Tachometer signal: doing better than the alternator

[socalrider]

I've done a bunch of reading on this & am about to try something, thought I'd pass it through the forum gauntlet first:

I've got twin Lehman 120's, one has a 120A Balmar, and the other has an internally regulated Motorola. Single 12V 700Ah FLA bank. Stock motorola tachometers.

My tachs are very inaccurate. I'm actually planning on replacing all my analog gauges with an N2K system, but that's another thread. In addition to being inaccurate, my tach signal on the Balmar unit drops out completely when my bank is fully charged - I suspect because the motorola float voltage set point is a bit above the Balmar set point.

Regardless, I'd like a more reliable source for my tach signal than the alternators. My FL120's don't have the mechanical drives installed, but Aetna Engineering makes drives and senders that will fit. Slick, $535 shipped seems excessive.

Has anyone tried just using a simple hall effect sensor like these for around $20? There's a good spot under the pulley to mount a bracket with 1/4-20 holes already tapped. Sensor would be wired to the N2K gateway, calibrated to 1 pulse per RPM. Seems like a good way to save $500. Am I missing anything?

 

[diver dave]

Dont forget the magnet[emoji106]

 

[STB]

In general, those units work.

That one doesn't seem to be adjustable, so I don't think you'll be able to have it scale the RPMs of the pulley to the RPMs of the engine, unless they are the same, e.g. the main pulley.

Mounting the sensor near the main pulley might be easy or might be hard, depending upon your set up. It needs to be close enough to the pulley to read it (the "gap" needs to be correct), but out of the way and secure from vibration and movement, also.

That magnet will need to be secured really, really well as it is going ofr a lot of spins. Keeping it in place will be a challenge.

Also, check the function of the unit before mounting anything. Hall-effect sensors are sensitive to the orientation of the magnet, e.g. N vs S pole. If the magnet is unlabelled, or if it is labelled backward and you don't find out until the glue has dried and things don't work, that might be sad. Sensors can be designed to work either way. But, the low price point units don't always, or even often, do that.

 

[catalinajack]

I've done a bunch of reading on this & am about to try something, thought I'd pass it through the forum gauntlet first:



I've got twin Lehman 120's, one has a 120A Balmar, and the other has an internally regulated Motorola. Single 12V 700Ah FLA bank. Stock motorola tachometers.



My tachs are very inaccurate. I'm actually planning on replacing all my analog gauges with an N2K system, but that's another thread. In addition to being inaccurate, my tach signal on the Balmar unit drops out completely when my bank is fully charged - I suspect because the motorola float voltage set point is a bit above the Balmar set point.



Regardless, I'd like a more reliable source for my tach signal than the alternators. My FL120's don't have the mechanical drives installed, but Aetna Engineering makes drives and senders that will fit. Slick, $535 shipped seems excessive.



Has anyone tried just using a simple hall effect sensor like these for around $20? There's a good spot under the pulley to mount a bracket with 1/4-20 holes already tapped. Sensor would be wired to the N2K gateway, calibrated to 1 pulse per RPM. Seems like a good way to save $500. Am I missing anything?

You do not need to use those very expensive Aetna signal generator. A Stewart Warner #82623B will generate 8 pulses per revolution. Available from many sources for around $130. VDO makes one for about $320. Or, an Autometer #5291 for about $110, 16 pulses. The product literature talks about speedometers but it is the number of pulses is what drives the tach.

 

[socalrider]

You do not need to use those very expensive Aetna signal generator. A Stewart Warner #82623B will generate 8 pulses per revolution. Available from many sources for around $130. VDO makes one for about $320. Or, an Autometer #5291 for about $110, 16 pulses. The product literature talks about speedometers but it is the number of pulses is what drives the tach.

Thanks again CatalinaJack!

I think (from the Aetna site) I need two pieces though: the sender itself (which is $120) and a drive adaptor ($130) which presumably fits into the plugged hole on the lehman and interfaces with the engine's internal mechanical tach drive interface.

