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Old 01-21-2007, 09:51 AM   #1 (permalink)
jrsavoie
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how to? alternator electronic tach

How do you add a wire to an alternator to run an electronic tach?

http://www.4crawler.com/Diesel/Cheap...er/index.shtml

http://www.beede.com/TS-Tach.htm
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96 GMC 6.5 Suburban 1500 4x4 200,000 mi; Fluidamper, remote PMD, Amsoil bypass, K47 air filter
95 Tahoe 2 door 4x4 6.5 200,000 mi; Fluidamper, Amsoil bypass filter, remote PMD, K47 air filter
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Last edited by jrsavoie; 01-21-2007 at 10:00 AM.
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Old 01-25-2007, 11:44 PM   #2 (permalink)
wlshrum
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jrsavoie;

To get a signal from your alternator that varies with engine speed, you will have to go inside the alternator and attach a wire to the AC side of the bridge rectifier. All the external terminals will have DC on them and while the level will vary a slight amount with engine speed, it will depend on the quality of your batteries. If your batteries are new it will vary very little, if your batteries are several years old the level will vary more, but it will still be slight.

The way most electronic tachometers work is by counting some kind of electrical pulses. In the case of a gasoline engine, the pulses produced by the ignition system work very well for triggering a tach, but as you no doubt know, diesel engines do not use spark plugs so there is no ignition system. An alternator is a three phase device, but each of the three phases taken by itself looks like the single phase AC electricity in your house. One way to calibrate a traditional automotive tachometer was to just connect the pickup leads to a 120 volt AC outlet. Since US AC power is 60 cycles per second, depending on whether the tach was set for 4,6 or 8 cylinders you would get a reading on the tach that would be the equivalent of 60 pulses per second coming from the ignition coil of a vehicle. For example, a 4 cylinder 4 cycle gasoline engine produces 2 pulses per revolution from the ignition coil, so a speed of 30 revs per second would produce 60 cycles per second for a tach indication of 1800 RPM. So, basically the tach does not care where the pulses come from, it just needs to know how many pulses the engine produces per second at a given speed (RPM).

It looks like the people at your second link have a unit that you can calibrate for any number of pulses per rev, so it should work if you open your alternator and add a wire to one of the 3 phase inputs to the bridge rectifier.

Hope I didn't put you to sleep with my reply,

Bill
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Old 01-26-2007, 07:50 AM   #3 (permalink)
jrsavoie
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Actually I found a shop in Chesterton, Indiana. John gave me a cheap education and sold me a tach pickup stud.
I've also found that you can install a mechanical tach drive on a lot of alternators. My backhoe originally came with a generator with a tach drive. The only problem is tach drives do not have standard gear ratios. You must figure the final output ratio of the generator - spin the generator 10 revolutions and count how many revolutions the tach drive turns.
Do the same to the alternator with the tach drive installed.
Now you can determine the size of pully required on the alternator.
The alternator pully size shouldn't have much effect on the final output of the alternator.

Thanks for your reply.
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98 GMC Suburban 2500 6.5 4x4 160,000 mi, Fluidamper, remote PMD, FS2500
96 GMC 6.5 Suburban 1500 4x4 200,000 mi; Fluidamper, remote PMD, Amsoil bypass, K47 air filter
95 Tahoe 2 door 4x4 6.5 200,000 mi; Fluidamper, Amsoil bypass filter, remote PMD, K47 air filter
1989 GMC Sierra 2500 2wd 5 spd get-rag trans had 108,000 mi. in 92 when the odometer broke. 6-800,000 miles on the truck. Installed a GM crate 6.5 non-turbo in 2000 about 300,000 miles on the motor, original IP
95 Chevy 2500 xtra-cab 2wd 6.5 190,000 miles,
91 international 4700 low pro, spicer 6 speed, DTA 360 with 18' van box 500,000 miles plus.
NRA Life Member for over 30 years
Vote Freedom First
1994 chev K3500, FS2500 bypass filter, remore PMD, K47 air filter
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