# Detroit Diesel 671N/J&T "Engine takeoff ratio and tachometer calibration"

Discussion in 'Diesel Engines' started by nnyerges, Jun 24, 2013.

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### nnyergesNicolas62

Gentlemen,

My boat engines DD 671N/J&T (1984) use mechanical takeoff for the tachometer sender. Questions:

1. The take-off is in the back side of the engine (see image). I need to know what is "Engine takeoff ratio" (1/2:1, 1:1, 1-1/2:1 or 2:1)? to set my new tachometers.

2. I will like to calibrate my tachometers, using my osciloscope. The sender is a SIGNAFLEX (8 pulse per turn).

Case study:

With the sender outside the engine, if I manually turn the needle at 1000 RPM, the sender will give me 1000x8 = 8000 pulses in one minute, equivalent 8000/60 = 133.3 pulses per second = 133.3Hhz.

With the sender installed in the engines takeoff port, do I have to consider the "engine take-off radio" to calculate the real engine RPM?. In other words, does the "engine take-off radio" affects the sender reading or just is to setup the tachometer?

Example:
Suppose that the engine take-off ratio is 2:1 and I measure 133.3Hhz in my osciloscope, does the final RPM is 1000/2=500RPM, 1000*2=2000RPM or 1000RPM?

Nicolas

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2. Joined: Nov 2011
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### nnyergesNicolas62

I'll make the question simpler:

If the DD 6-71 engine runs at real 1000RPM, how many pulses per minute is going to give me the SIGNAFLEX sender output?

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### CDKretired engineer

The take-off is mounted high on the engine, so it uses the camshaft; you get 4000 pulses/min at 1000 rpm.

Precise calibration is easy with a laptop and a simple program from the internet "rpmsetup.exe".

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### nnyergesNicolas62

Some confutions...

Some discrepancies, since one takeoff ratio is 1/2:1 and the other is 1:1

Bilgediver give me a nice an perfect idea (how not occurred to me before) !Thank you!:
I will give it a try this week.

Now, aside from all that, no matter what position I place the takeoff ratio switch selector on my two Faria tachometers, there is no reading for both engines at IDLE or LOW RPM. They start to “steadily move up” at +/-1000rpm (in both engines).

Any idea why?

Last edited: Jun 25, 2013
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### CDKretired engineer

Ok, I wasn't prepared for a 2 stroke engine.
I guess the pickup is a coil with a rotating magnet. Not only is the frequency a function of the rotational speed, the amplitude also increases with rpm. Below 1000 rpm the trigger level isn't reached, so there is no indication.

Could it be the tachometers and pickups do not belong to the same "family"?

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### nnyergesNicolas62

You are right. The amplitud also increases with rpm, and my actual sender is from Teleflex/Signaflex and the tachometers are Faria. Faria have they own mechanical takeoff senders.

They are physically identical but may have different responses. Does any body else can confirm this?

If these tachometers dependent voltage emitted by the sender, then I'm screwed. I'll have to change the senders. If the RPM reading is done by number of pulses, then no problem.

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### FAST FREDSenior Member

Almost every unit I have seen is a simple pulse counter.

With a good engine a warm DD will idle about 450 rpm , so simply slow the engine , set the rpm and then check.

Many DD are set to 2100 as high limit , run it up for a couple of seconds and see if the rpm is reasonable.

Pulse tachs are usually more accurate but less reliable than cable tachs, but either way the reading RPM is approximate and a good ear will sync two engines better than a tach.

All most folks need from any tach is good repeatability , so you can reset the desired rpm after a throttle change.

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that looks like its on the drive so the tacho generator is spinning at engine speed
you will need to ensure the tacho can accept the signal it generates

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### CDKretired engineer

Faria says in IS0100_Tachometers.pdf‎ there must be a signal exceeding 2 volts, measured at the tachometer input.

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### nnyergesNicolas62

SOLVED:

I have measure the ratio by rotating the crank wheel and counting the number of pulses of the sender. It’s 1:1. Tachometers are now calibrated... Thanks to all.

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### JudojimNew Member

Does anyone know how to safely remove the signal generators. I am going to us the AC tap on the alternators to run new tach, so I want to remove the old generators.
Jim

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### FAST FREDSenior Member

I am going to us the AC tap on the alternators to run new tach

DONT ,

an alt tach is not at all accurate , belt wear , alt load and moisture will change reading.

OK for repeatable setting a cruise RPM after a slowdown , but to really measure RPM , a waste of effort.

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### JudojimNew Member

The only other alternative left for me, I understand, is to install a signal generator that reads the teeth of the spinning flywheel. The problem with that plan is that there doesn't seem to be a place on the engine that has access to the teeth on the flywheel, other than the starter!

Do you have any other methods in mind?

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### FAST FREDSenior Member

Look again just below the housing that holds the starter.

There is a plug that is removed so a pry bar can be used to turn the engine when setting the rack , or adjusting the valves.

About an inch in diameter , under the starter and of course parallel to the ring gear.

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