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OMC Tachometer testing

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71V153

Regular Contributor
Anyway to test a tach? And/or test the signal coming into it?

Have both analog & digital multi-meters. Nuthin fancy but they work.
 
You can test the signal with a peak reading volt meter or a DVA adapter for your multi-meter. Should have about 8-9 volts.
 
Tachometer gets it's signal from the alternator. Use the multimeter you have to test to see if the alternator is working. Simply check the voltage at the battery terminals, comparing voltage when running to that when not running. Should be higher when running a few minutes, indicating the alternator is charging the battery..
 
Regardlsess of where the tachometer needle is sitting, it should zero out when the the key is turned to the ON position (votage applied to the + lead of the tachometer).

For what it's worth... I prefer the analog meter.

The tachometer operates off of the 6 pulse charging system. First make sure that the 3 wire rectifier is functional as follows.

(Small Rectifier Description & Location)
(J. Reeves)

On most 2,3,4,6 cylinder engines, the small rectifier is located on the starboard (right) side of the engine just in front of the engines electrical wiring strip. There are a few older V4 engines that have the wiring strip on the rear portion of the engine and the rectifier would be located just under that terminal strip. The smaller horsepower engines usually have the rectifier located on the starboard side of the powerhead close to the carburetor area.

The rectifier appears to be a round object approximately one inch (1") in diameter and also about one inch (1") high. The base of it is sort of triangular in appearance and is attached to the engine with two (2) screws/bolts..... usually one screw/bolt is larger than the other. The rectifier, depending on which one your engine uses, will have either:

One Red wire, one Yellow wire, and one Yellow/Gray wire, or One Red wire, and two Yellow wires.

Note that either of the above rectifiers could have a fourth wire which would be Yellow/Blue

Thousands of parts in my remaining stock. Not able to list them all. Let me know what you need and I'll look it up for you. Visit my eBay auction at:

http://shop.ebay.com/Joe_OMC32/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
********************
(Small Rectifier Test)
(J. Reeves)

Remove the rectifier wires from the terminal block. Using a ohm meter, connect the black lead of the ohm meter to the rectifier base (ground), then one by one, connect the red lead of the ohm meter to the yellow, yellow/gray, then the red wire (some rectifiers may also have a fourth yellow/blue wire. If so connect to that also). Now, reverse the ohm meter leads and check those same wires again. You should get a reading in one direction, and none at all in the other direction.

Now, connect the black lead of the ohm meter to the red wire. One by one, connect the red lead of the ohm meter to the yellow, yellow/gray, and if present, the yellow/blue wire. Then reverse the leads, checking the wires again. Once more, you should get a reading in one direction and none in the other.

Note that the reading obtained from the red rectifier wire will be lower then what is obtained from the other wires.

Any deviation from the "Reading", "No Reading" as above indicates a faulty rectifier. Note that a rectifier will not tolerate reverse polarity. Simply touching the battery with the cables in the reverse order or hooking up a battery charger backwards will blow the diodes in the rectifier assy immediately.

Thousands of parts in my remaining stock. Not able to list them all. Let me know what you need and I'll look it up for you. Visit my eBay auction at:

http://shop.ebay.com/Joe_OMC32/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
 
No problem with the rectumfryer.

Is there a way to check the Tachometer itself? It is old and has seen alotta use. Before it died it would act kinda erratic, and when it did 'work' I always thought it read low. Now nuthin. Part #174048. Has settings from 1 - 6 and is set on 6.

Was just given a old Merc tach w/4 settings. 1 - 3 are for 4,6, or 8 cyl four cycle engines. #4 is for 12 pole alternator. Any reason this shouldn't work?
 
The old tach does not have terminals. The wires come through a rubber grommet on the back.

There's a three pin connector that plugs into the control box. Wire colors are black, grey, and purple.

Assuming black is ground, purple is hot, and grey is the signal. Is this correct?
 

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That engine's charging system emits six (6) pulses which operates the tachometer.

As such, the tachometer must be one that functions off of a six (6) pulse charging system.

Normally, such a tachometer has four wires: Black (ground), Purple (voltage 12v), Gray (signal), Blue (light bulb v.). It would also have a dial which is to be set to number six (6).

Unlikely that Mercury tachometer will function with that system.... Mercury is not my line of expertise, just an opinion.
 
Joe, it would be fun to run a small compass around the flywheel that he showed and see what kind of results you get.
 
Been a lil chilly lately actually. Posed to warm up tomorrow.

Merc tach don't work worth a chit. For gits'n shiggles I changed the setting(s) & it started to read backwards ...? Whoops! Heh heh heh

Ah well. Reckon I'll have to play it by ear tomorrow. Cuz I'm goin boatin come hell or high water!
 
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