Digital Tachs

Steve S

Steve S

Well-Known Member
Jun 5, 2007
3,129
Northern IL.
Boat Info
2000 400 Sedan Bridge with twin CAT 3116's

2000 340 Sundancer - SOLD!
210 Monaco 1987 - SOLD!
Engines
Twin Caterpillar 3116's 350 HP straight drives
I know someone posted about a digital tach that would a drop in replacement for the tachs on my 400 DB but I cannot find it. Who was that manufacturer?
 
SKybolt said:
Yep, great tach's. The only issue is they are slightly smaller then the standard tach cutout. You might want to get the black bezel inserts with them as well, then they fit perfectly.

https://www.aetnaengineering.com/tachometer-accessories/ Part#-13705
Click to expand...

Love the red leds, the lcds that most diesel guy's have are reported to be awesome accurate which is of the upmost importance, but to me, they just scream old hat.
 
Red LEDs tend to wash out and be unreadable in sunlight. In an enclosed cabin they would be fine, but the guys at Aetna cautioned me against LED's when I was buying mine digital techs.
 
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fwebster said:
Red LEDs tend to wash out and be unreadable in sunlight. In an enclosed cabin they wold be fine but the guys at Aetna cautioned me against LED's when I was buying mine digital techs.
Click to expand...

I actually have these, the LCD ones and are totally readable in sunlight. But to your point Frank, yeah then red does not do well in sun for sure.
 
fwebster said:
I have the same tachs....LCD with red back lighting.

This is the tach I was cautioning because of wash out in the sun:

https://www.aetnaengineering.com/model-8402-led-tachometer/
Click to expand...

Yes, exactly. I had those tach's in a previous boat and they were horrible in sun light. I think they were of the original offerings from Aetna engineering back in the day. The ones we have are great tach's and very accurate as you know, even by today's standards.
 
Steve S said:
I know someone posted about a digital tach that would a drop in replacement for the tachs on my 400 DB but I cannot find it. Who was that manufacturer?
Click to expand...

Hi Steve, Mine are the 8905R-P-H-DS
R=round
P=polished (stainless bezel)
H=hour meter
DS=diesel
Install was easy and I am very pleased with them although the port side hour meter doesn't light up as much as the starboard. But I've been told by other members here that their hour meters don't light at all???
This is what they look like in our boats.
305BC6BD-92B5-4A39-8738-8E15BBFFC96D.jpeg

As mentioned, they are about 3/16” shy of being a perfect fit. 4 or 5 wraps of electrical tape around the gauge body will center it while you tighten down the U bracket.
The hour meter back lighting is not as drastically different as depicted in the picture, the crappy cell phone pic just makes it look bad.
 
Got mine today.
Looking forward to putting them in.

My old tachs read around 600 rpm high and my port one started to act up last season.
 
As I understand it, the Aetna's are a direct swap. How would I set the PPR (Dip) switches with the factory sender? According to the chart : SW2 = 0 0 0 0 (All off) and SW1 = 0 1 1 1 1 1 1 0.

What is confusing me is the 8*/126 listing by the 3116 on the selection guide.
 
You are setting the number of teeth on the ring gear. CAT 3126 and 3116 I believe are the same, 126. So your setting above are correct.

Set the DIP switches and start the engines, you should be dead on. You can confirm with a photo tach.

SW2 is used for some addition setting listed further back in the manual, and not necessary. However, as listed above you should have 1 rpm resolution.
 
Last edited:
dtfeld said:
You are setting the number of teeth on the ring gear. CAT 3126 and 3116 I believe are the same, 126. So your setting above are correct.

Set the DIP switches and start the engines, you should be dead on. You can confirm with a photo tach.

SW2 is used for some addition setting listed further back in the manual, and not necessary. However, as listed above you should have 1 rpm resolution.
Click to expand...

Dave, that matches the instructions. What Caught my eye was turn preceding “8*”.
 
8* yeah, a little confusing. I see what your talking about.

I guess for some applications you would use 8 pulses per revolution (ppr). Every SR I know with CAT engines, the setting is 126 ppr. So your settings above are correct. Manual could be a little clearer.

Note from the manual…

*Unless using sender other than 8901/8902 or a reduction gear.
 
dtfeld said:
8* yeah, a little confusing. I see what your talking about.

I guess for some applications you would use 8 pulses per revolution (ppr). ...
Click to expand...

Older two cycle engines like Detroit's have the take off from the cam which is 8 pulses/rev. They use the sender like Aetna Eng. sells.
 
SKybolt and Dave,

Thanks for the response! I’m looking forward to my new tachs.

Steve
 
my3sons said:
Hi Steve, Mine are the 8905R-P-H-DS
R=round
P=polished (stainless bezel)
H=hour meter
DS=diesel
Install was easy and I am very pleased with them although the port side hour meter doesn't light up as much as the starboard. But I've been told by other members here that their hour meters don't light at all???
This is what they look like in our boats.
View attachment 102503
As mentioned, they are about 3/16” shy of being a perfect fit. 4 or 5 wraps of electrical tape around the gauge body will center it while you tighten down the U bracket.
The hour meter back lighting is not as drastically different as depicted in the picture, the crappy cell phone pic just makes it look bad.
Click to expand...
my3sons,
Were you able to set your hour meters when you installed the new units from Aetna? Thank you
 
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