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So.... I just installed one of these tachometers from a 1990 Topkick in my 6.2 CUCV. I was previously running a VDO tach off the alternator.

The factory tach reads about 25% of the actual engine RPM's, and I checked it vs the known VDO tach. I tried changing the jumper from A to B, and running it without the jumper also. There is a slight change (I think) in reading, but it looks like A reads the fastest, and I'm talking maybe by 25-50 RPM max. At 2000 RPM's the factory tach is displaying just a tick under 500.

I tried getting one of the Dakota Digital Diesel Tach adapters, but that actually won't let it read over maybe 200 rpm max. The tech guy at Dakota says they don't work with a Diesel style tach, and he seems to have been correct.

Doing research it looks like the Cat engines that would have came with this tach would have been a 3126 or 3166, and previous to that they could be had with a 3208. From what I have found, the Caterpillar ECU takes a signal from a couple engine speed/timing sensors and then puts out a programmable tach signal the the tachometer. The reference doc I found states that GM vehicles would use a 17.5 pulse per rev signal from the ECM into the tach.

I really want to make this thing work, as it looks awesome. I guess I'm looking for some help. What I need, is to be able to take the alternator signal and multiply it by 4X, and ideally have some adjustable gain to get as accurate as possible. Anyone know how to do this, or does another type of tach trigger exist that would be compatible with this OEM tach?

What is the actual signal from the alternator? It looks like about 6.6 VAC when I put the meter on the tach terminal and grounded to the case. What is the difference between a tach designed for a diesel and one that is designed for a gas (other than the obvious of where they are hooked up)
 

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Oh man, guess I'm going to have to dig out the info I have on this. I've got one of these sitting in my barn I want to put on. I have the pickup sensor as well. I'll try to get back here with some info when I have a minute. I also have a wire schematic from the truck it came from.
 
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So here's what I know about this. The Topkick tachometer works off a ground switch. The switch is a magnetic pickup. It picks up the teeth on the flywheel. Looks a lot like the linked one below. I have the sensor around here somewhere.... The wiring diagram I have for a C5/6 with a diesel is below as well.

The wiring for the tack is pretty basic, 12v to the + terminal, a ground, and one that goes over to the magnetic pickup switch. I just havent found a good way to mount the pickup to my NV4500 bellhousing yet. I've also had about 1000 other projects and a crap ton of overtime, so sadly my tach cluster sits :(

https://www.summitracing.com/parts/dak-dsl2e?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-dakota-digital&gclid=Cj0KCQiAoJrfBRC0ARIsANqkS_4Vn11_bvmoWjlOU7w64T3yXZfSIxcyndqrYjN5IOMHES2CDW-tPmUaAjThEALw_wcB

 

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Can you repost the wiring diagram please? It just shows a broken image.

Is the magnetic pickup a one wire? And do you have any idea of the pulses per rev?
 

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Attached It's a two wire. I do not remember the pulses per revolution, but I believe it was a CAT 3208 I pulled this from. When I did my earlier research a 3208 and a 6.2 had almost exactly the same number of teeth on the flywheel.
 

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Awesome. Anyone have the part number for the pickup sensor?
Theoretically you could use any magnetic pickup sensor. Ideally the smaller the better. The one off the CAT is easily bigger than a quarter around. You just need something to pulse with the teeth of the flywheel.
 
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Well, I'm close to having it work correctly.

The tach is apparently designed to read at 134 pulses per revolution, as that is the tooth count on the Cat flywheels. I found a tach adapter box from Widget Man that will change the signal to different pulse counts. I tried a 1:4 (or 4x the pulses from the alternator) and it still reads slow. 1500 rpms at 2000 rpm actual engine speed. Next it tried a 1:6 ratio. That got me to 2250 rpm on the tach at 2000 actual. Using some algebra I calculated out that I need a ratio of 1:5.33 to make it accurate. The box I had doesn't have that ratio, but they are custom making one for me, hopefully have it next week.

134 pulses / 5.33 = 25.14 alternator pulses per revolution on the 6.2 is approximately what I'm getting with the stock CUCV pulley ratio.
 

