How does the tachometer unit "read" the RPMs - wire from the engine to tach? - JeepForum.com
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post #1 of 20 Old 07-07-2020, 02:49 PM Thread Starter
CJ7inNH
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How does the tachometer unit "read" the RPMs - wire from the engine to tach?

I know, I know, I know.* This has been covered hundreds of times in one way or another, but I still can't wrap my head around it. I've spent weeks reading threads about it and still don't get it.*

83' CJ7 258 - OEM tach

I understand the wiring from the tach itself, kind of.* Ground wire, lighting wire, and 2 additional wires.* What I'm trying to understand is which wire runs from the engine into the tach to give the RPM reading?**

I believe it's the green wire running off the neg terminal on the coil that is "reading" the RPMs.* Correct?**

***That wire ends in the ICM but I don't see anything going OUT of the ICM into where*it would hit the tach behind the dash.* This is what has me baffled.**

I've looked at wiring diagrams (even bought a magnifying glass to do this).* Just can't figure it out.**

And yes, this started with the fact that I've purchased a DUI distributor and was looking at trying to wire the tach directly to the "tach" lead on the new distributor - even though 99% of the posts say it isn't that simple.**And just want to be sure I'm removing the correct wires and leaving anything I may need in the future to hook up a tach.

Any help would be great.**

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post #2 of 20 Old 07-07-2020, 03:53 PM
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Quote:
Originally Posted by CJ7inNH View Post
I know, I know, I know.* This has been covered hundreds of times in one way or another, but I still can't wrap my head around it. I've spent weeks reading threads about it and still don't get it.*

83' CJ7 258 - OEM tach

I understand the wiring from the tach itself, kind of.* Ground wire, lighting wire, and 2 additional wires.* What I'm trying to understand is which wire runs from the engine into the tach to give the RPM reading?**

I believe it's the green wire running off the neg terminal on the coil that is "reading" the RPMs.* Correct?**

***That wire ends in the ICM but I don't see anything going OUT of the ICM into where*it would hit the tach behind the dash.* This is what has me baffled.**

I've looked at wiring diagrams (even bought a magnifying glass to do this).* Just can't figure it out.**

And yes, this started with the fact that I've purchased a DUI distributor and was looking at trying to wire the tach directly to the "tach" lead on the new distributor - even though 99% of the posts say it isn't that simple.**And just want to be sure I'm removing the correct wires and leaving anything I may need in the future to hook up a tach.

Any help would be great.**
How the OEM Tach works?

How technical do you want to get?

Basically the OEM tach looks at changes in the current going to the coil every time it charges up (or discharge?) the coil for the next time the spark plug fires.

There is a transformer in the tach that turns these pulses into an AC signal that goes into a resistor and capacitor to smooth out the signal, a couple of diodes that turns it into DC The values of the Resistor and Capacitor determine for what number of cylinders the tach is designed.

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Aftermarket tach have so many different designs that it would be impossible to post them all here.

Basically though you would have some type of input protection, signal conditioning, frequency to voltage conversion then output conditioning to milliamp meter.
There are some pretty neat IC to do that now.

You are going to see a lot of posts where they recommend a relay for maximum output to the plugs but this blocks the simple way the OEM tach can be hooked up as it is no longer seeing the surges in the current feeding the coil.

In my opinion, if you were running a V8 at 7000 RPM it's important to shove in as much current as you can so the coil has time to charge up in between firings. In a 6 cylinder tractor engine reving up to 3000 RPM? , I don't think it's that important. I've run a GM HEI system without the relay and it worked great for years.

I don't think you need any more than one wire for the distributor so the rest are unnecessary. If you are planning on adding a tach in the future and you decide to go with the relay method, You could buy an aftermarket tach and hook it into the TACH socket in the D.U.I. distributor or use a Tach converter that will drive the OEM tach.

If you decide not to run a relay, You can simply wire in the OEM tach as designed. Just use the wire that feeds power to the ICM.
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post #3 of 20 Old 07-07-2020, 04:18 PM
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hello
yea what he said. it think lol

oldschool
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post #4 of 20 Old 07-07-2020, 05:18 PM Thread Starter
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Quote:
Originally Posted by John Strenk View Post
How the OEM Tach works?

