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Old 01-08-2010, 02:10 PM   #1
NewSkewlHauler's Avatar
Join Date: Feb 2009
Location: Goodland, KS
Posts: 233
Year: 1988
Coachwork: Bluebird
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Engine: 8.2L Detroit N/A, Allison
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Adding a tach to a diesel...

I'm looking to add a tach to my bus. It didn't have one from the factory and I really think I kinda need one. Are there better tachometers out there than others? I think that there are different ways to activate them too. I have seen a few in the $125 price range that strap to the alternator. I'm thinking this is the Hall Effect type? Anyone know how accurate these would be? Is there a way to determine how accurate they are? I did a search and I didn't get much except that they are different from the tach on a gasser...which I've installed two before.

Any advise is appreciated.

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Old 01-09-2010, 01:59 AM   #2
Bus Crazy
Join Date: Jan 2008
Location: Upstate NY (Mohawk Valley)
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Re: Adding a tach to a diesel...

I haven't tried one, so this is just the theory, which is sound:

The alternator windings put out AC, which is turned into pulsed DC by going through the diodes. It should be no problem to count the pulses and determine accurate alternator RPM. To be accurate displaying engine RPM,there would need to be a calibration built in to compensate for the speed change of engine pulley versus alternator pulley size. A tach might just be preset for an average or common pulley pair. Also, if the alternator had one or more bad diodes, the number of pulses would be off (and your battery would have trouble charging).
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Old 01-09-2010, 09:35 AM   #3
Bus Crazy
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Re: Adding a tach to a diesel...

Most older diesels (and some newer ones, I drove a 1997 that had it) used the alternator-drive tach. Newer ones run off the ECM.
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Old 01-09-2010, 12:50 PM   #4
Bus Nut
Join Date: Dec 2006
Location: downriver, detroit mi
Posts: 794
Re: Adding a tach to a diesel...

marine diesels usually use an electronic tach using the alternator for the pickup signal, the other option (expensive) is to use a transducer that picks up a signal from a crank sensor or counts the teeth on the ring gear, both are more accurate than the alternator pickup version but are really only necessary for test and development work.
I use a handheld optical tach when checking/troubleshooting for governor set points or rpms, it uses a piece of reflector tape on the crank pulley or flywheel and a handheld reader that you point at the tape to read the signal, it's handy but not permanent installation friendly.
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