Diesel RPM Gauge & Sensor

[jfkal]

Is there a way to test the magnetic "gear counting" sensor. Is it supposed to produce a measurable voltage or current?

Got a spare alternator type gauge as well. Thought it need to be connected to an AC stud at the alternator. Mine does not have one . Would the ripple in the DC enough to trigger it as well?

 

[ChrisP]

Most prox. sensors act as a solid state switch operating when they come into close proximity with either a magnet of a piece of metal. They don't actually generate a signal. Best way to check them is to connect them to a source with a meter in the line and then wave a piece of metal close to the sensing area. you will see the deflection on the meter as the switch operates. It should be noted that some models are 3 wire type. 1+, 1- and one that goes to the supply -VE via the load (meter).

I believe that if you take your alternator to a Lucas / CAV dealer they can add the required connection for the tacho. You have to take into account the pully ratio's to give an accurate reading.

Good luck

ChrisP

 

[Gordonmc]

Is the three wire type sensor or alternator fed? At some point the mechanical tacho on my Thorneycroft T90 has been replaced with an electronic job with three wires, obvious - and + plus a third which goes onto a mounting stud.
The tacho needle flickers when the power goes on, indicating the supply is OK, but is dead thereafter.
Any ideas before I spend beer money on a new guage?

 

[Robin2]

I would be surprised if these sensors produce a noticeable output without some electronic circuit to amplify their signal. Without a technical specification for the part it is down to trial and (maybe expensive) error.

 

[HaraldS]

I have seen two different styles of magnetic rev senders on diesel engines.
If it is just one wire coming from it( ground plus one wire), then it is likely just a little coil, in which a small current is induced by the magnet flying by. With a meter that is sensitive enough on AC you should be able to notice some voltage when the engine is running.
The other type uses a hall-field sensor, which needs a 12V supply on one wire, and has an open-collector output (electronic switch to ground) for the signal. To measure something on this one, you would need to put supply voltage on the right wire and a small load like 2.2kOhm resistor from 12V to the signal output. Then again you would measure some AC.
It is not difficult to add a sense tab to your alternator yourself. Let me know if you want to take that route.
But usually your instrument is just built for only one of the methods above, so you might not have a choice.