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New tachometer. Still showing 0 rpm

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Bigdrago

Member
Hey there.
A few months ago I turned the battery polarities wrong way, started up the engine and the cable from the alternator to the battery burned up. I changed the cable and noticed the tilt monitor and tachometer was dead.

Anyway. Now I have changed the tachometer to a new one, just like the old one I had (the new having lcd hourmeter while the old one had analogue) The new one is Sierra 65129P Lido. My engine is a Mercruiser 1.7 dti (boat from 2006) with alpha one gen 2.

I have connected the new one as the old and turned the settings to 2P 4C. I turned on power and the tachometer needle fell down to 0 rpm. I can see the light in the tachometer working and so does the hourmeter. Turning the engine on and it still shows 0 rpm.

What can be wrong here? I noticed on the wiring diagram that there is something called tach amplify circuit.
On the parts overview picture there is something called tach sensor (nr. 22). Nr. 21 is "tach module".

I measured voltage on the signal cable conencted to the gauge and it is showing about 13.3V.

As long there is power on the signal cable it is working? Or can tach sensor or something on the alternator be defective and it still will send 13.3V without rpm frequenzy?

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Since this is diesel it doesn't have a coil and needs the engine speed sender to send the information to the tac.
It is very likely that the sender has failed.
 
the sender (signal) wire to the tach needs to have a pulsed frequency train on it for the tach to convert that frequency to drive the needle...having battery voltage on it, w/o any pulsing isn't good.

I doubt the sender unit failed before the amplifier with the crossed battery cables...if the key was on, I'd suspect the amplifier didn't like it and from the little looking I did, those are NLA...

I'd worry about getting the pulse train to the signal wire and then adjust the tach input options to see if one works. If you can get an answer on Sierra's tech support line, they should be able to tell you if that tach is compatible with your 1.7L diesel...
 
Thank you for all the answers. I guess the "tach module" may be faulty then. It is very narrow in the engine compartment but I guess I will have to try to dismantle it and have a look at it.
 
makomark said:
the sender (signal) wire to the tach needs to have a pulsed frequency train on it for the tach to convert that frequency to drive the needle...having battery voltage on it, w/o any pulsing isn't good.

I doubt the sender unit failed before the amplifier with the crossed battery cables...if the key was on, I'd suspect the amplifier didn't like it and from the little looking I did, those are NLA...

I'd worry about getting the pulse train to the signal wire and then adjust the tach input options to see if one works. If you can get an answer on Sierra's tech support line, they should be able to tell you if that tach is compatible with your 1.7L diesel...
Click to expand...

Do they have an e-mail adress? My english is not that good.
 
1. That is not the proper tachometer for a diesel. If that's what came on the boat original, then it was wrong from the factory. You need a diesel tachometer. The one you show in the picture is for gasoline engines.
2. The diesel tachometer needs a sensor to tell it how fast the engine is going. The item in the picture (#22) is that thing. If it is bad, the tachometer will not work. The yellow/green wire is for the tach signal.
I think.
 
Thanks. I bought it because it is 100% similar as the original one, 7k rpm and the back is the same. Picture of the original one attached. This type of bocould be delivered with gasoline and diesel engine. The boatbis a Uttern D55.

Pictures cant be inserted when using cellphone. Uploading later.

Picture here: https://ibb.co/1GMd6rz
 
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IF that's what WAS working, then it should be ok...just set it up the same as far as the selector dial...

I'd guess the amplifier (divider) isn't functioning...I'd suggest an oscilloscope as the toll to figure out where the issue is. I'd guess the engine speed sensor is probably making its pulses with the engine running and the amplifier isn't functioning due to the battery cable issue...
 
The engine speed sensor is a magnet that reacts to the pulses of the starter gear (Hall effect). It makes a very weak signal. The amplifier boosts the signal so that the tachometer can read.
 
That's pretty much how I see it...I'll add that I suspect the tach module also has a 'divider' circuit in it and that's how the 'gas engine' tach worked before...
 
Installed and tested new tach. When turning on power it goes to 0. When turning on the engine it goes up to 3000 rpm and quickly down to 0. This confirms that the tach module may be faulty. Ordering a new one now.
 
Seems like 884833A2 is a hard part to get. Trying the fourth shop now and I hope they have it.

What else could I use if 884833A2 doesnt work?
 
I'd say probably not much....

If you have repair shops for electronic control modules (used in autos) over there, maybe one of them may be able to fix it.

Other option would be to find an electronics wizard who can make a custom one or maybe 'copy' the one you have...
 
If you don't have any luck finding this obsolete part, there are a number of ways to work around it. The easiest is to take a tachometer signal from the alternator. There should be a tach signal wire connection on there somewhere. If so, then you are in pretty good shape.
The signal is in ppm, or pulses per minute. You will probably need to use an adapter such as the Dakota digital one that can take the alternator pulse signal and has adjustment and calibration that will allow it to work with your tach. Here is the manual for that device.
This appears to be less than the cost of just the replacement sensor, so it may be a good option to just ditch the factory sensor and go with something you can control yourself.
 
Thank you for your answer o2batsea, that will be my next step if this doesnt work.

Changed part 22 today (see first post for drawing) without no luck. Removed the tach module and opened it. Anyone can make this board? :)
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