tachometer

[bladecrazy]

Is it doable on W123 DIESEL. i,m sure we used the alternator but am going back 40 years , it would be helpfull to see what criuseing revs are at and also what revs gear changes appear at. can it be done via tapping into alternator somehow?.

 

[rifiki]

Just try google for W123 Diesel specification all info will be there.
I think the gearing in in top will be around 20-23 MPH per 1000 RPM.
Rev counter is a useless instrument for a non petrol head.
My 1960 190B still has original dynamo and still does the job, will change to alternator when it goes wrong

 

[bladecrazy]

Had a look on goog but am setting up the bowden cable and would find it usefull to have a tacho to set it upproperly, going to look at the alternator and hopefully find a signal cable.

 

[KennyN]

Had a look on goog but am setting up the bowden cable and would find it usefull to have a tacho to set it upproperly, going to look at the alternator and hopefully find a signal cable.

Cant you use a digital tacho with a piece of reflective tape on the crankshaft pulley , if it is only for setting up ?

I have used the coil (on petrol engines obviously) to get a trigger signal for a tacho , but never the alternator ??

K

 

[bladecrazy]

yes used the coils in the past and has worked well, do you mean the hand held digital tachos that show rpm?, i found a way but requires attaching a sensor to the alternator outer casing and seems to work well, going to speak to an auto electrician to see if its viable through the alternator.

 

[KennyN]

do you mean the hand held digital tachos that show rpm?, i found a way but requires attaching a sensor to the alternator outer casing and seems to work well, going to speak to an auto electrician to see if its viable through the alternator.

Yes , the ones that use a piece of reflective tape on the rotating part and picked up by the lens on the digital tacho.

I would think the alternator would spin much faster than the crank due to the larger diameter of crank pulley compared to the alternator so getting one alternator pulse per engine revolution may prove difficult.

K

 

[LostKiwi]

Yes , the ones that use a piece of reflective tape on the rotating part and picked up by the lens on the digital tacho.

I would think the alternator would spin much faster than the crank due to the larger diameter of crank pulley compared to the alternator so getting one alternator pulse per engine revolution may prove difficult.

K

You don't need one pulse per revolution, you just need to know what to divide by.
For example if the alternator frequency is 1kHz (it's an AC power source so will have a frequency related to rotation speed) at 500 rpm then you know to divide frequency by 2 to get rpm.
To get the frequency output you either need to top into a point before the regulator and diodes convert to DC or have something sensitive enough to read the frequency of voltage change post rectification.

 

[KennyN]

Could the OP not mount a proximity sensor next to the CS pulley and something onto the pulley (extended bolt etc) so that with each rotation the sensor outputs a 12v pulse that could be used a the trigger signal for an aftermarket tacho ?

K

 

[LostKiwi]

Could the OP not mount a proximity sensor next to the CS pulley and something onto the pulley (extended bolt etc) so that with each rotation the sensor outputs a 12v pulse that could be used a the trigger signal for an aftermarket tacho ?

K

Simplest is two small magnets glued to the crank pulley and a reed switch used as a sensor.

 

[AndySlade]

I am sure that I read in a Bosch alternator technical book I used to own, that there were alternators made for diesel engines with a specific electrical connection point to provide the pulses for a tachometer.

 

[bladecrazy]

yes thats what i remember, i,m sure it was a w terminal for signal?.

 

[AndySlade]

yes thats what i remember, i,m sure it was a w terminal for signal?.

Yes, I remember now. Found this - The W terminal generally outputs an unrectified AC voltage, the frequency of which is proportional to the engine speed.

 

[bladecrazy]

 

[bladecrazy]

sadly no w on this alternator, so looks like will have to look at another way.