Tach rpm sender location on engine

[LydieBP]

Good day ! I am troubleshooting my rpm gauge which has ceased working. The gauge display itself is getting 12 v. Now the cable to the sender needs to be checked .
However , I don't know where it is. old new to me boat . I've read on forums that it's on the port side on top of the bell housing of the flywheel or rather it's on top of the alternator ?!?. Either way, I can't locate it.
If anyone could please point it out to me in these pics or give me a clue as to where else it is , I would be very grateful , thank you .

 

[Ralph Johnstone]

Now, I know that YOU know that WE need to know the make, year and HP of your engine before we can even begin to tell you where the sender is located, so ............... how 'bout just a hint, a clue, a suggestion, maybe a tip, even a mere suggestion would likely go a long way ?

If you have a brand name of your gauge, that would also be helpful.

Give the above info, many people here should be able to virtually troubleshoot your tach problem from the comfort of their own homes.

 

[dlochner]

The reason you are getting confusing answers is engine manufacturers have different methods of obtaining the signal for the tachometer. Some receive the signal from the alternator, others (notably some Yanmar models) have a sensor that counts the teeth on the ring gear, the gear the starter engages with to start the engine. Without the make and model of the engine, we have no way of giving you reliable information.

Judging from the color of your engine, I'm guessing it is a Volvo, but I have no idea of the model number. If it has a sensor on the ring gear, the sensor will be on the engine block not the transmission.

So, get us the make and model of the engine and a photo of the aft end of the engine block where the starter is and we'll be able to help you.

 

[LydieBP]

Now, I know that YOU know that WE need to know the make, year and HP of your engine before we can even begin to tell you where the sender is located, so ............... how 'bout just a hint, a clue, a suggestion, maybe a tip, even a mere suggestion would likely go a long way ?

If you have a brand name of your gauge, that would also be helpful.

Give the above info, many people here should be able to virtually troubleshoot your tach problem from the comfort of their own homes.

Thank you for responding .
the engine : Mercedes OM 636 42HP 4 cylinder circa 1979
the gauge: Teleflex 4000RPM

 

[Ralph Johnstone]

Sorry @LydieBP , the Mercedes OM 636 42HP is an engine I have never seen much less heard of. I did come across a site which has a maintenance and operation manual on the engine for download :

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www.scribd.com

This will be your best bet on locating the location and type of RPM sender the engine uses.

Once you have located the sender, you may find this item helpful IF and only if your tach assembly is similar in basic operation to this writeup.

Best of luck with this

 

[LydieBP]

Sorry @LydieBP , the Mercedes OM 636 42HP is an engine I have never seen much less heard of. I did come across a site which has a maintenance and operation manual on the engine for download :

Loading…

www.scribd.com

This will be your best bet on locating the location and type of RPM sender the engine uses.

Once you have located the sender, you may find this item helpful IF and only if your tach assembly is similar in basic operation to this writeup.

Best of luck with this

The OM636 is the grandfather of all Mercedes diesel , revised into many variations over the decades, has been used in automobiles, farm vehicles and machinery and marinised for boats. there's a nice wiki on it.
I actually have a huge mechanics binder containing 2 inches thick ream of paper with instructions to install and service the OM636 . Unfortunately. senders and their cables, isn't part of the service manual scope.
thank you for your help .
A few more pictures .

 

[heritage]

Although I can't help you with the sender location, it is so rare to see a question about the Mercedes 636 that I have to comment. We had a 636 in our 1976 Westerly that was by far my favorite engine. Sounded like a sewing machine. The oil filter was unique in that it used a finned metal element that never needed changing and fairly easy to clean. Follow Dlochner's instruction and you should be able to find the sender.

 

[jssailem]

I have no personal experience with your engine or tach. Suggestions on places to look.

• Trace from the tach - either a wire or a cable, to the source on the engine.
• Possibilities include
  • the alternator is sending an electrical pulse to the tach (likely source)
  • There is a magnet on the cam pulley that is sending a pulse to the tach as the cam pully rotates
  • there is a cable spinning that is transmitting the tach data from a sensor on the side of the engine block (rare)

 

[LydieBP]

I have no personal experience with your engine or tach. Suggestions on places to look.

