sev marchal alternator wiring no rpm tach help

N

Nuggs2020

New Member
Joined
30 Sep 2020
Messages
5
Visit site
Hi all can anyone help please..
My rpm tacho (vdo) is not working i think it is because i have a blue wire which either goes on the alternator or external regulator.
Upon reading many forum messages i keep seeing that the rpm goes to a 'W' terminal which i dont have?
i have 3 wires from my external regulator which i presume goes onto the alternator,
but not sure as to where the blue wire goes, i have attached images to show
many thanks

rsz_sev_1 (1).jpgrsz_sev_2.jpg
 
Nuggs2020 said:
Hi all can anyone help please..
My rpm tacho (vdo) is not working i think it is because i have a blue wire which either goes on the alternator or external regulator.
Upon reading many forum messages i keep seeing that the rpm goes to a 'W' terminal which i dont have?
i have 3 wires from my external regulator which i presume goes onto the alternator,
but not sure as to where the blue wire goes, i have attached images to show
many thanks

View attachment 100466View attachment 100467
Click to expand...

Welcome to the forum.

You give no history of the problem, has it just stopped working ? Is it a new to you boat and it doesn't work ? Have you just fitted a new component etc etc ?

I doubt very much that alternator is capable of operating a tacho.

You also don't say what the engine is, which would surely help as your engine may well have a tacho sender somewhere.
 
I'm sorry ok the engine is a penta md2b,I just bought the boat but previous owner said he didn't worry about the rpm not working, but just needed wiring up, the engine does have a sender coming from the rear of the engine
 
The W terminal connects to one of the internal field phases prior to the rectifier. So if you can open the unit you should be able to find one end of one of the three coils. As the supply at this point is AC it won't matter which one of these six individual connections you connect to. This does assume your Tacho is looking for a pulse signal to count, and note the peak voltage is likely > 18 or 20 volts, indeed should the alternator ever lose its load it could reach a far higher level.

However, beware because you then mention a sender. These work by a magnet generating an (very weak / low voltage) electrical pulse from a passing gear tooth, commonly on the starter ring.

If so the hook up should be very straightforward.

But do not attempt to hook your tacho up to a W connection if it is looking for a really weak induced pulse, as this would likely wreck the tacho.
 
Nuggs2020 said:
I'm sorry ok the engine is a penta md2b,I just bought the boat but previous owner said he didn't worry about the rpm not working, but just needed wiring up, the engine does have a sender coming from the rear of the engine
Click to expand...

The MD2B does indeed have an engine sensor, this is where the tacho connects. If you can post a pic of the back of the tacho or the tacho model it might help.
 
Nuggs2020 said:
I'm sorry ok the engine is a penta md2b,I just bought the boat but previous owner said he didn't worry about the rpm not working, but just needed wiring up, the engine does have a sender coming from the rear of the engine
Click to expand...

The tacho on a MD2B should be connected to a sensor mounted on the cover of the hand start gear

You should find two terminals on the back of the tacho marked G and W ... these are the connections for the sensor. Both wires attached to these terminals should be coloured grey
( you should be able to find this in the relevant wiring diagram in your Instruction Book/ Owners Manual )

These snips from the parts catalogue show the location of the sensor ( shown as #21 in the first diagram and #38 in the second )

Hopefully the sensor is present and functional because it costs more than 250 euros. Hopefully you will also find the wiring present and useable )

1602596676627.png1602596948708.png
 
Thank you guys my mistake I found out the blue wire was just a ground also vicS many thanks im getting there now I did check the terminals on the rev counter which seem fine and have power etc,I took of the rev gauge took it home gave it a shake (like you do) and the needle seems to be free il know better this weekend
 
Nuggs2020 said:
Thank you guys my mistake I found out the blue wire was just a ground also vicS many thanks im getting there now I did check the terminals on the rev counter which seem fine and have power etc,I took of the rev gauge took it home gave it a shake (like you do) and the needle seems to be free il know better this weekend
Click to expand...

You could have 3 wires on the back of the tacho, positive, negative and signal. Positive and negative are easy to identify with a multimeter. Chances are the blue wire is the ground (G) wie in the sender, the W wire on the sender goes to the signal connection on the tacho. You can check the signal wire for continuity with your multimeter.
 
Last edited:
Hi Paul thank you for replying,ok my tach is still not working attached is the back of the tach there is no labeling on the contacts but looking in the instument book it says the positive wire goes on the first post,negative last post and 2 ivory in between
20201013_121123 (1).jpg
 
You must log in or register to reply here.

Share this page

Top