How technical do you want to get?

Basically the OEM tach looks at changes in the current going to the coil every time it charges up (or discharge?) the coil for the next time the spark plug fires.

There is a transformer in the tach that turns these pulses into an AC signal that goes into a resistor and capacitor to smooth out the signal, a couple of diodes that turns it into DC The values of the Resistor and Capacitor determine for what number of cylinders the tach is designed.

Attachment 3920239

Aftermarket tach have so many different designs that it would be impossible to post them all hear.

Basically though you would have some type of input protection, signal conditioning, frequency to voltage conversion then output conditioning to milliamp meter.
There are some pretty neat IC to do that now.

You are going to see a lot of posts where they recommend a relay for maximum output to the plugs but this blocks the simple way the OEM tach can be hooked up as it is no longer seeing the surges in the current feeding the coil.

In my opinion, if you were running a V8 at 7000 RPM it's important to shove in as much current as you can so the coil has time to charge up in between firings. In a 6 cylinder tractor engine reving up to 3000 RPM? , I don't think it's that important. I've run a GM HEI system without the relay and it worked great for years.

I don't think you need any more than one wire for the distributor so the rest are unnecessary. If you are planning on adding a tach in the future and you decide to go with the relay method, You could buy an aftermarket tach and hook it into the TACH socket in the D.U.I. distributor or use a Tach converter that will drive the OEM tach.

If you decide not to run a relay, You can simply wire in the OEM tach as designed. Just use the wire that feeds power to the ICM.
Thanks for the quick reply John. Anytime I see your name on a post I know there is good info coming.

The function of how the tach works is clearer now - I think I remember seeing a post somewhere that you wrote in talking about the V8 vs 6.

I don't know if I'm overthinking this or maybe severely underthinking it. Where does the tach dial itself get fed the information from? I know (now know) it's the coil that is providing that information, but at the coil there is the red power connected to the positive terminal that is run off of the main power wire, and the green wire on the negative that runs into the ICU. I don't see anything running out of the ICU and into the cab where it would attach to the tach itself.

I have been running the OEM tach for years with no issues. But I've started to install the new DUI, so not much of anything working at the moment.

I feel an "ah-ha" moment coming.

Thanks again.
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post #5 of 20 Old 07-07-2020, 08:31 PM
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Quote:
Originally Posted by CJ7inNH View Post
Thanks for the quick reply John. Anytime I see your name on a post I know there is good info coming.

The function of how the tach works is clearer now - I think I remember seeing a post somewhere that you wrote in talking about the V8 vs 6.

I don't know if I'm overthinking this or maybe severely underthinking it. Where does the tach dial itself get fed the information from?
I know (now know) it's the coil that is providing that information, but at the coil there is the red power connected to the positive terminal that is run off of the main power wire,
and the green wire on the negative that runs into the ICU.
I don't see anything running out of the ICU and into the cab where it would attach to the tach itself.
The ICM doesn't need to feedback a signal .to the tachometer.
The current running through the wire tat is feeding power to the coil and the ICM has bursts of power that the tachometer can pickup as it passes through the transformer
inside tachometer.
This will cause the tach to move depending how fast the bursts are coming.

Here is a simplified schematic of the ignition system, the HEI is actually quite similar but all the wires are contained inside the distributor.
durasparkii-simplified.jpg

As the trigger wheel in the distributor passes the pickup coil near the wheel, a little pulse is sent to the ICM.
The ICM then detects this little pulse and amplifies it enough to trigger a transistor, this transistor then fires a powerful transistor that is capable to discharge the ignition coil.
As the coil discharges quickly, this generates high voltage that is able to jump the gap in the spark plug.

Here is a schematic of the Duraspark control box.

duraspark2-modified.jpg


Quote:
I have been running the OEM tach for years with no issues. But I've started to install the new DUI, so not much of anything working at the moment.

I feel an "ah-ha" moment coming.

