Tachometer hook up

Rrent

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I think I have the Blazer somewhat ready to drive (fingers crossed/knock on wood). Now I’m thinking gauges. Water temp & oil pressure seem pretty straight forward.
Will I need to know (somehow) the RPM’s so I can calibrate the tach… so I’ll know the RPM’s?
Thinking of this tach - unless someone has a more “military vehicle” style tach for sale?48612881-8F2A-4E49-8AFC-FEF815CE5707.jpeg
 

Keith_J

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I don't have the specifics on this brand/model. I had success with a VDO brand which can use multiple inputs including an alternator AC output. This tach has multiple switches and a potentiometer to fine tune.

The driver side alternator has this output as a push on connector with a brown wire. It goes into the cab into a little plastic box called the tach buffer module, used for the STE-ICE diagnostics. Brown wire, circuit 104 in the TM. Appendix E11 in the -34 TM.

I believe the 27SI alternator is a 16 pole and the drive ratio is 2.583:1.
 

Barrman

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I thought I answered this on Sunday. But, I don't see my reply.

Hooking it up like Keith just wrote about is part one of your job. Then, you get to calibrate it which he also wrote about. There is also an clamp on pressure sensor that can be used to get an rpm signal through a timing light that displays rpm. Along with the Snap-On machines.

Just set it to 800 at fast idle and it should be 650-700 and warmed up idle. If you have that, you will be pretty close for going down the road until you get a way to calibrate it.

One "feature" of an alternator driven tach is you can tell when you belt is slipping and needs adjustment.
 

Rrent

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Looks like the Dakota digital interface will allow me to use the terminal on the alternator. Hopefully I’ll figure out how to calibrate it - if need be.
 

Spot_ll

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@patracy can probably tell more, but the Tach in his M1008 says Diesel on the face(like you have pictured), and is band clamped to the passenger side alternator housing. It reads off the alternator, I’m guessing similar to how a reluctor wheel works. How it was calibrated, he’d have to answer that.
 

Keith_J

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Alternators have at least two poles, most common are 8 to 20, always an even number as there are in pairs, a north and a south pole. With each pole pair, the voltage in the stator coils changes direction but more importantly, this change represents power generation. Higher RPM alternators require fewer poles, this is why the 27SI was chosen for this engine as it has 16 poles.

With the voltage switching 8 times per revolution, this alternator achieves full current at an engine speed of 1800 RPM. And this voltage switching is used by some tachometers as a much more precise engine speed signal than the traditional breaker point signal which for a V8 is 4 pulses per engine revolution.

If you were to use this signal from the alternator to drive a regular 8 cylinder tach, it would read 5 times faster than actual engine speed..if it could resolve the alternating current.
 

patracy

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@patracy can probably tell more, but the Tach in his M1008 says Diesel on the face(like you have pictured), and is band clamped to the passenger side alternator housing. It reads off the alternator, I’m guessing similar to how a reluctor wheel works. How it was calibrated, he’d have to answer that.
Correct. The pickup is to pick up the field changes to act as a signal for the tach. There's an adjustment method to these tachs. You will need a optical tach to set it up to start with.
 

Barrman

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The Dakota Digital interface works great. I have it on two of my vehicles. I have seen the clamp around the alternator method used on a few M1009's as well. They all need to be calibrated.
 

Keith_J

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I calibrated mine by first calibrating the speedometer using stop watch and interstate mile markers. Knowing the speedometer is spot on, I then bumped the speed on the flat to 75 MPH. That is 3650 RPM governed with 255 85 16s and stock 4.56 axles. Yes, screaming and dragging the SECM box.

Close enough for former government work 😋
 

Rrent

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I calibrated mine by first calibrating the speedometer using stop watch and interstate mile markers. Knowing the speedometer is spot on, I then bumped the speed on the flat to 75 MPH. That is 3650 RPM governed with 255 85 16s and stock 4.56 axles. Yes, screaming and dragging the SECM box.

Close enough for former government work 😋
I have an M1009 - 3.08, so not so much screaming for me.
 

chevymike

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I calibrated my tach (Speedhut gauge), which runs off the "W" terminal off one of my alts by using this unit. You put a piece of reflective tape on the balancer (comes with the tape) and using this, it will give you the RPM's. Then adjust the tach to match. Easy peasy.

 

Camolad

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I was thinking of installing a tach. I was thinking of using an app that calculated the rpm by listening to the engine.
Also slightly off topic but related to the Alt. Right now as I'm trying to locate a replacement I found that these Alts are either listed as out of stock or obsolete. Does anyone have the part # for the non-isolated version? I need to replace # one Gen. I can always Isolate it myself.
 

Sharecropper

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I was thinking of installing a tach. I was thinking of using an app that calculated the rpm by listening to the engine.
Also slightly off topic but related to the Alt. Right now as I'm trying to locate a replacement I found that these Alts are either listed as out of stock or obsolete. Does anyone have the part # for the non-isolated version? I need to replace # one Gen. I can always Isolate it myself.
It is bad manners to hijack one's thread. If you would search this forum you would learn exactly where to obtain a replacement alternator. Just saying.....
 

cucvmule

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I was thinking of installing a tach. I was thinking of using an app that calculated the rpm by listening to the engine.
Also slightly off topic but related to the Alt. Right now as I'm trying to locate a replacement I found that these Alts are either listed as out of stock or obsolete. Does anyone have the part # for the non-isolated version? I need to replace # one Gen. I can always Isolate it myself.

Would you like to define listening to the engine?

Continuing the discussion to further the thread is good, but to have ones own set of defining circumstances may or may not be releant to this thread. Starting a thread to get to your particular set of questions is good so that it may not muddy up someone else's thread. Or to begin a want to buy if you meet the criteria to do such.
 
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patracy

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I used a optical tach sensor like chevymike shared. The tach by sound, well, isn't that precise.

@Camolad best to start up a new thread or post in another alternator thread about replacement options. I don't think there's a readily available one at most parts stores anymore. But if you have ONE good alternator, you can move it to the 24v side. Then use a standard 27SI alternator on the 12v side. If memory serves a 1978 Cadillac Sedan DeVille is what I have in mine. The isolated ground is really only needed on the 24v side of the setup.
 
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