MD7 Tachometer blues

[vmelendezlugo]

Hi. The engine of our 1982 CD30 C is a Volvo MD7 that still runs quite well. Problem is that a very weird accident totaled the tachometer. It was a VDO original unit. I have tried to get another one to no avail. Does anyone know where to find a suitable replacement _ Thanks a lot.

VML

[Neil Gordon]

I sold my MD7A, including the control panel, two years ago, to Joe Demers at Sound Marine in CT. No idea what parts he might still have, but you can email him at jedsail@mindspring.com.

[mgphl52]

Problem is that a very weird accident totaled the tachometer.
VML

Oh my, there must be an interesting story here...

[sgbernd]

I had the MD7 up until last year, and while searching for spares, I occasionally found instrument panels and entire scrap engines on ebay or CraigsList. These engines are being replaced now and the tach doesn't; wear out, so I would watch for somebody parting one out or scrapping one, and see if he has the instrument panel. I sold mine on Craigslist and at that time, a couple of other MD7s were for sale. I believe it is the same instrument used for the MD2B, MD6, etc., so you may have a bigger field in which to hunt. Finally, keep an eye on the equipment list for sale in this web site as I've seen scrapped MD7s there as well.

Good luck.

Steve Bernd
Former MD7B owner

[Steve Laume]

How are these tachometers not like any other one that you could buy? I replaced the Tach on Raven which has a Universal engine. I simply bought a new one and hooked it up to the Alt. I did have to make one adjustment to calibrate it. I forget the details but it was changing the setting on the back of the Tach with a little switch that was all explained in the installation instructions. You might be running some sort of mechanical Tach now but there is any reason you couldn't switch over to an electrical one?

Replacing a gauge doesn't seem like it should be that hard, Steve.

[mgphl52]

From Google: www.egauges.com

[Steve Laume]

From Google: www.egauges.com

I think that one is controlled by the coil. Note that it is for 4,6 or 8 cylinder engines. I believe this is the one that I installed on Raven. It hooks up to the alternator so it doesn't care how many cylinders the engine has and will work with a diesel engine,

http://www.defender.com/product3.jsp?pa ... id=1383892

[Jerry Hammernik]

My tach stopped working in 2000. I replaced it with a VDO 333-163. It is still working although the hour meter cut out near the end of the season. That unit is still available on the internet.

[Keith]

When I bought MOON DANCE 5 years ago the Tach needle would stick at 1200 and 1900 RPMs. Rapidly tapping on the glass face would free it up and it would work in the 2000-2800 RPM range just fine. I remedies the situation by removing the face and manually exercising the needle/shaft many many times until the catch points were removed or burnishes down. It has worked flawlessly ever since. I learned this technique for working on old Porsche VDO gauges. It is a very slow and laborious process to get the face plate off and back on. It involves very gently prying the bezel material open with a small screw driver. Keep working your way around the bezel until it comes free of the main body of the tach. Reverse the process to put the bezel back on.

Keith

[Maine_Buzzard]

Yes you can buy a generic diesel tach to replace it with. An automotive one with 4-6-8 cylinder selection will not work on a diesel.

Look for one with an hour meter, and this one on ebay would be about perfect.

http://www.ebay.com/itm/DATCON-0-4000-R ... M~&vxp=mtr

On the back, there is a small rotary switch for selecting the number of poles on the alternator and a small hole for an adjusting screwdriver. Instructions are usually included or on the mfgr's web site.

Optical tachometers are cheap and the adjustment process is easy. I have this $10 one.

http://www.ebay.com/itm/New-Mini-LCD-Di ... SwXeJXewkX

https://www.youtube.com/watch?v=hA0ANrX8nIo

I broke the wire on my alternator and had to open it up to fix it. All it does is to grab one of the three stator windings, which will then deliver an AC sine wave to the tach (at ~40V and low current). It converts the frequency of the sine to the reported engine speed. The rotary switch and trimmer let one adjust for combinations of pulley ratio and alternator construction. Comparing to the old tach or a suggestion from a current (and spry) owner will get you started on a proper range.

Good winter project! You can leave the tachometer +12V and ground connected to a deep cycle battery to make the hours match the old one. If you go over, Ferris Buhler says turn it upside down to reverse it.

[Wayne Grenier]

Main_Buzzard is correct. You need a (any) diesel tach. Diesels do not have coils. The tach gets the signal from the alternator. You do need to figure out how many poles your alternator has and set the switch on the back of the tach.

or you can sail for years and years without one. Other than not having an hour gauge there is little need for a tach on a sailboat.

Someone once said you need a tachometer to go sailing like you need a French horn to go hunting. I think it was me.

[Steve Laume]

Main_Buzzard is correct. You need a (any) diesel tach. Diesels do not have coils. The tach gets the signal from the alternator. You do need to figure out how many poles your alternator has and set the switch on the back of the tach.

or you can sail for years and years without one. Other than not having an hour gauge there is little need for a tach on a sailboat.

Someone once said you need a tachometer to go sailing like you need a French horn to go hunting. I think it was me.

When I hooked up the new Tach I bought, it was pretty apparent that it was set wrong. You will have a rough idea of how fast your engine is turning and if it is set wrong it will be way off. There are only a couple of setting options so the process of elimination goes pretty quickly.

I would argue with not needing a Tach on your boat. I run the engine at aprox 80% of Max RPMs for a decent compromise between getting somewhere and best fuel consumption. If I am not too worried about getting anywhere but need to charge the battery, I will run at about 1500 RPM. Knowing how fast your boat should be going at 2250 RPM in calm seas with no wind will give some idea of what the current is doing when you check the speed over the ground. I am sure there are other reasons but it is good to have a working Tach, Steve.

[mgphl52]

And hour meters can be added stand-alone... KAYLA had no meter in her tach when I bought (the 1st time:) so I added one.
After awhile, you really shouldn't need a tach, just your ear...

[Maine_Buzzard]

The largest reason I wanted the tach accurate was for repropping. Getting the Beta wide open throttle (WOT) engine speed to 3550 is desirable, so knowing it was accurate is a good thing. Too flat (as I started with) lets the governor limit to 3600 and lacks thrust, too steep and the engine will bog. A Goldilocks size/pitch will keep it just under the max RPM.

A shout-out to Accutech Marine Propeller (603- 617-3626) in NH is appropriate. They repitched and tuned my tiny 3 blade for about $225, best I can recall. The prop was way out of tune and they did a great job. Maine locals can drop it off at Hamilton Marine stores and they will truck it back and forth.

WOT engine revs at 3530, so I'm happy. Now 13x9 on at 14HP Beta. It does run noticeably smoother too.


Early in the summer, I'd take my bowrider with a 135 two stroke to a flat empty lake and blow the carbon out... If I hit 48 on the GPS, I felt like it was still running well. I do the same with the tach, but 6 kts does not have the equivalent thrill.