Thanks again.


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post #6 of 20 Old 07-08-2020, 08:37 AM Thread Starter
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YES!!! This is exactly what I was trying to understand. The visuals are very helpful as well. Thank you!

My set up is/was a bit different as I only have one power wire going into the coil, and the one green wire running to the ICM. There is no second power wire or resistor.* The hot lead that the coil power wire is run off of has another wire split off that goes to the I terminal. That's how it WAS set up, before I started the DUI swap.

So in theory, running the power wire that was on the coil to the spot marked coil on the new DUI should work? But I've read countless posts saying it will not work and not to do it.

?Will the fact that I've eliminated the ICM make it more difficult to get the OEM tach back up and running?

?I've seen instructions on installing a HEI show putting in a fusible link from the hot lead to the distributor - is this something you recommend?

From the timestamps on your posts, I wonder when you sleep, but am glad you don't.
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post #7 of 20 Old 07-08-2020, 09:12 AM
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Quote:
Originally Posted by CJ7inNH View Post
YES!!! This is exactly what I was trying to understand. The visuals are very helpful as well. Thank you!

My set up is/was a bit different as I only have one power wire going into the coil, and the one green wire running to the ICM. There is no second power wire or resistor.* The hot lead that the coil power wire is run off of has another wire split off that goes to the I terminal. That's how it WAS set up, before I started the DUI swap.
The 'power' wire from the bulkhead connector to the coil is actually a 'resistance wire'. It takes the place of the ballast resistor. If you put an ohm meter on it it will read about 1.3 ohms.

But I'm not sure what you are describing about where the wire going to the "I" terminal comes from.
Does it come from the splice near the fire wall or the (+) terminal on the coil.

Normally, there is a splice by the bulkhead connector where the resistance wire is joined. One wire goes to the ignition switch or Tach if there is one installed. The other wires are a resistance wire to the coil and another 'normal' wire
going to the ICM.

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If you are installing a D.U.I. distributor, you can power the distributor with the wire that use to go to the ICM. Just remove the resistance wire and you are good to go.

Quote:
So in theory, running the power wire that was on the coil to the spot marked coil on the new DUI should work? But I've read countless posts saying it will not work and not to do it.
Interesting,

No, don't use the the OEM wire going to the OEM coil (+). That's a looser.
As I mentioned. This is a resistance wire and will cut the voltage to the distributer down to around 6- 7 volts. It will run , barely, at that voltage but when cranking the engine to start it, the voltage will drop to below 5 volts giving you lots of problems.

I don't think you should hook anything up the the terminal marked 'coil' on the HEI distributor. Normally power is applied to a terminal marked BAT+ or something like that?
Interesting, I'll have to look into that more.

In 'Theory' you will be supplying power to the coil from two sources. There might be enough signal there, One would have to experiment,
Which ever one supplies the most power wins. And if you use the OEM resistance wire, that's a sure looser.

Dang it, now you got me wondering....

Quote:


?Will the fact that I've eliminated the ICM make it more difficult to get the OEM tach back up and running?
No, well, depends If you use the wire that use to go to the ICM to power the distributor, you won't have any problems with the OEM tach working.
If you use a relay to power the distributor, then you will need many more wires/parts to make it work.

Quote:

?I've seen instructions on installing a HEI show putting in a fusible link from the hot lead to the distributor - is this something you recommend?
For a Jeep? in my humble opinion, I don't think it's necessary.
But if you are running sand drags with a hopped up V8 running 7000 RPM, then that probably will be a good idea.

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Quote:
From the timestamps on your posts, I wonder when you sleep, but am glad you don't.
I'm in San Diego for a week for work so my sleep cycle is off.
My kids are staying at my house all dying to borrow the jeep.


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post #8 of 20 Old 07-08-2020, 04:18 PM Thread Starter
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Thanks John. We're getting there.

I cleaned off the wire that is spliced into the wire running out of the bulkhead. Yes, it is a resistance wire. That wire then runs across to the passenger side. Once at the passenger side it is spliced off - one wire goes to what was the positive terminal on the coil, the other goes to the i-post on the solenoid. The plan is to remove the resistance wire and replace it with a 12 gage to the new HEI. Should I go directly to the HEI with the 12 gage or splice in with something less?