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Well, I'm close to having it work correctly.

The tach is apparently designed to read at 134 pulses per revolution, as that is the tooth count on the Cat flywheels. I found a tach adapter box from Widget Man that will change the signal to different pulse counts. I tried a 1:4 (or 4x the pulses from the alternator) and it still reads slow. 1500 rpms at 2000 rpm actual engine speed. Next it tried a 1:6 ratio. That got me to 2250 rpm on the tach at 2000 actual. Using some algebra I calculated out that I need a ratio of 1:5.33 to make it accurate. The box I had doesn't have that ratio, but they are custom making one for me, hopefully have it next week.

134 pulses / 5.33 = 25.14 alternator pulses per revolution on the 6.2 is approximately what I'm getting with the stock CUCV pulley ratio.
Hold on. You've got a diesel (4k) tach and it works with the alternator? That usually doesn't work. The alternator I believe puts out an AC voltage signal. The diesel tach from a topkick wants a ground signal. Now I'm curious.
 

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Hold on. You've got a diesel (4k) tach and it works with the alternator? That usually doesn't work. The alternator I believe puts out an AC voltage signal. The diesel tach from a topkick wants a ground signal. Now I'm curious.
Yes, it's a 4000 rpm diesel tach.

The tach isn't looking for a ground signal, it's looking for a voltage pulse. A magnetic pickup is basically a self contained voltage generator. The pickup is putting out an AC voltage that varries in both voltage and frequency.
 

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Well sh*t, you're right. I looked at it like a square wave signal (hall effect sensor). If you can get the electrical part of this working you may of finally cracked the tach questions for the 6.2!
 

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1990 chevy cheyenne r30 6.2 diesel granny 4 speed.
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Hey was reading this forum. Im also looking to do this setup for a 6.2 diesel but im don’t want to go off the alternator. Im wanting to use engine speed sensor that goes in the spot where the vacuum pump goes. Im wondering how would i wire the sensor to the tach and it be accurate.
 

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Heres what i have. The tach has three wires and the sensor has a 2 wire plug. How would i go about this. Any help is appreciated.
 

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Im not sure but you pulled up the right thread. Half the guys on this thread are not around anymore but hopefully someone will be along to help. Nit sure if Glagulator is still active but he may know. Good luck.
 

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Im not sure but you pulled up the right thread. Half the guys on this thread are not around anymore but hopefully someone will be along to help. Nit sure if Glagulator is still active but he may know. Good luck.
I took the wiring diagram that i found on the tread and wired my system up before putting it in the truck. With everything hooked up the way the diagram showed. I spun the sensor and the tach would read how fast the sensor was spinning. Not sure if its accurate or not but I’m going to try to put it in the truck and do the same and see if the idle speed of the engine will show up on the tach correctly. Hopefully this solved my problem that i had all along. Thank you for replying back. @JCDiesel
 

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Aight! Hold the presses on this one. Within the last week I dove WAY into this again for my Cummins swap. I've got one of these sitting in my barn working its way into my truck shortly. The diesel tach uses a magnetic pickup that runs off the ring gear. 3208 CAT has 134 ring gear teeth, so this tach is looking for 134 pulses per revolution. V8 gas tach is looking for 4, 6 cylinder 3, 4 cylinder 2 pules. I do not know how many pulses the pickup you have puts out per rev. Is it cut in half because it's driven off the cam? 4 pulses per crank rev? If you had an o-scope it would be really easy to find out, without you could probably do it with a multimeter.

What I figured out is you can take the back board from a V8 gas tach (5k prefered) and bolt it up to the diesel tach. From here with a little wiring depending on application you can get the tach dialed in. Below is the article I started into with this as a reference.

If you read back a ways our good friend Diaric had to do similar but a little more complicated way. I found a guy on ebay who sells the boards and will even calibrate your speedo if you send it to him. Second link below

Hopefully that helps a bit with this! Really the biggest issue you're going to have with this is how many pulses the tach is looking for. It's going to be hard to compensate on that one.


 
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