Mercury Rev Counter

[The Hoff]

Hi Ribnet,

I've just bought a Apex RIB with a 2001 Mercury 75hp 4 Stroke engine which i'm really pleased with but was wondered if anyone could help me with a wiring issue regards the rev counter, please?

I've got power as the back light works, when the ignition key is turned on, the needle slowly rises until it stops at 8k rpm. When the engine is turned over, the needle goes off the scale!!
When power is off the needle then goes back to 1.5k rpm!

I'm really no wiring expert, just managed to change the switch panel and wire the nav lights working which gave me great delight.

Any help would really be appriciated. I've attached a few photo's so you may spot the problem straight away :-)

Thanks in anticipation

Andy

[spartacus]

Got the right polarity setting on the tacho?

[The Hoff]

Thanks spartacus for your reply.
If i'm honest I am not sure what the polarity should be set at so am unsure if it is right, any ideas?
Cheers Andy

[gotchiguy]

Originally Posted by The Hoff

Thanks spartacus for your reply.
If i'm honest I am not sure what the polarity should be set at so am unsure if it is right, any ideas?
Cheers Andy

There will be a little switch on the rear of the dial which can be turned ~2-6. Try each setting, one should work. Basically the dial is given a certain number of pulses per revolution but it needs to know how many pulses it will get per revolution so it has to be told otherwise it won't work. It depends how many magnets there are on the flywheel and therefore how many pulses the dial receives per count.

But I may be talking complete rubbish

[spartacus]

Originally Posted by gotchiguy

Basically the dial is given a certain number of pulses per revolution but it needs to know how many pulses it will get per revolution so it has to be told otherwise it won't work. It depends how many magnets there are on the flywheel and therefore how many pulses the dial receives per count.

But I may be talking complete rubbish

Sounds convincing. Ever thought of going into politics!

[doggypaddle]

summat strange going on there, dont think its the switch setting though, if the ign is on and the engine not running the needle should go to zero not 8K, with the ign off the needle could stop anywhere, mine usually goes to about 2krpm too.
I would say if you have a 12v supply getting to it of the correct polarity then it may be faulty.

[doggypaddle]

Originally Posted by gotchiguy

There will be a little switch on the rear of the dial which can be turned ~2-6. Try each setting, one should work. Basically the dial is given a certain number of pulses per revolution but it needs to know how many pulses it will get per revolution so it has to be told otherwise it won't work. It depends how many magnets there are on the flywheel and therefore how many pulses the dial receives per count.

But I may be talking complete rubbish

pointing to 8K without the engine running though??????
must be set to the 0 pulse setting
seriously though I dont think the switch position should effect where it sits at 0RPM. zero rpm is zero pulses whatever the setting.
if it reads 8K its either got the wiring wrong or its fekkt

[Poly]

Originally Posted by gotchiguy

But I may be talking complete rubbish

Unusually accurate but it will probably be the signal from the alternate rather than fly wheel. I think on this engine you should try position 4p on the little black dial on the back.

[Poly]

Originally Posted by doggypaddle

pointing to 8K without the engine running though??????
must be set to the 0 pulse setting
seriously though I dont think the switch position should effect where it sits at 0RPM. zero rpm is zero pulses whatever the setting.
if it reads 8K its either got the wiring wrong or its fekkt

Agreed, perhaps wired wrong, can't see in pic.

[doggypaddle]

Originally Posted by Polwart

Unusually accurate but it will probably be the signal from the alternate rather than fly wheel. I think on this engine you should try position 4p on the little black dial on the back.

Isnt the alternator rotor the flywheel on a 9.9? and the stator inside it

[Poly]

Originally Posted by doggypaddle

Isnt the alternator rotor the flywheel on a 9.9? and the stator inside it

Possibly, but the op is asking about a 75hp

[doggypaddle]

Originally Posted by Polwart

Possibly, but the op is asking about a 75hp

FEK got me posts mixed up
isnt a 75 the same though? seem to remember the stator has alternator coils and trigger coils for the ign module under the flywheel?
having said that the last 75 i worked on was 20 years ago.....

[gotchiguy]

Originally Posted by Polwart

Possibly

[Poly]

Originally Posted by gotchiguy

Silly boy Doggypaddle is showing his age. Modern 75 HP Mercury 4 strokes have belt driven alternators...

[doggypaddle]

Originally Posted by Polwart

Silly boy Doggypaddle is showing his age. Modern 75 HP Mercury 4 strokes have belt driven alternators...

I did know that .........I missed the 4 stroke bit in the o/p must be a sign of getting old

[The Hoff]

Hi,
Just want to say a big thanks to all that have posted their thoughts.

I'll start with the polarity switch at the rear, setting it to 4P then look at the wiring.

If all fails then it'll have to be a new gauge.

Thanks again everyone.

The Hoff

[9D280]

For polarity checking, buy yourself a £9.99 multimeter from Maplin, set the switch to "volts - any number above 12". Make sure the red lead is i nthe + terminal on the meter, black to - (if the meter has built in leadsthey wil lbe the right way round already)

Now touch the probes against the two marked "ign" and "gnd", red to "Ign", black to "gnd". Your needle should go to somewhere around the 12V mark on the dial, or if it;s digital show a positive number n the scale. If the needle tries to jump off the scale or you get a -ve number, the supply is wired @rse about face.

Next job: Follow the wire connected to "send" & check every connector between tacho & engine. It should finish at a small black or grey box with three or four terminals, two of the other wires of which dissappear under your flywheel the other will probably be a red which makes its way to the big cable from the battery. The wire from the tacho should be on the same connection as one of the wires from under the flywheel.

Note that was an educated guess about how your engine is wired. If it's a nice shiny new computer controlled thing the chances are that will all be done in the engine management box, so shout back if it doesn't look like what I described.