Sorry, I miss spoke on part of the last post. The new HEI has one terminal marked "Bat", that's the one that the new wire from the bulkhead will power (still wondering if I should run the 12 g right into it or splice something less). The other terminal on the HEI is marked "Tach" not coil.

Going backward off the old coil, a red wire went off of the positive side of the coil, then spliced into the resistor wire, then the resistor wire spliced into the large red wire running into the bulkhead (that wire also spliced off down into the ICM).

From what you explained "the current running through the wire the tach is feeding power to the coil" is what the tach is reading. So is that large wire coming from the bulkhead running into the tach at some point? It must.

Now confusing me again is that with what I thought would work, former resistance wire that is now a 12 g from the bulkhead would splice off on the passenger side having one wire run to the "bat" terminal and the other to the "tach" terminal.* But it's the same wire.

Remember the show Welcome Back Kotter?* "I'm sooooo confused".

I wonder if this is why I've read posts saying you can't run the OEM tach and need to get a newer replacement one.

I feel good about getting the HEI hooked up now. Thank you!

I'll let you think through the tach situation.

Enjoy San D!

Last edited by CJ7inNH; 07-08-2020 at 06:00 PM.
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post #9 of 20 Old 07-08-2020, 06:52 PM
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Quote:
Originally Posted by CJ7inNH View Post
Thanks John. We're getting there.

I cleaned off the wire that is spliced into the wire running out of the bulkhead. Yes, it is a resistance wire. That wire then runs across to the passenger side. Once at the passenger side it is spliced off - one wire goes to what was the positive terminal on the coil, the other goes to the i-post on the solenoid. The plan is to remove the resistance wire and replace it with a 12 gage to the new HEI. Should I go directly to the HEI with the 12 gage or splice in with something less?
The wire coming out the of the bulkhead connector is a 10 Gauge so using a 10 Gauge wire is better.

Quote:
Sorry, I miss spoke on part of the last post. The new HEI has one terminal marked "Bat", that's the one that the new wire from the bulkhead will power (still wondering if I should run the 12 g right into it or splice something less). The other terminal on the HEI is marked "Tach" not coil.
Run a 10 gauge wire right into the "Bat" terminal.

Quote:
Going backward off the old coil, a red wire went off of the positive side of the coil, then spliced into the resistor wire, then the resistor wire spliced into the large red wire running into the bulkhead (that wire also spliced off down into the ICM).

From what you explained "the current running through the wire the tach is feeding power to the coil" is what the tach is reading. So is that large wire coming from the bulkhead running into the tach at some point? It must.
Yes it is. It's coming right from the ignition switch , through the tach to the bulkhead connector

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Quote:
Now confusing me again is that with what I thought would work, former resistance wire that is now a 12 g from the bulkhead would splice off on the passenger side having one wire run to the "bat" terminal and the other to the "tach" terminal.* But it's the same wire.
Connect nothing to the "Tach" terminal.

This is the simplest way to connect your HEI distributor into your jeep with or without the OEM tach.

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They sell a nice pigtail to connect it also.

61qjbl7ea-l._ac_sl1000_.jpg

https://www.amazon.com/Xislet-Distri...4256046&sr=8-1

Just don't use the green wire with the OEM tach.
The green wire is only for use with an aftermarket tachometer.

If you wan't to get real crazy you could pull that wire and terminal right out the bulkhead plug and replace it with an new terminal and a 10 ga. wire and run it right to your distributors "Bat" terminal. You really don't need the wire connected to the "I" terminal. It's only there to bypass the resistance wire in the Duraspark system.



Quote:
Remember the show Welcome Back Kotter?* "I'm sooooo confused".

I wonder if this is why I've read posts saying you can't run the OEM tach and need to get a newer replacement one.

I feel good about getting the HEI hooked up now. Thank you!

I'll let you think through the tach situation.