Please let me know if I'm misinterpreting this - I imagine you either needed this piece for your engines, or they came with the piece installed.

 

[rgano]

Call me Captain Obvious, but have you asked Brian at American Diesel about this?

 

[catalinajack]

Thanks again CatalinaJack!



I think (from the Aetna site) I need two pieces though: the sender itself (which is $120) and a drive adaptor ($130) which presumably fits into the plugged hole on the lehman and interfaces with the engine's internal mechanical tach drive interface.



Please let me know if I'm misinterpreting this - I imagine you either needed this piece for your engines, or they came with the piece installed.

I looked at the Aetna website and see that they sell an adapter for the Lehman. I think you do need that adapter. See the picture. I purchased their #8902 signal generator. The adapter came with my engine because I used to have a drive cable there for a Glendinning engine synchronizer so all I needed was the signal generator. What is nice about the Aetna product for me is that the top of the generator, covered by the red cap you see in the picture, is actually an extension port to which I can attach the drive cable for the Glendinning. That bluee coolant hose is in the way, however. Another future project.

 

[sbman]

...
My tachs are very inaccurate. I'm actually planning on replacing all my analog gauges with an N2K system, but that's another thread. In addition to being inaccurate, my tach signal on the Balmar unit drops out completely when my bank is fully charged - I suspect because the motorola float voltage set point is a bit above the Balmar set point.

...

Has anyone tried just using a simple hall effect sensor like these for around $20? There's a good spot under the pulley to mount a bracket with 1/4-20 holes already tapped. Sensor would be wired to the N2K gateway, calibrated to 1 pulse per RPM. Seems like a good way to save $500. Am I missing anything?

The tach signal on the Balmars drops out eventually no matter what you do in my experience when connected to an analog conversion gateway. The Balmar manual states "When the tachometer is connected via the MC-614, the regulator will ensure that the tachometer will not discontinue supplying field current when the batteries are fully charged".

I have not found this to be the case, I think the unit is not detecting the connection to the gateway (a standard tach probably has a lower resistance than the gateway) and the field is turned off at the end of the program. I have found the signal to be very accurate when it is present. This is the same issue on both FL-120's.

I see a couple of issues with the inexpensive hall tach.

First, there is no output to connect to a converter to get it into the N2K system, are you thinking of just hooking up to the signal line of the device?

Second, I'll bet the sensor housing is made of poorly plated pot metal and going to corrode in the bilge environment in days or weeks. The circuitry in the display is probably not conformally coated for the marine environment either.

 

[sbman]

My plan is to use one of these on my Detroit Diesel:

https://www.diyautotune.com/product/crank-position-sensor-hall-effect-tab-mount/

I'll use some existing embosses on the main pulley to be the trigger wheel which will net me 3 pulses per revolution, I'll just have to fabricate a stout bracket for the sensor. The advantage of this sensor is that it's self contained with all the trigger electronics built in, a clean square wave output to hook up to your gateway of choice and I can leave the existing tach in place while I experiment. No additional magnet is necessary.

Aside from the mounting tab hole reinforcement, it's fully encased in plastic so no corrosion issues to worry about.

 

[socalrider]

My plan is to use one of these on my Detroit Diesel:

https://www.diyautotune.com/product/crank-position-sensor-hall-effect-tab-mount/

I'll use some existing embosses on the main pulley to be the trigger wheel which will net me 3 pulses per revolution, I'll just have to fabricate a stout bracket for the sensor. The advantage of this sensor is that it's self contained with all the trigger electronics built in, a clean square wave output to hook up to your gateway of choice and I can leave the existing tach in place while I experiment. No additional magnet is necessary.

Aside from the mounting tab hole reinforcement, it's fully encased in plastic so no corrosion issues to worry about.

