Author Topic: "bench test" tach  (Read 632 times)

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Offline hada76

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"bench test" tach
« on: April 12, 2010, 10:14:38 PM »
is there a way to bench test a tach ..i want to be sure mine is bad before i buy another one   thx
'76 T/A 455 4sp   sold
'78 T/A Y88 auto  sold
'79 T/A WS6 4sp
'70 Z/28 4sp

jeff

Offline Rick

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Re: "bench test" tach
« Reply #1 on: April 12, 2010, 10:54:28 PM »
ta78w72 (Russ) is your man -- he can fill you in when he drops by! ;)

Offline ta78w72

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Re: "bench test" tach
« Reply #2 on: April 13, 2010, 09:15:36 AM »
I think you could do it with a pulse generator, but I haven't tried it.  You would need to know the characteristics of the signal at certain speeds though so you could generate the signal.  Or you could run a wire from another car that has a working tach.

The best way to determine if your tach is broken is to connect one lead of a light bulb to the signal wire from the distributor and the other light bulb lead would go to ground.  Start the engine and increase the RPM's.  The bulb should dim.  If it does, you've got a proper signal and the tach is bad.

The other way is to connect a hand held tach to the distributor signal wire and see if that works.  If it does, you've got a bad tach.  We used to use the combination Dwell/Tach meters to adjust the points on the old type distributors.  I happen to still have mine....it comes in handy.  I know they really don't make this meters anymore, but someone you know might have one collecting dust in their garage.

Offline hada76

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Re: "bench test" tach
« Reply #3 on: April 13, 2010, 09:37:12 AM »
so you are saying the bulb should get dimmer as rpms go up? can you use a test light?    cant help thinking its a loose/bad wire somewhere   would any fuses affect the tach?   thx for replys
'76 T/A 455 4sp   sold
'78 T/A Y88 auto  sold
'79 T/A WS6 4sp
'70 Z/28 4sp

jeff

Offline ta78w72

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Re: "bench test" tach
« Reply #4 on: April 13, 2010, 10:22:37 AM »
You could use a test light.  Yes, it gets dimmer as the RPM's increase.

Offline hada76

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Re: "bench test" tach
« Reply #5 on: April 13, 2010, 10:26:18 AM »
afraid youd say that...the reason i asked about fuses is i used to get a click when i turned the key (choke?) now there is no click   also how about a bad ground?
« Last Edit: April 13, 2010, 11:40:29 AM by hada76 »
'76 T/A 455 4sp   sold
'78 T/A Y88 auto  sold
'79 T/A WS6 4sp
'70 Z/28 4sp

jeff

Offline ta78w72

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Re: "bench test" tach
« Reply #6 on: April 13, 2010, 11:52:54 AM »
The tach is grounded to the cluster can itself.  If you had a bad ground there, your lights wouldn't work.

I don't know about electric chokes.  But there could be a fuse on the pink wire coming up from the fuse box to the tach.  You could measure that wire at the connector for voltage.  You could have blown the fuse.

Offline hada76

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Re: "bench test" tach
« Reply #7 on: April 14, 2010, 12:58:40 AM »
found my old tach/dwell meter and hooked it up...worked fine. guess i have to face the facts.  thx for info russ
'76 T/A 455 4sp   sold
'78 T/A Y88 auto  sold
'79 T/A WS6 4sp
'70 Z/28 4sp

jeff

Offline ta78w72

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Re: "bench test" tach
« Reply #8 on: April 14, 2010, 10:46:35 AM »
Well, before you chuck the old tach, measure for voltage at the pink wire of the tach connector just to make sure you've got power to the tach.

Keep that old tach/dwell meter.  They come in handy!