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Detmar tach

mikehopkins

New member
"gentlemen, tach in an '81

"gentlemen, tach in an '81 Tempest not working. Det-mar square body, 6000rpm unit with 3 posts; 1 to coil, 1 to power and a ground. there are also 3 sunken posts in the center tapped for screws,1 at 12:00, 1 at 6:00 and 1 at 3:00. there is a small brass jumper plate going between 6:00 and 3:00 but with only 1 screw attached at 6:00. wondering if the 3:00 screw fell out; reluctant to install and find out to prevent shorting system. no replacement tach found, planning on seperate dash mount if this one is toast. understand it is possible to test using a battery charger but unclear as to how? any help welcome."
 
"Mike, if there's a jumper

"Mike, if there's a jumper plate, it is very UNLIKELY you will hurt anything by installing a brass screw. Worst case is that it may configure the tach incorrectly and it will be "off'.

I don't know who smoked what when they told you a battery charger could test a tach reliably - too many variables. Best way is to use a photo-optical tach on the actual installation.

If you know someone into electronics that has a pulse generator, you could use that, too. Alternatively, you could pull the tach and do a temp install on a friends boat or an older auto using a distributor.

If you are into electronics, you can build a test circuit pretty cheaply using a 555 IC with a few components."
 
"Mike...sounds like that jumpe

"Mike...sounds like that jumper is to tell the tach how many cylinders???

Anyway here is a link to the battery charger procedure (I've never tried it myself), keep in mind it may not work with the really new chargers that need to be hooked to a battery before they will turn on.
http://www.cadvision.com/blanchas/54pontiac/tach.html

Not sure what yours looks like, might be similar to this one for sale???

http://www.oldboatgauges.com/detmar.html"
 
Bob:
thanks for the link to t


Bob:
thanks for the link to the battery charger test. It makes the assumption that all chargers use a full wave circuit. I see no reason why those won't work assuming the tach's input conditioning circuitry handles the sine waves ok. The tach will be off by a factor by 50% if the charger only uses a half wave circuit.

I've built a few of the 555 circuits - much easier to get the 33% duty cycle that represents the 30 deg dwell on a V-8.

Concur with the strap defining the configuration of the engine.
 
"Makomark....like I said...I&#

"Makomark....like I said...I'm not sure if this would work with some of the newer "Smart" chargers but if you've got one of the old kind kicking around (one that hums) with a xformer, diodes then it couldn't hurt to try it."
 
"thanks guys, gave it a try ev

"thanks guys, gave it a try every which way and no response, will assume tach is shot. issue now is that cavity is square and all current replacements round; will put back original and go to solo dash mount."
 
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