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Diesel alternator vs tacho - different question

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Globbits View Drop Down
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    Posted: 14 May 09 at 09:26
Hopefully a simple one this time!

I have a petrol rev-counter in my instrument cluster and, when the van was petrol-engined, it worked fine. Now I've put an AAZ engine in it doesn't work at all. Well, there's the tiniest blip when you first turn the glowplugs on and that's it.

So I hooked up an aftermarket rev-counter to the feed on the back of the instrument cluster and there's a signal, but the gauge read about 9,000 rpm at idle and rushed higher at a fair old rate of knots as I blipped the accelerator. So I have a signal from the alternator, but the pulse rate is about 10*higher than it should be Confused

I don't know the origin of the alternator - although I bought it in a donor T3, it wasn't the original engine in there and they had not made use of the rev-counter output from the alternator. All I can tell you is it's a 70Amp unit

My guess is that it's the wrong alternator for the van, hence why the pulse rate is so far off what I was expecting? But if anyone else can shed any light, I'd be grateful

Many thanks
Richard
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Post Options Post Options   Thanks (0) Thanks(0)   Quote timmythedog Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 09 at 09:45
If you have an after market rev counter it should not be an issue? Maybe you have taken the wiring off the alternator to the wrong wires on the loom? Is your battery charge light working okay? If the aaz has it's own alternator which it sounds like it is, 70amp, and running on a wide pulley? It would probably have always given a false reading on a petrol rev counter, but it would run okay on a diesel rev counter(though slightly inaccurate).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Globbits Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 09 at 09:52
Sorry, I'll explain better. I have a factory rev-counter in the instrument cluster which, when the van was petrol-engined, worked fine. I've now put an AAZ engine in the van. I've checked the connections and they are the right way round - the charge warning light works correctly anyway.

I used the aftermarket rev-counter to try and check through what was going on and, whether I connect the rev-counter to the dash or to the alternator itself, I get the same thing - about 10* the actual engine speed

But it does sound like I have the right alternator - it's a 70Amp, wide pulley job. Does that mean that petrol and diesel rev-counters are different? I know you can recalibrate a diesel rev-counter for an AAZ engine, but does anyone know if it's possible to recalibrate a petrol rev-counter? Cry
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Post Options Post Options   Thanks (0) Thanks(0)   Quote illzlee Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 09 at 13:06
yup, diesel rev counters are different to petrol ones. On the bay of fleas i bought a diesel-spec rev counter and it's tat. (avoid seller JDM-trade).
 
You're right where the signal comes from though, the 'W' signal from the alternator produces a pulse.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Globbits Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 09 at 13:25
Thanks for that illzlee
Just to check, what was wrong with the diesel-spec rev counter you bought? Is it not a diesel one, or is it f**ked? A friend and I are both reasonably knowledgeable on electronics and if we can find a f**ked diseasel one, might be able to:
1) work out the difference between the petrol and diesel one and modify my petrol one to diesely-weasely mode
2) fix yours in the process

Just a thought.... Wink
Richard
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 09 at 14:26
There is always   tinytach which takes the pulses from an injector pipe with a transducer and converts it to a signal.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote timmythedog Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 09 at 14:34
So it sounds like your alternator is putting out the wrong current potentially?  My aaz alternator has a double spade plug off the side. Top spade is red/black, and lower spade is mid blue. This matches the vans original wiring.
 
Out of interest are you sure I can recalibrate my rev counter to match the alternators output? I undestood you could adjust the tickover reading to be true, but that as you went up through the revs it would go out of sync? Is there a device you can fit in line?
Cheers for now, and hope you get it sorted.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote badger Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 09 at 14:43
Probably not much help, but the petrol rev counter probably works in a competely different way - either sensing TDC/hall effect etc. The alternator is obviously rotating faster than the engine due to the belt drive pulley diameter differential (woudn't have thought it was by a factor of 10 though). So feeding the output signal from the alternator direct to the existing rev counter will obviously give incorrect reading.
I've modified rev counters previously but on non VW stuff by inserting resistors into the circuitry on the back of the rev counter (changing from a 4 cylinder to a V8 engine), but no idea what goes on behind a diesel rev counter on a T3.
So, not much help really?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Globbits Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 09 at 15:14
A rev-counter under normal circumstances (certainly the VW ones) works by counting pulses - either from the coil or from the pulsed output from the alternator. These "counted" pulses are used to charge up a capacitor. The more pulses per second, the higher the voltage on the capacitor. The rev counter gauge is then simply a voltmeter, displaying the voltage across the capacitor in terms of "rpm"
In the case of the diesel alternator, you have a pulsed output which is directly related to engine speed, but not the 2-1 ratio you get on a petrol engine (2 sparks per revolution on a 4-cylinder engine). So all you have to do is either:
1) reduce the number of pulses passed to the gauge (awkward, lots of circuitry and mostly pointless)
2) reduce the charge given for each pulse (much easier Big smile)

My (partly) educated guess is that the input resistance on the diesel rev-counter, so that the pulse has less effect on the charge. If this is the case, there is likely to be only one different component between a petrol and diesel gauge and, by replacing this component (resistor) the petrol gauge can be calibrated to work with a diesel

Obviously, since I only have a petrol one, I'm stabbing in the dark though. Anyone got a diesel gauge I can borrow (working or not - doesn't matter)? Wink

Richard
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Baxter Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 09 at 21:48
Diesel one works from W terminal and it will output  about half charge, or about 7v.
Alternators are 3 phase, the W terminal is simly a tap into one of the phases.
Petrol tachos work from the coild wire "1" and it gives a tachometric signal as the coil is switched on and off by the ignition amplifier.
Diesel rev counters "expect" to see a sinewave of around 0-7v, a petrol "expects" a more or less digital signal.
 
See "RegCheesmans" thread on the GTD forums, plenty on there about converting petrol to work with Diesel.
Or search for Dakota Digital as they do a doohickey that interfaces the to to make them work together.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tee3 Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 09 at 22:36
"doohickey"  Thumbs Up

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Post Options Post Options   Thanks (0) Thanks(0)   Quote pearce Quote  Post ReplyReply Direct Link To This Post Posted: 14 May 09 at 22:38
Forget the Dakota Digital unit Simon,as thats what i got to use with my petrol rev counter and getting the signal from the W terminal on a tdi alternator,but no luck so i emailed Dakota and they said "We haven't had much luck with  VW Vanagon's" Great!!,i spoke to Busboy(on the yard) and he said to use a Diesel rev counter as it needs a lower voltage signal.So I'm now looking for a Diesel dash unit.anyone got one for saleWink.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote timmythedog Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 09 at 07:51
The code on my diesel rev counter id 2,39 and it's a VDO unit.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Globbits Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 09 at 10:29
Thanks for the info guys. I haven't had any joy searching for Reg Cheesman, but I did find this link
http://www.4crawler.com/Diesel/CheapTricks/Tachometer/vwdslfaq.txt
which seems to cover adapting "any" VW petrol rev counter to pick up a diesel signal?

Does anyone have any experience of this, or can anyone point me in the right direction for Reg's guides?

Thanks as always!
Richard
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Baxter Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 09 at 21:18
GTD forums are down at the moment, which is a shame.
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