Testing Tach? -Now with pics!

[NW Redneck]

The tach on my 1975 OMC powered boat is the only gauge that isn't working right. It seems fine at idle up to about 1800rpm or so. (matched with tach/dwell meter when setting timing at home). Past 1800+/- it 'slows down' and stops about 2600 with plenty left to go on the throttle.

I removed and cleaned (again) the connections on the coil. My manual lists 2 types of OMC tach, and this one doesn't match either!:/ There's no name on it, but just under the needle pivot there is a small stylised 'MW' The ones in the book only have 2 wires-one black (ground), one grey(coil).

Mine has a bundle of 4 small wires- red, white, black & green. As is, I have black gong to common ground, red to 12v+, green to coil, and white to nothing(looks like it never has been connected- all bundled up) It is also the only guage without a light. Maybe what the white wire is for?

First- is the wiring on this thing right?
Second- is there any way to test the tach to see if it works properly? I don't have a spare tach available to swap in. All info greatly appreciated.

Colin

 

[ricksrster]

Re: Testing Tach?

Re: Testing Tach?

OMC what? I/O or outboard? What size motor?

 

[NW Redneck]

Re: Testing Tach?

Re: Testing Tach?

Sorry! Its an OMC 175 I/O, ford 302 engine.

 

[ricksrster]

Re: Testing Tach?

Re: Testing Tach?

Make sure the little selector switch on the back of the tach is set for 8c if it is an eight cylinder. Here is a link to for onfo that Teleflex has for tachs. http://ww2.tflx.com/PDF/99320.pdf

 

[NW Redneck]

Re: Testing Tach?

Re: Testing Tach?

Here's some pics to help out. I removed the rear cover and found a slide switch (at bottom, 2nd pic) labeled 4c 6c 8c, it was set to 8c. There are three 'terminals' on the left that have turnable 'rings' with a tab. (Third pic, tabs don't show too well) On the tabs are K9 on the top one, and C9 on the other 2. All the tabs are pointing away from the tach. Blue and yellow wires go to light. (Bulb burnt out)

 

[b00tstrap]

Re: Testing Tach?

Re: Testing Tach?

Replace the dang guage already! d

Seriously, it looks a bit "weathered from the pics"

 

[NW Redneck]

Re: Testing Tach? -Now with pics!

BS- I know, I know.....
Money's a little tight right now so just trying to see if this one will work. The bezel trim is the only thing that shows any corrosion. Inside is bright & shiny with no signs of moisture. Was hoping it would just be a wiring/setting issue.

So IS there any way for me to test the tach? Can/do shops have a way to test them?

Colin
Makin' wire outta pennies here.

 

[b00tstrap]

Re: Testing Tach? -Now with pics!

So IS there any way for me to test the tach? Can/do shops have a way to test them?

I hear you on the saving costs part. A tach works off of small millivolt pulses. I am not aware of a shop test other than connecting the instrument to another known good motor. Do you have the ability to test off of another 8cyl block? It would eliminate wiring, etc..

 

[NW Redneck]

Re: Testing Tach? -Now with pics!

Unfortunately I don't have another engine available to test it on. I'm gonna go over my whole wiring harness again bow to stern to see if I might have missed a bad wire/connection. Either end seems good so it might be in the middle somewhere.

If I can't find anything I'll just bite the bullet and get a new tach as soon as funds are available.

Have a piece of Aztec gold for your trouble...........:devil:d

 

[ricksrster]

Re: Testing Tach? -Now with pics!

To check I/O tachs another PDF file from Teleflex http://ww2.tflx.com/pdf/stdign~1.pdf

 

[b00tstrap]

Re: Testing Tach? -Now with pics!

I am a fan of running temporary jumper wires to do the test. Sometimes it rules out hard to find wiring issues that you probably need a long jumper wire to be able to test correctly with a DVM anyway.

Just a thought..

 

[Barnacle_Bill]

Re: Testing Tach? -Now with pics!

Somebody makes a box that will test tachs. At least outboard tachs. I seen one used just today on my teleflex tach. (at Bass Pro). Unfortunately, he couldn't calibrate it with it so they ordered me a replacement. If I remember, when I picj my tach up I will try to get more info on that box he was using.

 

[NW Redneck]

Re: Testing Tach? -Now with pics!

BS- So try running red to battery +post, black to engine block, green to coil sound right? Or should i run the black to the battery -post? Thanks again.

 

[Barnacle_Bill]

Re: Testing Tach? -Now with pics!

Here is the tester they used. http://www.teleflexmarine.com/document/productpdf/document1002_5_1051.pdf

 

[b00tstrap]

Re: Testing Tach? -Now with pics!

BS- So try running red to battery +post, black to engine block, green to coil sound right? Or should i run the black to the battery -post? Thanks again.

I am not familiar with the exact instrument, but that sounds right. I would run the black to the battery negative as it is a better ground for testing.

 

[KaGee]

Re: Testing Tach? -Now with pics!

I agree, The coiled up wires appear to be your dash light.

Keep an eye out on EBay.... tachs can be bought pretty cheap.

 

[NW Redneck]

Re: Testing Tach? -Now with pics!

I'll try that next time I go out. I've already got the wires cut with clips for the tach end!

 

[BLU LUNCH]

Re: Testing Tach? -Now with pics!

That Teleflex PDF file from teleflex is real helpful I used to find out that my tach has all the right voltages but it's junk.....

 

[JohnRuff]

Re: Testing Tach? -Now with pics!

I can't find this tester for sale anywhere on the internet. Suggestions?

John

Here is the tester they used. http://www.teleflexmarine.com/document/productpdf/document1002_5_1051.pdf

 

[b00tstrap]

Re: Testing Tach? -Now with pics!

Faria instrument tester

PN: 91203
PN: 91202 includes OMC option


http://faria-instruments.com/site_manuals/IS0087C.pdf