Perkins mechanical tachometer

[SeaDogAK]

I have twin Perkins T6.3544 engines, approximately 1991 vintage. Currently my tachometers run from the Balmar voltage regulator. I’m thinking of switching to mechanical tachometers using the tach drive on the front of the engines.

The current tachs work intermittently. As far as I can tell, they quit working when the alternators aren’t charging. When they do work, they always read 500 rpms or so high at cruising speed. I’ve gotten to where I go down to the engine room with a laser tach when I need to know rpm’s, because the tachs are pretty well useless.

I’ve checked all the contacts, and I’ve had a tech look at the situation, but I’m not sure what else to do to try to make the existing tachs work. There isn’t really anyone in Juneau that I can have look at the problem. I’m thinking the solution is to just switch to mechanical tachs.

I’ve seen a lot of threads with people complaining about alternator tachs not working, but not a lot of solutions. I suppose there’s a reason why people switched from mechanical to alternator tachs years ago, but I’m not sure if there are any disadvantages to going back to a mechanical tach.

There is what I think is a tach port on the front of the engine (picture below). I’m thinking I can put a mechanical sender like an Aetna Engineering 8901on there. I’m not quite sure, though, about the gear ratios; I don’t know if the tach port turns at the same rpm’s as the engine, or if it’s geared differently, and how to deal with that. The Aetna says it gives 8 pulses per rotation, but I don’t know if that corresponds to one rotation of the engine.

Has anyone done this? Does anyone have a recommendation for a tach to use?

 

[KnotYet]

You can use a non-contact sending unit like this:
https://www.amazon.com/GlowShift-Di...mzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc

Intended for an electronic tach gauge, it may be compatible with your current tachs,
and so much cheaper than the ones you are looking at, too. It's worth giving it a try.

 

[jgwinks]

I just replaced two of the four Stewart Warner tachs for my 1973 Perkins 6.354MGT's. On that engine they're driven by Hall effect senders attached to that same tach drive. The new SW tachs have DIP switches on the back and the instructions have a long table of possible settings based on the number of pulses per rotation of the sender. I didn't know that number so I just hooked one up to see what it would do. It read half speed so I reset for half the pulses. That fixed it. Any new tach you buy will probably be similar. I believe the tach drive is rotating at the same speed as the crankshaft. Hint on setting the switches: if the tach reads slow, it thinks you have a higher pulse count than you actually do. Reset for a lower one and repeat till it's right.

 

[STB]

I replaced those in my last boat. Scott at S&W Diesel in Wilmington (310) 835-3155 can sell the right generators and has spares (used) for all of the rest of the parts.

When he sold me the generators he was teasing me saying he should give me all of his spare parts (the used ones) as they were taking them out not putting them in -- everyone wanted to come off the alternator.

Personally, I liked the mechanical take-offs and almost converted my new boat to use them.

 

[jgwinks]

I think the alternator feed is just cheaper for the builder. No sender needed and most tachs have the required output. But they're dependant on the alt pulley ratios. Change a pulley and the alt is way off, maybe beyond the adjustment range. And they don't work if the batteries are fully charged. That's a common problem on boats with a lot of solar, the panels can cause the alts to shut down. If you turn on a big electric draw, they start working again.

 

[KnotYet]

X2 regarding S&W Diesel. A very good Perkins expert shop and friendly to boot.

 

[CaptTom]

I converted to the Aetna sending units almost 10 years ago. Very satisfied customer. I connected the Aetna to my NoLand RS11 NMEA 2000 engine data converter. The tach display on my chartplotter is primary. I left the old alternator-based tachs as backup. No reason you can't use your existing tach with the Aetna though.

 

[SeaDogAK]

I converted to the Aetna sending units almost 10 years ago. Very satisfied customer. I connected the Aetna to my NoLand RS11 NMEA 2000 engine data converter. The tach display on my chartplotter is primary. I left the old alternator-based tachs as backup. No reason you can't use your existing tach with the Aetna though.

Thank you for this, I think Im just going to order two of these. I want to see if they’ll work with my existing tachs, that would be good.

 

[CaptTom]

Just one tip: There are all kinds of new gauges these days, with illuminated dials and letters instead of the old-school single backlight bulb. Might be a nice time to upgrade the tach gauges, too. They'd come with instructions on calibration, too. No searching around to see how to match up the old tachs with the new senders

 

[Barking Sands]

Here is a cheap way to do it. And its been rock solid. If you use a fully adjustable tach like the Aetna you wont need the Dakota tach adapter in the video. The Dakota adapter was just needed to adapt a 2 magnet signal and hall effect sensor output to the W signal since its such a wide gap in signal count.

and this one with the Aetna tachs. I LOVE my Aetna tachometer!

 

[jgwinks]

Technically a "mechanical" tach is driven by a spinning cable attached to the tach drive. Very old school. If you need to replace your tachs and specify mechanical, you'll be disappointed. When I was shopping for mine I ran into three types: the mechanical cable drive, magnetic driven by a sensor on the flywheel, and electronic which could be Hall effect or alternator driven. The two electronic versions do not appear to be interchangeable.

 

[CaptTom]

For the record, I had the Perkins without the sending unit on the engine. I bought the Aetna model 8875, found here, which was the whole bolt-on plate, with the sensor installed in it. I'm not sure of the part number for just the sensor part in the center.