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Installing Digital Tachometers

From the Factory

Our 2000 Bayliner 4087 came with Faria Analog gauges. I'm sure they were very inexpensive for Bayliner, and they look fine, but they are amongst the least accurate I've ever used.  It's not just one faulty unit -- all four and even the warrantee replacements are simply not very accurate.

During the first year I had a couple of the tachs replaced on warrantee.  In the year after warrantee, I sent a couple back to Faria for service.  Their customer services is excellent, they repair the units fast, and the charge was quite reasonable.

In the next year, I started to calibrate them myself using a optical tachometer. The problem is that it's difficult to get them closer than +/-50 RPM across the entire range and within weeks they are back to only +/-100RPM and quickly worsen from there.  After chasing this for years, I grew tired and decided to replace them.

The Replacement: Aetna 8402-B-SH

After some research I ended up talking to Aetna Engineering (http://www.aetnaengineering.com/) of Grand Rapids Michigan. Want I wanted to do was to replace the two lower helm tachs, but not the upper.  Aetna was unconcerned and said these would wire up without change or difficulty and would operate accurately in circuit with the upper helm Faria's should I chose to keep them.  I'm totally tired of constantly calibrating so I asked how frequently they would need calibration.  They replied never and they would always be accurate to +/-1 RPM.  I'm a bit skeptical but like the sound of it.  I decided to go with the LED display pictured above although they also have LCD displays available.  I chose the 8402-B-SH.  The 8402 is the LED display unit, The B signified black Bezel, and the SH models are designed for a high number of pulses per revolution. The -SM units can be set to sense between 28 and 90 pulses per revolution whereas the -SH units can be set for 91 through 255 pulses per revolution.  I have Cummins 6BTA5.9M engines with a magnetic pickup on the bell housing that counts fly wheel teeth. Since Cummins B-Series engines have 159 teeth around the fly wheel, I need the -SH unit.  To set them up for 159 pulses per revolution, remove the black circular plastic vent plug and you can see an 8 DIP switch mounted below it.  Set it to ON, OFF, OFF, ON, ON, ON, ON, ON and that's it.

Unfortunately, the existing Faria's are rear mounted units requiring a non-standard 4" dash panel opening.  Aetna's come in the industry standard 3 1/8" diameter and don't have a unit design for a 4" opening although they do sell a bezel that can allow them to be used with openings somewhat larger than 3 1/8".  However, there is no help available to adapt them to a 4" diameter hole.  I didn't find any better choices so I decided to go with them anyway.

Aetna's local distributor is Fisheries supply (http://www.fisheriessupply.com/online/). Fisheries got the two units in quickly and they are now installed at just over $200 each.  Prior to installation I checked them for accuracy using an optical tach and they are indeed accurate to +/-1 RPM up and down the range I checked from 650 through 2000.  Simply amazing.  I love them already but installing them is more than just dropping them in.  Wiring was simple: all they need is power, ground, and signal.  You'll likely have a tach light power wire as well but it's not required with the Aetna's.

The final mounting I ended up selecting is to get a small aluminum plate, drill four holes into to allow them to pass over the old tach mounting studs (see picture below). I temporarily mounted the aluminum plate and then drilled out the two 3 1/4" holes to allow the mounting of the 3 1/8" diameter tachs. 

Final Installation

Once I had the aluminum plate cut as needed, the next step was to finish the aluminum in black.  Rather than paint it, I decided to use a padded black material that is both matte black and the slight padding helps fill the gap formed by back mounting the plate. It's actually shelf liner but don't tell anyone.  The end result is presentable, the installation was fairly easy and the tachs are absolutely amazing.  I just love them.  By the way, with many boats these units are straight drop ins.  If you are so lucky as to have standard sized gauges from the factory, making the change can be done in well under 30 min and, either way, you'll be glad you did. They are amazingly accurate and I just love them.  I should have done this years ago.

Last update: 2006-11-21 (James Hamilton)