bouncing tach needle @ idle

[Matthew A.]

I was wondering what might cause the needle on my tach to jump around at idle. The tach does not read accurately at idle. It shows 1000 to 1100 rpm at idle which I know is not correct. Plus the needle will bounce plus or minus 100 rpm or so while reading an idle speed of 1000 or so rpms. The tach has a selectable 20 pole setting. The tach, going by feel of the motor, seems to be accurate once the motor is above idle speed. However, WOT rpm reads 4600 yet motor reaches an rpm that feels and sounds closer to 5000 or higher. At WOT motor reaches the last 300 rpm or so one can defineately feel the powerband drop off as the additional rpms do little to nothing as far as additional speed goes. The tach is made by Medallion. As far as wiring, I run the tachs signal wire directly from one of the AC terminals of the rectifier/regulator. I purchased the tach as old but brand new still in original box but without installation instructions on ebay. According to what info I've been able to find on the net concerning these tachs, there may be 3 pots that can be adjustedto fine tune the tach. According to the info I found, only one of the 3 pots should be adjusted. As far as how one determines which pot to adjust...is where my search for information came to a dead end. Might one of you many wise and knowlegdable people have any information that may be of help?

 

[mthieme]

Re: bouncing tach needle @ idle

I assume you have no problems with the motor (?) and it runs good - no rough idle, etc.
You need to get a shop tach to verify.
If it does the same thing, I would suspect the voltage regulator.
If it doesn't, it's most likely and installation issue.

 

[kinggabbo]

Re: bouncing tach needle @ idle

I have the same problem with my tach. Bouncing around at idle, sometimes not registering RPMs at all, etc... The motor is running great. I plan on getting a hand held tach and checking that way.

 

[john from md]

Re: bouncing tach needle @ idle

I suggest that you try cleaning the contacts associated with the tach. That means all of the contacts including the buss bar under the dash if you have one. I would disconnect them and use bronze wool to clean them up. After re-attaching them, I would spray the joint with Boeshield T9 or LPS corrosion protector.

This has worked for me on a couple of boats. It has not worked for me on my last boat. It reads good at low and high rpm but not the mid range. I have checked mine with an induction type tach and find that the lower and high ranges are correct. Go figure.

John