installing tachometer

[ttomas]

Okay guys, I am already over my head with this one so try and take it easy on me. I have a new Datcom tach. If my alternator has an ac tap I am in good shape. it is a mci9 6v92ta NJ transit bus. Now two questions: do you know if my alternator has a tap? I am going to install the 12volt  alternator out of my old bus. It will go where the AC compressor was in the rear. Can I use the 12Volt auxiliary alternator to feed my Tachhanks tomas

[RTS/Daytona]

see my reply (Pete RTS daytona):

Bruce,

Last year I sold a number of salvaged Monaco diesel dashboards at the Bussin 2004 rally - here is my tach installation info:

If you have the VERY Common gear driven / oil cooled Detroit Diesel Delco 50DN 270 Amp 24 volt The alternator's "R or Relay" terminal puts out a 12 volt signal at the following pulse rate - (2.93 alternator drive gear ratio x the alternators 3 pole stator x 2 because of the (full wave rectified) = 17.58 pulses/rev)

Many Tachs that are available for 12 Pulse / rev (Detroit DDEC's put out 12 pulse/rev as well as most modern computer controled cars) - there is usually a fine adjustment device behind the tach.

To re-calibrate for 17.58 pulses/rev - first start the engine & record the default 12 p/rev RPM reading at a steady idle-with a screwdriver, carefully turn the adjustment device until the tach registers approx 2/3 (actually 12 / 17.58) the original recorded RPMs.

Note 3 -> For you engineers - You can use a 12 volt A/C transformer with an unfiltered full wave rectifier to test - 60 cycles via a full wave bridge=120 pules/sec

120 pulses per second tach input at:
17.58 p/rev (D/D Delco 270 ALT) = 410 RPMs
12.00 p/rev (Computerized DDEC) = 600 RPMs

use the gear driven 24 volt alternator's "R" terminal  (It's actually a 12 volt pulse (inverter sine) wave on a 24 volt generator)

The R terminal on the 12 v alternator is also 12volt pulse - BUT because it will be BELT DRIVEN - and may slip ?? it wouldn't be as accurate - and you have to figure out what the pulses per rev will be with the pully ratio and the PTO or engine pully you would be using to drive it -

Pete's instructions copied and pasted from another board to keep everything together. Your Friendly Moderator 

[Cary and Don]

Our Datcon tach had differant settings on the back to get the right rpms.  If you don't have the directions it might be trial and error.  It took us twice with the directions. That was using the second tach.  The one that was sent with our new dash wasn't the right type and it took us a long time to figure that one out.

Don and Cary
GMC4107

[TomC]

Many 2 strokers also have a mechanical tach drive off the rear of the blower opposite of the drive.  You can run a synchronous generator for the pulse.  Good Luck, TomC

[DavidInWilmNC]

Many 2 strokers also have a mechanical tach drive off the rear of the blower opposite of the drive.  You can run a synchronous generator for the pulse.  Good Luck, TomC

Tom,
Where would one find this on an 8V-71 (if it's equipped with one)?  Thanks.

David

[TomC]

The tach drive on mine is on the rear side of the blower (opposite end from the governor).  The blower has two shafts in it-on the rear the right side has the drive and on the left is where my tach drive is. You may or may not have it though.

[Stan]

DavidInWilmNC: On the 8V71 engines in MCI that I have had, it is staring you in the face between the fan belts. About the middle of the engine, a screw-on cap about 1" in diameter - similar to a speedo cable connector. Just remove the cap to access the connector. Most people don't use it because of the reliability of the newer electronic tachs that run of the alternator or count the flywheel ring gear teeth. HTH