Boat Tach on car

[frankr]

quote Mike Stroz:

I might be wrong, but I was under the impression that these tachs took a signal from just one pole so it doesn’t matter how many, it’ll just get a signal once every rotation

Sorry, but most OMC motors have six pulses per revolution. But some have five. The ones with two yellow wires to the rectifier are six pulses, while the ones with three yellow wires to the rectifier are 5 pulses. Most tachs have a switch on the back to select, but not some of the early ones, which were six pulse only. Anyhoo, we don’t know what specific tach he has, not that it matters.

[mike-stroz]

Sounds like I’ll be better off finding a tach meant for a car. Or maybe I can do what Garry said and just make a new face plate!

[outbdnut2]

quote Mike Stroz:

I know this is a pretty unusual question, but I’ve got a 1950’s OMC accessory tach from the 50’s

I thought the first OMC alternator was on 1960 75 HP, and all had generators before that. Are you sure your tach is from the 1950s? or did some of the fat 50s have an alternator?

[frankr]

quote Mike Stroz:

and on the tag it says that one wire goes to ground and the other goes to the “non regulated output side of alternator”. I can post some pictures if that helps

Good point by outbdnut2. I was just going by what the OP said about the connections.

[mike-stroz]

Here are a few pictures of the tach and the tag on the back. I’ve honestly got no idea what era it’s from, it just looked to me like it might have been from the 50’s. I did unscrew the back and it has a potentiometer for adjusting it for different motors inside

[dave-bernard]

Just hook it up and see if it is close. should be 600-700 rpm at warmed up idle.

[outboard315]

You could always gut the tach and find a car tach with the same sweep and convert it.

[mike-stroz]

quote Outboard315:

You could always gut the tach and find a car tach with the same sweep and convert it.

That’s not a bad idea, I might try that if it’s not compatible.

I’m definitely going to try it out first just to see if it’s anywhere close to working, but where should I hook it up to on the alternator? It’s got 4 posts coming out of it, one being ground. I’d post a picture, but it’s pushed in the garage now and I can’t get around it to open the hood…

[garry-in-michigan]

The 1958 fifty horse Starflite was the first alternator equipped Evinrude. It was built by Motorola and I saw it show over a 35 amp output with a low battery. It would drop back to two or three amps when the battery was fully charged. They had some problems with overheating regulators. The under hood temperateure was about 150 degrees running, but could go much higher when just setting in the hot sun. We replaced some regulators as Motorola tightened up there burn in and testing. Later models were better protected from static electricity. Like OMC always did, when they got it perfected it was discontinued for a cheaper model. . . 🙂

[frankr]

quote Mike Stroz:

quote Outboard315:

You could always gut the tach and find a car tach with the same sweep and convert it.

That’s not a bad idea, I might try that if it’s not compatible.

I’m definitely going to try it out first just to see if it’s anywhere close to working, but where should I hook it up to on the alternator? It’s got 4 posts coming out of it, one being ground. I’d post a picture, but it’s pushed in the garage now and I can’t get around it to open the hood…

Not being familiar with your specific alternator, it’s a pretty darn safe bet that you would have to take it apart to access the AC voltages (ahead of the diodes). I’m guessing there are six diodes in there, 3 negative and 3 positive. After the diodes would be the DC output at one of those posts. Another post probably is voltage regulator input and the third maybe is reference voltage to the regulator. Most of this is just uninformed guessing though. Except for the diodes’ location. You can almost take it to the bank that they are internal. That tach is designed for OMC outboards that have an external bridge rectifier (4 diodes), and access to the AC side is easy. BTW, the tach on my own boat looks exactly like yours but it has a pulse selector switch on the back. My motor is 5 pulses per rev.

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