Old Digital Tachs are Way Off

The friendliest place on the web for anyone who enjoys boating.
If you have answers, please help by responding to the unanswered posts.
See posts with no answers
CaptTom

CaptTom

Guru
Joined
Dec 2, 2012
Messages
2,728
Location
USA
Vessel Make
Prairie 36 Coastal Cruiser
The upper helm has two tachs that are both obviously way off. The lower helm (original analog tachs) appear correct.

They upper tachs say "Ensign Digital Tachometer / Aqua Meter" by Rule Industries, which apparently no longer makes them, or even has their own web site since being taken over by ITT some years ago.

Replacing them is on my "to do" list, along with a total re-make and re-wire of the upper helm station.

Meanwhile, I'd like to see if I could fiddle with these enough to get a rough approximation of my real RPM.

I'm running twin Perkins 3.544's with the tach signal coming from the alternators.

I don't see any set screw on the back of the tachs, anybody have a manual or diagram for this, or at least a clue as to what to look for?

Thanks!
 
I have the a problem with an older style tacho, one of 2 on the FB reads 400rpm below its equivalent below. Could you tech guys cover those too when helping CaptTom?
 
CaptTom I'ld replace with Aetna digital tach, I had a set on a previous boat they worked great & guaranteed correct + or - 1 rpm. Check all connections & you may get the faulty tach working correctly, some have adjustment switches on the back so they can be calibrated. To check the accuracy of tachs you can use a photo tach to check them and then adjust them if they can be calibrated. Harbor freight has cheap photo tachs or check eBay.
 
river cruiser said:
to check the accuracy of tachs you can use a photo tach to check them and then adjust them if they can be calibrated. Harbor freight has cheap photo tachs or check ebay.
Click to expand...


^^^^this^^^^
 
Thanks guys, I ordered a photo tach (from Amazon) and will probably end up taking the old ones out to examine, maybe there's a set screw somewhere.

I was hoping someone knew of or had these, and maybe had a manual or at least a recollection as to where the adjustment might be.

Since they run off the alternator signal, I assume there must be a way to adjust them or they'd be useless. Not that it would be the first useless thing I've found in this boat.

They look just like this dual tach, but in my case it's two single tachs:
142501d1291035741-ensign-aqua-meter-digital-tach-sync-price-drop-49-99-sold-img_4066.jpg
 
Here's the back of the tach in question:
Tach_Back.jpg


Anyone know what "Threshold Adjustment" means? I turn one of them too far and the RPMs go down as the throttle goes up. I assume that's the pulse width or minimum voltage, and maybe as I rev up those values get smaller. The other seems inconsistent; the reading goes from 1040 to 1500 just with a little tweak, but then a large change in the other direction does nothing.

I don't have any manual to show what the DIP switches do.

Still looking for a manual, or any experience with similar tachs.

Thank you!!
 
The dip switches are set to certain positions depending on the number of cylinders. They should be set correctly if the person who did the original install followed the instructions & no one has messed with them since.
 
Very good trick Craig, thanks! But as Ron pointed out, *IF* the installer was competent, the switches should be correct. Big "if" on this boat, which has had at least one clueless PO.

I played with the two threshold adjustment screws. I think under some conditions, the signal gets noisy, which the tachs interpret as multiple sikes. This would double or triple the reading. That is consistent with what I saw. The threshold adjustment apparently helps suppress this.

Tweaking got me to a point where they're within 20 RPM of what the hand-held tach says. They're set to a 20 RPM resolution, so I think I'm on the right track.

All of this is only at the dock. It remains to be seen how noisy the signal gets underway, under load, and with the other electronics operating.
 
CaptTom said:
Very good trick Craig, thanks! But as Ron pointed out, *IF* the installer was competent, the switches should be correct. Big "if" on this boat, which has had at least one clueless PO.

I played with the two threshold adjustment screws. I think under some conditions, the signal gets noisy, which the tachs interpret as multiple sikes. This would double or triple the reading. That is consistent with what I saw. The threshold adjustment apparently helps suppress this.

Tweaking got me to a point where they're within 20 RPM of what the hand-held tach says. They're set to a 20 RPM resolution, so I think I'm on the right track.

All of this is only at the dock. It remains to be seen how noisy the signal gets underway, under load, and with the other electronics operating.
Click to expand...

Sounds like you are on the right track. Not sure if this is your exact model, but I found these references over my morning coffee.

Here is the troubleshooting section from the Aquaguide 735 manual that describes the trigger threshold adjustments. The switch settings table is scrambled though

Here is a thread that has the switch settings table intact, toward the end/bottom of the thread.

Have fun!

LarryM
m/v Boomarang
 
Thanks Larry!

I actually found those same links since my last post, sorry I didn't get back here and post the links myself. Once I pulled the tachs out of the dash and could google the exact words and abbreviations on the back, those two popped up. Unfortunately they reference a different model. Mostly they have the same settings, but my tachs have TWO adjustment screws, so I'm still sort of winging it trying to figure out what each one does. Maybe the one is threshold and the other is the normal fine adjustment.
 
You must log in or register to reply here.

Similar threads

Barking Sands
The long route to a steady diesel tach
Replies
13
Views
3K
Barking Sands
Barking Sands
angus99
Wiring new tachs
Replies
1
Views
795
Comodave
Comodave
mcarthur
Bilge pumps with digital switching
Replies
15
Views
1K
Iggy
I
P
Digital vs Analog Instrumentation at the upper helm
Replies
1
Views
389
CaptTom
CaptTom
Barking Sands
I need some help and advice on Radar mast placement and radar operation
2
Replies
34
Views
2K
Jeff F
Jeff F

Latest posts

Back
Top Bottom