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Discussion Starter #1
Hey,

this may be a long shot but i wonder if any of you can help me out?

I am restoring/modifying a 1303 beetle into a track car and the interior is totally stripped. I need a rev counter for obvious reasons and the 156 rev counter is perfect for my needs - easy to mount, clear, contains nearly all warning lights etc..

Anyway, i have bought one and found the schematic so i know what pins do what, but my problem is that i don't know what type of signal it is expecting for revs.

On the alfa software i found, all it says is that the ecu sends a signal, based on RPM to the tacho.

Does anyone know what this signal should be? or does anyone have an 'scope and is at a loose end? a long shot, i know.

So far i have found that a sine/square/sawtooth with 10v p2p and ~2kHz will get the needle sat at 2krevs but changing the frequency/amplitude does nothing (although the waveform gen i have access to only goes up to 10v so i can't try any higher)

Any help on this would be much appreciated!

Regards,
Mark B.
 

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Discussion Starter #2
wow, fast moving forum!!

any ideas anyone? can anyone suggest any way of finding this information out?

Cheers,
Mark.
 

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I've had a look through eLearn (haynes manual for alfa's) and come across this ... although it sounds like you already know this ... I'll keep having a look see what I come across

Connector A, 20-way
1, Right rear side light
2, Left front side light output
3, Side light signal
4, Left rear fog lamp signal
5, Brake light signal
6, Side light for trailer
7, Rear fog lamp for trailer
8, Dipped beam enablement
9, Left rear side light
10, Direct power supply for check (left branch)
11, Left number plate light
12, Right number plate light
13, Right brake light
14, Earth
15, Supply controlled by the ignition
16, Direct supply for check (right branch)
17, Right rear fog lamp
18, Left brake light
19, Brake light for trailer
20, Right front side light

Connector B, 16-way

1, Rear fog lamps warning light
2, Trailer direction indicators warning light
3, Rev counter module lighting
4, Main beam headlamps warning light
5, Low oil pressure warning light
6, Engine rpm signal input
7, Recharging warning light
8, Engine rpm signal output
9, Left direction indicators warning light
10, Rev counter module warning light test
11, N.C.
12, Automatic transmission display (N.C.)
13, N.C.
14, Fog lamp warning light
15, Fuel injection/EOBD fault warning light
16, Right turn signal warning light
 

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and found this statement


The engine rpm signal, which is different for each engine type, comes from the engine management control unit M10 to pin 6 of connector B.

Although that is probably because the rev limit for different 156's are different so 4000 rpm will be in a different position between a V6 and a JTD ...
 

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Just had a look at this thread ... http://www.alfaowner.com/Forum/alfa-33-sud-and-sprint/114627-sud-rev-counter-problem.html ... and ok it's for an old Alfa, but Alfa don't tend to change things unless they are forced to by the eurocrats ...

... it seems to indicate that the rev needle position is based upon the "average" voltage of the pulse train so it all depends on the duty cycle (i.e. ratio to on/off), from the sounds of it you just increase the frequency which means that the duty cycle will be the same, if you increase the "on time" and reduce the "off time" then what effect do you get?
 

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Discussion Starter #6
thanks for all the replies! much appreciated.

Yep, i have tried changing the duty cycle also - this has no effect until the duty cycle is too low, then the rev counter drops back to zero.
 

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My only thought then is that it must be a more complex pulse train, but then how complex could it be?? If you're anywhere near me (camberley) then you could pop round one night and we could pull out the rev counter and attach and oscilloscope ...
 

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If you're not local to me then one of my mates has got an ocillascope, and I might be tempted to have a go working it out ... not just to help you, but also because I want to fit a rev limit light (so I don't have to spend my time looking at the rev counter when accelerating due to the silentness of the diesel!!!) ... I won't have a chance this week as I have to fit a new set of upper arms and drop links (with powerflex bushes :thumbs:), and also sort out the dodge connection to my BOSE amp ... but maybe next week ...


... just make sure you've got an email notification set up on this thread, or PM me your email address so I can keep you informed ...



(obviously the best thing to do would be to rip out that rubbish german engine and put in a decent alfa one ;))
 

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Discussion Starter #9
cool, i shall pm you my email address shortly.

you laugh, but it has actually got an alfa engine in at the mo!! a 1.5 boxer lump from a sprint!

sadly this is only my test engine to check the brakes/suspension/water cooling setup, then it's on to the engine i'm building now - a twin charged (super and turbo) scooby engine :)

i'm not in work today so won't get a chance to have another play, but i'll give it another go tomorrow and keep you informed if i find anything else also.

Mark.
 

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Right ... here we go ... took to rev counter out and attached the scope to pin 6, there didn't seem to be a good earth lying around but a floating one seemed to work(ish) ...

This picture is at idle



This one is at about 2000 rpm (don't know exactly because to attach the scope you have to disconnect the rev counter)



This one is at about 3000 rpm



Didn't go any higher ... as quiet as a diesel is I didin't really want to p my neighbours off too much as it is 9 O'clock at night ... All pictures were taken with the same scale/setting on the scope which were ...



and just for good measure here's a pic of my rev counter ...




and I know owe my mate a few beers for the borrow of his scope ... just as well I drunk a couple of his when I went over the collect it :thumbs:
 

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Discussion Starter #11
Hey, thanks loads for doing that. I actually worked it out yesterday morning at work!

For the petrol rev counter it works out to be 33Hz per 1k revs. when i was testing with square waves i was at much higher frequencies. I had tried at these frequencies, but with much narrower pulses so discounted it, DOH!

trying to make life complicated for myself!

Anyway, after looking at your scope traces it looks like (but don't quote me on this as i can hardly see the divisions on the scope) idle is 110Hz, ~2k = 250Hz and ~3k = 400Hz. I'm not sure why the diesel's signal would be higher frequency though...

Just as a note, the rev counter accepts quite a wide range of p2p voltages, pulse widths and wave types and still reads the correct value.
 

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:thumbs: glad you've got it working, the pictures were taken outside at night so I couldn't get particularly good pictures ... I think the reason why diesels is so much higher is because the only difference between a petrol and diesel rev counter is the scale, so 5000rpm on a diesel is in the same position as 7000rpm on a petrol (therefore the frequency that the ECU has to generate must be the same for both of these rpms for the different models)
 

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Discussion Starter #13
yeah, but 8k on the petrol is only ~260Hz, whereas the diesel hits this at ~2k, hmm never mind, on to designing your shift light now then?
 
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