Enjoy San D!
Just being 'Tactful'

Anyone here in San D need some help?


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post #10 of 20 Old 07-10-2020, 06:07 AM Thread Starter
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Hugely helpful as usual. Thank you John!

I've got the new 10 g wire ready to go. Going to start snipping out the old tonight and get the new HEI wired in. I'll test to be sure things are running all right, then address the tach as needed.

I'll provide an update as well.

* Found an older post of yours showing the bulkhead wiring. That was also very helpful in figuring out what the "extra" wires all went to at one time.

Stay tuned.
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post #11 of 20 Old 07-12-2020, 10:12 AM Thread Starter
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Well there's good and there's bad.

The good is after some turning of the distributor she started and would idle, the bad is that the key does nothing to turn the engine off.

Some background:
1- When I removed the old distributor I did have the 1st cylinder at TDC on the compression stroke. I lost my reference point from the old one so put the new one in as close to possible as I had the old one with the rotor pointing at the 1 post on the distributor. On the first turn of the key I had the loudest backfire I've ever heard (first backfire since I've owned the Jeep). I'm assuming this was a result of the timing being way off. A few turns of the distributor and she started and idled.

2- During one of the attempted starts I went back to adjust the distributor and saw some smoke near the passenger side fender - couldn't tell where it came from. On the next try there was no power. I remembered the main power feed that attaches to the side of the selenoid felt like it may be rotted away inside. The right side of the solenoid has the positive battery cable running to it, the main power feed (E/Y from John's bulkhead pic), and a wire running to the alternator. The main power feed and the alt wire both had a fusible link spliced in. The main feed line was definitely rotted out. I could feel it broken inside the insulation and was able to kind of wiggle it back together and got power. I cut the fusible link wire out, reattached to the selenoid, and was able to get the motor running.

Now she runs, but will not turn off at the ignition/key.

? Could that main power feed not getting a good connection caused the selenoid to become damaged and result in the no shut off issue? When I cut the fusible link wire out it was pretty rusty and crappy all the way along the inside wire. I don't think the fusible link burnt up during one of the attempted starts, but it's possible.

? If I get a new selenoid are there any brands to steer clear of? Autozone has a Duralast for $12, Quadratech has an Omix for $22 and a Crown for $30. For the price it doesn't hurt to have a spare ready to go if needed.


There seem to be a lot of posts regarding the "can't turn off" issue. I'll do some reading up as well.

FYI. I thought this was a good video that explains how the selenoid works

Thanks.
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post #12 of 20 Old 07-12-2020, 12:52 PM
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Is the engine not turning off or not stop cranking?

That video wasn't bad but the part about the solenoid 'pushing' kind of threw me....


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post #13 of 20 Old 07-12-2020, 01:48 PM Thread Starter
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I actually didn't watch the entire video. I liked how it shows how it sort of completes a circuit.


The engine is fully running at idle, not really well since I still need to adjust the timing, but 100% running and not turning off.

I started reading some posts about this. Tested to see if the solenoid was getting power with the key off (while not running and key off) it is not.


Any thoughts/help would be great.

Thanks!
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post #14 of 20 Old 07-12-2020, 05:41 PM
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Is the engine not turning off or not stop cranking?


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post #15 of 20 Old 07-13-2020, 05:02 AM Thread Starter
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The engine is not turning off.

I read this post (need to read it again) that talks about installing a diode on the brown wire running to the alternator. With no local Radio Shacks around anymore I'll most likely have to order online once I get details on this.

Here's the post:


https://www.jeepforum.com/forum/f8/2...stall-3866297/

I have run in to one problem with this install.. The switch no longer shuts the jeep off. I have to stall it out. From what I have read, there is an in-line diode I need to install in the brown wire coming from the alternator harness to the red w/ tracer wire for the switch. From my understanding, the brown wire is important with the system charging. After the alternator starts spinning and you shut the switch off, the alternator sends power back up the wire resulting in the engine not shutting off. The diode is basically a check valve preventing back-flow of electric to the relay. If anyone has ever heard of this and can offer some input, I'd appreciate it.
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