That looks great. I'm not sure if my crankshaft pulley's existing ridges would trigger it (looks like 2mm wide 2mm deep is required) but I'll check them next time. I suppose I could always epoxy a magnet or piece of steel for that purpose if necessary. Certainly like the plastic encasement & agree with your previous comments.

Edit: I wonder if the threaded body version might make for an easier bracket fabrication given the desired 1mm clearance.

https://www.diyautotune.com/product/crankshaft-position-sensor-hall-effect-threaded-body/

 

[sbman]

I suppose the form factor of the sensor depends on your specific bracket fabrication. I still prefer the plastic version just from a corrosion perspective. Anodized aluminum should be OK too I suppose.

I think that the specific size of the ridges don't matter as long as there is a minimum width and sufficient gap when the metal is not present. On my Detroit, the bosses on the pulley are about 3/8" wide, and then the blanks are at least 2 inches deep. I think that will work OK assuming I can get the gap correct without any physical contact.

All the hall effect sensors that screw into the Aetna adapter are basically the same type sensor with similar triggering electronics, but mounted inside a convenient, sealed casing that has a trigger wheel and the hall effect sensor and trigger electronics all ready to go. It's easily mounted with the proper adapter and no fabrication or screwing around required.

The same Stewart warner, VDO and Autometer sensors would also screw into the existing tach drive ports that I currently have on my Detroits.

 

[koliver]

I have just ordered some of these; https://www.amazon.com/EEEKit-Digit...d=1&keywords=rpm+sensor&qid=1630713650&sr=8-4
Thanks for the tip!

 

[Barking Sands]

Tiny-tach has diesel tacks that use a sensor that clamps on the #1 fuel injector hard line. Seems to work reliably and is around $120. They also have a converter to amplify the signal to a regular square wave to supply signal to other systems.

 

[socalrider]

Tiny-tach has diesel tacks that use a sensor that clamps on the #1 fuel injector hard line. Seems to work reliably and is around $120. They also have a converter to amplify the signal to a regular square wave to supply signal to other systems.

Thanks! I’ve seen the tiny tach but not the converter - is there a link?

 

[Barking Sands]

diesel tach https://tinytach.com/diesel-tinytach

Converter https://tinytach.com/other-instruments

 

[socalrider]

diesel tach https://tinytach.com/diesel-tinytach

Converter https://tinytach.com/other-instruments

Thanks! I hadn't seen that. Ends up being the same price as the Aetna gear though, which is likely a bit more robust. I am kind of tempted just to spend the $$ rather than trying to fab a bracket that places a sensor within 1mm of the correct clearance (when I'm being honest about my fab skills)...

 

[sbman]

Thanks! I hadn't seen that. Ends up being the same price as the Aetna gear though, which is likely a bit more robust. I am kind of tempted just to spend the $$ rather than trying to fab a bracket that places a sensor within 1mm of the correct clearance (when I'm being honest about my fab skills)...

Understand that. I may just end up with a couple of sensors screwed into the existing RPM drive ports as well. I was hoping to retain my existing old 1 wire senders while I mess with the A-D conversion, but it may be more effort than I want to go to as well. The sensor needs to be placed so that it isn't subjected to the crankshaft endplay to achieve that kind of tolerance, so it needs to be on the inside or outside arc of whatever surface is being used to drive it.

 

[hgd]

Piezoelectric Tach

I have been using a piezoelectric system on my Beta 85 very successfully over the past two years. I had the problem of unreliable tach readings using pulses from the alternator. This is because of reasons that were previously cited. When the batteries are full the field voltage drops out and this is the source of the tach pulses. The piezoelectric system by passes all of this with a piezoelectric sensor clamped on to a injector tubbing measuring the liquid diesel pressure pulse through the tubing. The company is Design Technologies in Indiana 630-920-1300 Steve is the technical guy. The system consists of a small clamp on piezoelectric sensor plus a Piezoelectric Amplifier, which provides the shaped and amplified pulse to your tack gauge.