HEI TACH LEAD TO IT, THE TACH JUMPS TO 5200 RPM. I KNOW FOR A FACT THAT IT IS IDLING, AT ABOUT 800-1000 RPM. ANY SUGGESTIONS.

NINÅ

The electronics for the tach are behind the dash.

The tach needs 12 volt pulses to function.

Even tho you have a “high energy” distributor make sure you still have 12 volt pulses going to the tach.

83z28camaro

That is what NINA said to do.

mShock43

How do you know if 12 volts are going to the tach? I was reading about a tach filter. Would this have anything to do with it? Thanks

NINÅ

The best way would be to throw a scope on it.

But you can get an indication with a 12 volt light bulb.

Connect the bulb from the tach wire located in the engine compartment to ground.

When the engine is running the bulb will be light as the pulses cycle.

A low wattage bulb will work best.

At low RPM you might able to see flickering.

The tach filter smooths the square wave of the pulses and removes spikes.

mShock43

Okay I will try that. One thing--Does there need to be a tach filter between the tach wire and the distributor for the tach to read the rpms? Thank you very much for your help.

NINÅ

mShock43

Hi Nina,
Sorry for coming back to this tach subject after a few months, but I have not gotten my stock tach to work on my HEI distributor. I read what you and 92RSSTUD had to say but I don't fully understand because with the HEI distributor I got rid of the old coil. I just don't know where to turn for help on this thing. Anything is appreciated.

NINÅ

What is the make and model number of your new coil?

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mShock43

I will try to find that out and let you know as soon as I can. Is there a difference between HEI distributors?

mShock43

I'm not sure about the model, but it's a ProComp HEI distributor. I hope that helps. Thanks

NINÅ

OK if you have the one with the coil in the cap the RED wire from the coil goes to the tach.

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mShock43

Ok, so you're saying you splice into this red wire and lead it to the tach? So the tach input right next to the ignition input on the cap is not used? Once again, I appreciate you helping out a rookie like me.

NINÅ

In POST 1 You said you’re using an after market coil and distributor but didn’t know the model number.

However now that you know you have a tach terminal on the distributor as you said in POST 18, that is where you should connect the tach wire.

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mShock43

Sorry for the miscommunication, but you might have me mixed up with 92RSStud because he wrote POST 1, not me. I tried hooking up the tach to the tach terminal on the cap (before I even started writing on this message board) and the tach read 0. I don't know where to turn next...

NINÅ

You can find out if the tach terminal is providing a signal using a test light.

Start the engine, then put one lead of the test light to ground and the other end to the tach terminal.

If you’re getting a strong signal the test light will light, and might flicker because you’re only at idle.

If the test light does NOT light use a voltmeter in the same way to read the voltage. Use the 12 volt DC scale.

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mShock43

Ok, well I have a signal at the tach terminal and I hooked the white wire from the tach to it and still nothing. Is there another wire that needs to be running from the tach to complete an electrical circuit? (since there is the white wire for power and one for a ground so there should be a third for a negative connection?) I was reading something about needing a buffer box if you go from a V6 swap to a V8. Could that have anything to do with it? Thanks again

NINÅ

The white wire that was originally hooked to the stock distributor is the signal wire.

Because your tach is stock, it should be still connected to it’s original power and grounds.

The buffer should be in it’s original location in the dash.

When you turn the key to RUN after the car has been sitting overnight, does the tach needle move even a hair?

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mShock43

Well I guess you can say it moves a little. It seems to hover right around 1500 the whole time, even when the engine is off. Is there a way to run a new wire from the "coil" pin on the back side of the tach to the tach terminal on the distributor?

NINÅ

With the key in RUN, engine off, the tach should be at 0.

Either it isn’t getting power or has a mechanical problem.

The dash has 2 connectors on it.

You want to investigate the left one as viewed from the back of the dash.

With the key in the OFF position, connect one lead of an ohmmeter from a ground away from the dash, and with the other test lead probe pin 8.

Are you reading near 0Ω?

