I probably should know this

**Lionelgee** · 18th November 2022, 08:34 PM

Hello All,

I asked an online retailer if a VDO tachometer they are advertising as a Truck tachometer would suit my application. I received the following reply back from the retailer...

"it would run off alternator W point no problem"

Just wondering what a "W" point on an alternator might refer to?

The tachometer is a VDO Cockpit International Electrical 12V 80mm Tachometer 0-4000rpm 333 035 002 #234739305387 described as

Suitable for most petrol and diesel engines. Field programmable to suit 4, 6 or 8 cyl./4 stroke ignition and alternator pick-up (terminal "W"). Single phase, not suitable for star point.

Next question .... what is a star point.

The intended application hooking up to a Perkins 6354 = six cylinder diesel with 354 cubic inch capacity.

Wouldn't the VDO unit need to come with a type of sender that actually reads the number of revolutions per minute. My understanding is that hooking up to an alternator would provide a power source. Although things might have changed from the original purely mechanical tachometers that the old Perkins first came out with. These were made up of a driver, a cable and then the gauge itself. Each part NOS currently sold separately for a motza for each part. This why I am actively seeking alternatives to the original Perkins tachometer arrangement.

Kind regards
Lionel

**shack** · 18th November 2022, 08:42 PM

The "W" terminal on an alternator is a universal take off point for a tacho.

Check if yours has it and you should be ok.

**Graeme** · 18th November 2022, 08:43 PM

The W point is a tap onto 1 phase. I fitted one to an alternator that didn't have it to get a tacho working.

**p38arover** · 18th November 2022, 08:52 PM

The W point is a feed from one phase of the 3-phase alternator. This sine wave feed doesn’t actually give you the engine RPM, it give the RPM of the alternator hence the need to calibrate it.

I’m not an electrician so I could be wrong on this bit…
There are two ways to wire 3-phase - delta (triangular) and star where the 3 phases connect at a single point and there will be no useful sine wave output at that point. I think that’s the star point they mention.

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**Lionelgee** · 18th November 2022, 08:58 PM

Originally Posted by shack

The "W" terminal on an alternator is a universal take off point for a tacho.

Check if yours has it and your should be ok.

Hello Shack and Graeme and Ron,

So unlike the original Perkins Tachometer everything the VDO unit needs to read the revolutions per minute is sourced all from just the "W" terminal? If it is then that is a very big bonus. Happy days ... well as long as the alternator has a W terminal - I will have to look in the shipping container tomorrow when it is daylight

The NOS original parts was close to $400 for the driver/sender and $400 for the gauge itself. Plus I would still have needed to source a cable to connect the driver/sender to the gauge which would not have been cheap venture either.

Kind regards
Lionel

**p38arover** · 18th November 2022, 09:02 PM

You will still need a 12v DC feed from the ignition circuit. The W terminal supplies an AC signal. If your alternator doesn’t have a W terminal, it’s easy to add a flylead to do the same job.

**Lionelgee** · 18th November 2022, 09:07 PM

Originally Posted by p38arover

You will still need a 12v DC feed from the ignition circuit. The W terminal supplies an AC signal. If your alternator doesn’t have a W terminal, it’s easy to add a flylead to do the same job.

Hello Ron, Thank you for the clarification about the ignition. My only comment is "wow" - in relation to how much technology can change since the 1960/70s when these trucks came out.

Umm next question Ron, since my degree is not in electrical engineering .... what is a Flylead?

Kind regards
Lionel

**p38arover** · 18th November 2022, 09:13 PM

Most alternators with a W lead probably have a stud to bolt the tachometer lead to. Adding one to an alternator without one might not be possible so a way around that is to just have a wire come out of the alternator to which you can connect the tacho. That wire coming out from the tacho would be the flylead.

I’m (was) just an electronics tech so automotive electrickery is not my area of expertise

**BradC** · 18th November 2022, 09:15 PM

It'll need to be calibrated also, because it's really measuring the RPM of the Alternator and that varies depending on pulley sizes.

When we re-powered the boat the new alternators had slightly smaller pulleys and we had to adjust both Tachs to suit.

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