• Trace from the tach - either a wire or a cable, to the source on the engine.
• Possibilities include
  • the alternator is sending an electrical pulse to the tach (likely source)
  • There is a magnet on the cam pulley that is sending a pulse to the tach as the cam pully rotates
  • there is a cable spinning that is transmitting the tach data from a sensor on the side of the engine block (rare)

Okay . thank you . I'll try to search that way .
Today I cleaned and reconnected the ring terminals on the back of the RPM gauge . Then I started the engine, and the needle moved. I was about to get excited , until I noticed that whatever the amount of throttle I gave it, the RPM needle remained at 1500, whether at low idle or high. oh well ...

Fun fact about all the gauges, there is not the usual labeling on the back of the gauges due to boat paint sprayed all over. it adds to the challenge . I tried scraping the paint off around the connector poles but it revealed nothing .

Thank you everyone for your tips .

 

[LydieBP]

Although I can't help you with the sender location, it is so rare to see a question about the Mercedes 636 that I have to comment. We had a 636 in our 1976 Westerly that was by far my favorite engine. Sounded like a sewing machine. The oil filter was unique in that it used a finned metal element that never needed changing and fairly easy to clean. Follow Dlochner's instruction and you should be able to find the sender.

Yes , the OM636 is quite easy going.

 

[Ralph Johnstone]

Once you have located the sender, you may find this item helpful IF and only if your tach assembly is similar in basic operation to this writeup.

Expanding on "this item" which I mentioned above and managed to forget:

You may find some use in this after you locate your sender.

I noticed that whatever the amount of throttle I gave it, the RPM needle remained at 1500, whether at low idle or high. oh well ...

It's right firstly when speeding up and secondly when slowing own .

 

[Indysailor]

Okay . thank you . I'll try to search that way .
Today I cleaned and reconnected the ring terminals on the back of the RPM gauge . Then I started the engine, and the needle moved. I was about to get excited , until I noticed that whatever the amount of throttle I gave it, the RPM needle remained at 1500, whether at low idle or high. oh well ...

Fun fact about all the gauges, there is not the usual labeling on the back of the gauges due to boat paint sprayed all over. it adds to the challenge . I tried scraping the paint off around the connector poles but it revealed nothing .

Thank you everyone for your tips .

Trace every wire to its source and you'll find it. Also, it's good to have a complete diagram of your engine wiring and this is a great place to start, since so much of it goes to the gauge panel.

 

[LydieBP]

I found SEND, IGN and GND markings after much more scratching on the tachometer back . It's like a scratch lottery ticket I used to see at tobacco shops in Spain, scratch, scratch until you get a winning ticket
i removed and cleaned the SEND pole, the cable ring terminal and washer and reconnected. I attempted to trace the blue send wire but no luck so far. In this boat, wires often change colors when peaking out from behind a bulkhead lining down the line . I've done alot of wiring charting on the batteries and equipment side already.
This display gauge has been working fine until now it doesn't .

I started the engine, the needle moved nicely up and down as I throttled. Unfortunately , after about 5 minutes running , the needle stuck around 900 RPM and never moved again .
perhaps a connection on the other end that needs tightening. we shall see.

Thank you, Ralph Johnstone , for sharing the article about a good tach reading .

Happy New Year 2025 Sailing !

 

[Ralph Johnstone]

If you get desperate for locating that sender wire, I reluctantly suggest taking a look at this little gem. I purchased mine about 15 years ago and the technology may have improved over the years. In using mine, it's about 95% Ju-ju and 5% technology. The transmitter sends a signal into the unattached wire where you are starting from and then you run a receiver along the wire as it progresses in bundles or through decks and bulkheads following the induced signal as you go. Only problem is ....................... the lost wire induces the signal into nearby wires and soon every wire in your boat has a signal in it. It's a combination of balancing the receiver against the strength of the original signal from the transmitter.

I've managed to find a couple of lost wires with this unit which were in nasty locations. I had had no luck in locating by the grunting, cursing and tracking method.

The only saving grace is that they're cheap.