Dead tach

[Rocinante33]

My tachometer stopped working the other day. So far, my troubleshooting hasn't revealed the cause, but I could troubleshoot much more efficiently if it was an old gas burner I was dealing with....its a Universal M25. The tach receives it's signal from the alternator.

I recently upgraded my alternator to a new 90 amp, and the tach did work for about 45 minutes of engine time. Now, at idle, it shows ~500 rpm & drops to ~ 200 as I rev the engine up. So, possible causes are quite numerous; 1. I could have caused a problem with my installation, 2. old, tired wiring or connections could have failed, 3. Tach itself could have failed.

When I investigate, I get ~6-7 volts DC or AC measured at the alt. output with the engine running - it varies slightly when reving the engine. The same at the tach, so I think that wire is OK. The alt is producing 13 - 14.5 volts from the DC output terminal, so it appears to be working OK.

Can anybody advise me on troubleshooting this problem?

Thanks in advance,
Keith

 

[JohnK]

I upgraded the alternator on my M40 recently and had similar problems. Is your tach a Teleflex? If so, I found the pdf from the Teleflex site useful since it shows how to set and calibrate the tach for the new alternator:

http://www.downloadingfiles.co.uk/teleflexfiles/library/instructionmanuals/Tachometer%20Diesel%20Alternator.pdf

You'll prolly also need an optical tachometer to determine true engine speed. One like this off of eBay works great:

http://cgi.ebay.com/Laser-optical-digital-tachometer-test-motor-9-Volt_W0QQitemZ290138178626QQihZ019QQcategoryZ1504QQrdZ1QQcmdZViewItem

Good luck!

 

[Rocinante33]

Tachy

Thanks, John
The diagram looks helpful & you've given me some ideas. I don't know what brand the tach is, but it is a standard Universal control panel. I will look at the back of the tach to see if there is a brand name on it. The diagram says, "3.5 VAC minimum at idle, increases with RPM." I think that is good for me. Maybe the tach has failed, or else there is a poor ground connection. I will investigate.

The optical tach on E Bay looks good & would be useful, but I am leery of buying from Hong Kong.
Regards,
Keith

 

[Chris Miller]

Couple of things...

If your tach is bad, you can get a new one on ebay for about $40 or so... or Boaters world had them for like $80. My experiences buying from Hong Kong or China have been positive except that it takes FOREVER to get stuff here via mail. Lots of other people selling laser tachs on ebay. Original Tach was most likely a teleflex red. Just make sure you get the diesel one (lower rpms) if you need to replace it. I did this replacement late last year with very positive results and no alternator change, so I didn't have to adjust for different alternator.
Chris

 

[Geoff Johnson]

High output alternators will throw off the tach (although I found that my readings, while off, are off by a predictable amount, so I can still use it). If you want a cheap accurate tach, get a TinyTach which measures the pulses in the high pressure fuel line. Easy to install and works as advertised:

http://www.tinytach.com/tinytach/diesel.php