Club 80-90 Forums

Does anyone know how the VW diesel rev counter works?

I know it picks up a signal or feed from the alternator to identify the engine revs but what drives the meter? Is it an electric motor turning a magnetic drive similar to a speedo? Does the signal act as a speed controller for this motor? Any ideas?

Mine has been playing about which I thought was related to bad contacts at the instrument pod but now I have repaired the ribbon loom and the plastic around the connector it is still quite variable. It seems OK up to about 2,000rpm but then flickers about or drops back to zero. It starts to pick itself up again approaching 2,500 to 3,000 but I haven't done much at this speed/revs today.

I am wondering if there is a servicable component in the alternator which needs attention to provide a good clean signal. All help appreciated.

thanks
Garyd

I'm not familiar with a diesel tacho but I shoudn't think it would be anything to do with the alternator. You may simply have a poor connection somewhere.
Petrol engine tacho's are driven by the pulses at the coil so I would guess that diesels have a sensor on the flywheel or pulley ???
I have looked in the Bently manual but can find no mention of it.
Would be interesting to find out how it works though

Diesel tachos do indeed run from the alternator.

I've no hands on experience, but I'd imagine the meter will be a charge pump.
(each pulse charges a capacitor, the more pulses per second the greater the voltage across the capacitor.

I wouldnt think anything is serviceable, unless you are an electronics wiz.

just check for bad contacts or maybe the meter needle is physically sticking.
T

tonytech wrote:Diesel tachos do indeed run from the alternator.


T

I stand corrected. Sounds very complicated

They're a mystery to me too. I don't have a standard tach on my dash (just a huge clock) so I attepted to wire up a 52mm Tim electonic rev-counter. Took the pick off from the correct terminal from the alternator and nothing from gauge apart from one single flicker. Anyone know why an after market gauge won't work on a diesel?
1.9Td AAZ

Going back to my original posting, I have now had the time to investigate.

I asked 'is there a serviceable component at the alternator end?' and the answer is yes. he wiring connects through a standard 6mm spade terminal which was dirty & loose - therefore intermittent signal!

Fixed it yesterday and the rev counter was smooth & steady on the run to Stonor & back today.

Thanks for your thoughts.
Garyd

brookie wrote:They're a mystery to me too. I don't have a standard tach on my dash (just a huge clock) so I attepted to wire up a 52mm Tim electonic rev-counter. Took the pick off from the correct terminal from the alternator and nothing from gauge apart from one single flicker. Anyone know why an after market gauge won't work on a diesel?
1.9Td AAZ

I think the TIM rev counters are petrol only. As said above, they work in a different way to diesel ones. You will need a diesel specific rev counter. Just fitted a TIM 52mm one in my van (1.9 petrol) and it says on the instructions 'not for use on diesels'.