Wiring for Tacho Sender

[pcatterall]

I have a choice of 2 senders for the tachometer on my rebuilt Perkins 4108.
The first unit (Black on photo) has 2 terminals marked Z and G respectively with a third terminal position ( but no terminal in place) around the back.
The second unit has just a single terminal marked G ( there is a terminal in place around the back).
I had assumed that G meant ground but now wonder as the one around the back is direct to the body of the sender.

I guess that the difference may be that the first unit just grounds via the block though I could fit the missing terminal around the back for a better earth connection
but in that case the second unit would seem to have one terminal marked G and a second terminal around the back which is just to the body of the unit.

(Earth?)

Hopefully someone can tell me which connections do what, it would be interesting to know if there is any standard colour code for these connections especially for sender to gauge.
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[rogerthebodger]

The problem is there are several different types of tach sender.

I have a VDO tachogenerator that ad 2 wies from the sender to the tachometer and requires no external power.

My tachogenerator is driven direct off the back of my injector pump.

There are others that pick up pulses from a gear or toothed wheel.

The G I would think is gauge but what type yor sender is I cannot identify but could be a tachogenerator as it looks like it had a mechanical drive like mine.

https://vdo-webshop.nl/en/revolutio...-vdo-generator-tacho-speed-sender-7-8-18.html

 

[Yngmar]

RPM on a diesel engine typically is measured at the alternator ("W" terminal), although if there is no alternator for some reason, VDO makes two alternative sensor types, an inductive one measuring at the flywheel and a "generator" one on either the injector pump or the camshaft.

All three types come in two variants, with either a separate ground contact (typical in marine engines where the engine block isn't grounded) and ones with just the one contact, with ground via the engine block it is inserted into.

Yours looks like a generator type sensor and thus the "G" terminal is wired to the gauge, where the "W" terminal on an alternator would go. "Z" is therefore ground (often also just labeled "-").

 

[pcatterall]

Thanks. I confirm that the unit is engine mounted with a keyed drive from 'inside' the engine to the sender.
G for gauge sounds logical ( now you tell me!)
So it would seem that the first unit has a terminal to Gauge and an isolated terminal to earth while the second unit uses the G terminal to Gauge and rellies on the rear terminal as the earth point?
Cheers

 

[Yngmar]

G for gauge sounds logical ( now you tell me!)

"G" stands for Generator (which is German for... alternator). The little keyed shaft actually drives a tiny alternator, which generates just enough power for a tacho signal.

So it would seem that the first unit has a terminal to Gauge and an isolated terminal to earth while the second unit uses the G terminal to Gauge and rellies on the rear terminal as the earth point?

Yes.

 

[pcatterall]

Ahh!! the clouds have lifted! I was going to ask where the power to show the rpm on the gauge came from! …… generator ! ………. thanks.

So.…. ( danger the brain is working again!) I could test the sender out put using low revs on a drill?

 

[Billjratt]

Is the 4108 islolated from ground? Check by looking for a separate ground circuit on the startermotor and alternator. If the answer is 'yes', all the senders will have a separate return cable as well. That means you need the sender with multiple terminals. Any marked "G" are 'geben' which is 'giver' or 'sender' in German as previously mentioned. The benefit of course is the lack of anodes in the engine (unless you have old zinc spigots on a TMP gearbox - now replaced by plastic). Remember the G-for-giver phrase if you ever haul the harness off the instrument panel as well!

 

[Yngmar]

So.…. ( danger the brain is working again!) I could test the sender out put using low revs on a drill?

Yup.

 

[pcatterall]

Thanks again for all of your replies.
I note that G=geben/giver or Gauge or generator according to 3 versions!! However the purpose remains the same so the exact definition is not vital!!