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mShock43

I won't be able to check any time very soon, but I will let you know. Can you test lead probe pin 8 with the instrument cluster in the dash or will I have to remove it? Thanks again for your patience.

NINÅ

I believe you will have to remove the dash.

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mShock43

Alright thanks. I will let you know the results when I can test it, which won't be for awhile. I'm sure I will be asking you more questions in the future Thanks again for your time

mShock43

One more thought. Is there a way to connect the tach to the distributor by bypassing the instrument panel? Such as running a new wire from the "coil" pin on the back side of the tach to the tach terminal on the distributor?

NINÅ

No because that is what is being done in the first place.

The tach terminal on the distributor is internally connected the coil primary.

Also, due to the fact the tach is inside the dash there is no way to connect it to anything without using the wires installed by the factory.

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mShock43

Oh ok. Well thanks for your help and I'll be in touch.

NINÅ

Yes the circuit board is compatible.

However you will have to recalibrate.

That means you will have to leave the dash out or open till calibration is complete.

Here’s how to recalibrate:

There is circuitry in the dash near the tach.

It’s contained on a small plug-in PCB or integrated in the main board.

On the PCB you’ll see a resistor chip.

Desolder pins 4 & 10.

Use a 500K potentiometer.

Solder it across traces 4 & 10.

Move the potentiometer to half way.

Start the engine and compare the tach reading with a test equipment quality tach.

Dial it in, unsolder one side of the potentiometer, measure it and install a matching resistor.

I might be able post a photo of the calibration area Monday.

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mShock43

Thanks for your help. It somewhat seems like a foreign language to me, but hopefully I can figure it out. Any pictures would be appreciated. Thanks again

NINÅ

The arrows are pointing to the pins which need to be de-soldered.



After removing the solder you can pull the pins out of the holes.

Now you can tack a potentiometer to the free lands.

(Don’t solder the potentiometer or resistor to the pins, solder it to foil on the PCB.)

Move the potentiometer to half way.

Start the engine and compare the tach reading with a test equipment quality tach.

Dial it in, unsolder the potentiometer, measure it and install a matching resistor.

If you have never soldered you should practice on a surplus printed circuit board available at many electronics outlets.

When you can remove components from the surplus PCB without causing damage you’re ready.

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mShock43

Thanks for the reply and the picture. Re-calibrating my circuit board won't necessarily make my tach work though will it? There might be some other issues that are the problem? I was just thinking that since my stock tach doesn't work it will be obviously difficult to compare the stock tach reading with a test equipment quality tach like you said.

NINÅ

That’s correct.

If it’s stuck due to a mechanical problem that needs to be fixed first.

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mShock43

OK, so let me get this straight. Even though my circuit board is calibrated for a V6, the stock tach should still read SOMETHING (even though it won't be a correct reading of RPMs) when I hook up the WHITE wire from the tach to the tach terminal on the distributor? In that case this is where I would need to test probe pin 8 to see if it's reading 0 like you said in POST 25? Just making sure we are on the same page. Thanks.

NINÅ

Quote:

Originally Posted by mShock43

Even though my circuit board is calibrated for a V6, the stock tach should still read SOMETHING (even though it won't be a correct reading of RPM) when I hook up the WHITE wire from the tach to the tach terminal on the distributor?

Yes, provided that it isn’t mechanically stuck.

Quote:

Originally Posted by mShock43

where I would need to test probe pin 8 to see if it's reading 0 ohms like you said in POST 25?

After the dash is removed you will see a board which looks similar to this representation.



This pic shows the half on the board where the tach is.

The top portion of the pic of the shows the board as viewed from the rear, the bottom portion of the pic of the shows the board as viewed from the front (as if you’re in the driver’s seat looking at it.)

The yellow line points to terminal 8.

That’s the one which should go to ground.

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mShock43

Thank you very much for the diagrams - they are very helpful. Well the first thing I will do then is test to see if that is reading 0 ohms and I will let you know the results. I won't be able to work on it for a couple months though because I will be leaving the country. I just wanted to make sure I had some answers when I got back. Thanks again and if I have any more questions I know where to turn. Thanks again.