Tach pegged with engine off

[cobra63]

I've got a problem with a 85 hp Evinrude pegging the tach every time I turn the key on. When I actually start the engine, the tach stays pegged at 7000 rpm.

It is a VDO 3 wire tachometer, and I've doublechecked that the gray wire from the engine is hooked up the right terminal on the back of the tach, as well as Ignition switched power and ground.

I've checked and cleaned all my grounds, and they seem good. The battery has not been holding a charge very well lately, and I'm thinking I may have a rectifier gone bad.

Could a bad rectifier cause the tach to peg the way mine is doing, with the engine not running and the key turned to "On?"

Gotta get this fixed!

 

[jonesg]

Re: Tach pegged with engine off

Forget the tach for now,
check voltage engine off then start it and compare, is it charging?

if not swap the rectifier out,

the tach will probably fix itself when you do this.

 

[cobra63]

Re: Tach pegged with engine off

I got a new rectifier. However, it has two yellow wires, and a red, just like the old rectifier, but neither of the yellow wires on the new one is marked with a gray stripe, like the old one. How do I know which yellow wire to hook up to the terminal that has the yellow wire with gray stripe? Thanks.

 

[ezeke]

Re: Tach pegged with engine off

Either yellow wire on the rectifier will work on the gray striped one at the terminal block.

Check the ground and positive wiring to the tachometer.

 

[cobra63]

Re: Tach pegged with engine off

OK, I completely disconnected power and ground wires from the tach, and then wired it to a different battery using two known good test leads. Kept the tach signal wire hooked up. The tach still pegs to 7000 rpm as soon as the power is connected.

I am beginning to suspect a bad tach. I have completely removed it from the boat now. Is there a good way to test the tach on the bench using my multimeter? Thanks.

 

[jonesg]

Re: Tach pegged with engine off

I suppose you could try running at different rpms and seeing if the meter varies in voltage, the service book just says replace with known good tach.

I think they operate on a chip which converts a pulse to variable current, you'd have to chop it open to get at the chip, probably not worth the bother.

 

[cobra63]

Re: Tach pegged with engine off

Update: Replaced rectifier, and checked voltage at the battery terminals with engine off: 12.44v steady. Then started the engine, and noted reading of
12.67v, which increased slightly as I advanced the throttle. Levelled off around 12.77v.

I then checked the tach signal lead at the console, and it increased as I advanced the throttle. I am pretty confident now that at some point in time, I accidentally reversed polarity when I installed the battery, and fried the rectifier and tach.

I also noticed last time out (when I got stranded), that as the battery got weaker and weaker as I attemmpted to start the engine in vain, the tachometer needle registered almost like a voltage indicator, starting at about 4000 rpm, and dropping lower and lower as the battery drained. Does this indicate a short across the tach.? Whatever the case, I have replaced the rectifier, and will soon have a new tach, but I'd just like someone else more knowledgeble to help me confirm what might have been the problem.

 

[jonesg]

Re: Tach pegged with engine off

reverse polarity definately will fry the rectifier instantly, I know that much.

 

[ezeke]

Re: Tach pegged with engine off

The lack of a working rectifier would cause the problems, but would not necessarily damage the tachometer.

Rectifiers will be damaged by any voltage flowing the wrong way to one of the diodes. That can be caused among other factors, by loose cables, by charging batteries while connected to the motor, by a bad ignition switch and by reversing polarity. Disconnecting batteries on a running motor will also kill the rectifier.

 

[cobra63]

Re: Tach pegged with engine off

Yep. The tach is still a mystery. It pegs as soon as current is applied, and won't budge off the 7000 rpm limit until power is shut off. VDO tach, probably 7 or 8 years old. There is a diagram on the back showing 2-4-6-8 (which I understand is for the number of cylinders), but there are no dipswitches or screws to set for the proper # of cylinders. It appears to have a sealed face, so unless I'm missing something, there is no way to take it apart without destroying it.

 

[ezeke]

Re: Tach pegged with engine off

Some VDO tachs are electronically programmable and some are set by switches; typical instructions: http://www.egauges.com/pdf/vdo/0-511-012-253.pdf

 

[henrye718]

Re: Tach pegged with engine off

Yep. The tach is still a mystery. It pegs as soon as current is applied, and won't budge off the 7000 rpm limit until power is shut off. VDO tach, probably 7 or 8 years old. There is a diagram on the back showing 2-4-6-8 (which I understand is for the number of cylinders), but there are no dipswitches or screws to set for the proper # of cylinders. It appears to have a sealed face, so unless I'm missing something, there is no way to take it apart without destroying it.

If it pegs with out the gray wire connected then the tach is just fried assuming there is no funny bussiness going on with the power wires. How about just connecting to a car battery all by itself off the boat. If it pegs then just get a new tach and be done with it.

 

[emdsapmgr]

Re: Tach pegged with engine off

You may want to double check the tach with the mfr. The markings on the back of a typical outboard marine tach correspond to the number of pulses coming from the stator, not the number of cylinders.