This Tach Problem Is Killing Me

[Salty Tricks]

I 've posted before and thought it was solved but...

1991 200 hp Rude. I replaced the Rectifier with an Ebay purchased used one. Tach is a VDO. Dip switches are set for 12 pole. I get 12+ DC volts on the purple wire at the gauges. I have continuity on the black wire to ground. While engine running I do not have a negligible AC voltage reading from the grey wire (mV maybe ...certainly not 5 to 7 volts as I believe I should have) The gauge did register a rpm reading last week although it was erratic, Now I have nothing...pegged at zero. I tried another tach...nothing. I am certain the motor is charging the batteries as I have a Xantrex Link 20 battery gauge that accurately tracks voltage and amperage used and being put back in. At WOT I am registering about 25 amps plus going back into the battery.
What am I doing wrong?

 

[R.Johnson]

Re: This Tach Problem Is Killing Me

What's a used rectifier going for these days' ?

 

[Barnacle_Bill]

Re: This Tach Problem Is Killing Me

I recently replaced my tach thinking it was bad. It turned out to be an intermittant problem with the rectifier.

 

[Salty Tricks]

Re: This Tach Problem Is Killing Me

I think I paid about $50 for a bunch of electrical component for my year engine including the Power Pack bracket, starter solenoid , connectors and the water cooled rectifier.

I was thinking about running a separate wire directly from the terminal strip under the power pack to the S terminal on the gauge.

Any thoughts would be appreciated.


Thanks
Michael

 

[Seasport]

Re: This Tach Problem Is Killing Me

I don't think you're doing anything wrong. Can you measure a voltage on the gray wire right back near the rectifier? If so you might have a break in the cable somewhere. Can you measure a voltage on the gray wire with the tach disconnected? Maybe the tach is loading this output too much.

Otherwise, you should also be able to run the tach directly from the yellow or yellow/gray wire going from the stator to the rectifier. Try hooking the gray wire to either of these terminals.

 

[Salty Tricks]

Re: This Tach Problem Is Killing Me

I ran a wire from the grey terminal at the motor directly to the tach....nothing, I then tried a wire from the yellow/grey terminal and still nothing.

I am thinking the rectifier from Ebay is no good. I am confused as to why the charging seems to be fine.

 

[Joe Reeves]

Re: This Tach Problem Is Killing Me

(Testing Tachometer With Water Cooled Regulator/Rectifier)
(J. Reeves)

A quick check is to simply plug in a another new tachometer as a piece of test equipment. If the new tach works properly and the old tach didn't, obviously the old tach is faulty.... but usually boaters don't carry around a spare tach (see below).

A faulty rectifier wouldn't damage the tachometer, the tachometer simply wouldn't work. This is due to the fact that the tachometer operates off of the charging system and the rectifier converts AC voltage to DC voltage, enabling the charging system. A faulty rectifier disables the charging system, and the tachometer simply doesn't register.

However.... those watercooled regulator/rectifiers that are used on the 35amp charging systems (and some others) bring into play a different type problem, and as you've probably found out, they are really a pain to troubleshoot via the proper procedure. There's an easier way.

The tachometer sending/receiving setup operates off of the gray wire at the tachometer. That same gray wire exists at the engine wiring harness which is connected to the engine electrical terminal strip. You'll see that there is a gray wire leading from the regulator/rectifier to that terminal strip, and that there is another gray wire attached to it. That other gray wire is the wire leading to the tachometer which is the one you're looking for.

Remove that gray wire that leads to the tachometer. Now, find the two (2) yellow wires leading from the stator to that terminal strip. Hopefully one of them is either yellow/gray or is connected to a yellow/gray wire at the terminal strip. If so, connect the gray wire you removed previously to that yellow/gray terminal. Start the engine and check the tachometers operation, and if the tachometer operates as it should, then the regulator/rectifier is faulty and will require replacing. If the tachometer is still faulty, replace the tachometer.

If neither of the yellow wires from the stator is yellow/gray, and neither is attached to a yellow/gray wire, then attach that gray tachometer wire to either yellow stator wire, then the other yellow wire, checking the tachometer operation on both connections.

I've found this method to be a quick and efficent way of finding out which component is faulty.... the tachometer or the regulator/rectifier. It sounds drawn out but really only takes a very short time to run through. If the water cooled regulator/rectifier proves to be faulty, don't put off replacing it as they have been known to catch on fire with disasterous consequences.