Club 80-90 Forums

AngeloEvs wrote:If you don't have the DOPs board installed then you could use a Diesel Alternator (Petrol and Diesel DOPs pcb are different).

This is a god point but you still have the coil tacho pulse for the DOP, it's just that you probably need to adjust the PCB to seperate the two signals and make a sepaerate connection from the W connection to where the diesel tacho needs it.

AngeloEvs wrote: dashwp3 by Angelo Evans, on Flickr

Those are both very inventive solutions. I like how you moved the LEDs to the part of the dash that looks like it was intended for them. You're right - my dash PCB sits behind the area where you mounted the 52mm tacho, so there's probably not enough depth.

If you search on EBAY you will find a lot of petrol rev counters, all sizes, with prices starting as little as £10 and averaging around £20 - £30 depending on wether they are new or second hand. Price wise, petrol tachos are cheap.

I used a Smiths Spitfire Tacho, cost me £10-00 and been in service for the last 10 years in my van.

937carrera wrote:Looks like you can add a W terminal to an alternator.

There is an unused terminal on my DG's 45A alternator, In always thought it was a W but maybe not.
It's marked +

bigbadbob76 wrote: There is an unused terminal on my DG's 45A alternator, In always thought it was a W but maybe not.
It's marked +

I believe that terminal is for a suppressor. Aside: I once killed my alternator by accidentally shorting that terminal to earth - needed a new diode pack after that. I now leave an insulated blade terminal on it permanently.

It seems like a tachometer is basically a volt meter, and both petrol and diesel tachos seem to produce the same needle deflection for a given voltage, i.e. they have common hardware. The difference between diesel and petrol tachos is the circuitry that converts the input pulses (which differ in voltage and frequency) to 'needle' voltage. So one way to convert a diesel tacho to petrol might be to buy one of those £10 petrol tachos off ebay, and extract the circuit board to connect up to a VW diesel tacho, bypassing the diesel circuitry. But there's no guarantee that the output voltage from one of those £10 petrol tachos is compatible with the VW tacho hardware. I'm going to persevere with making a petrol circuit board unless there's an obvious 'gotcha'.

http://forum.club8090.co.uk/viewtopic.php?f=39&t=163521

I can confirm that the £10 Chinese tacho hardware is compatible with the VW petrol hardware

I'm not sure that they are voltage driven I understood they are pulse / frequency driven, but others know much more than me.

Yep - pulses in, circuit converts pulses to voltage, voltage makes the needle swing (just like a voltmeter). The 'hardware' that I was referring to is the 'voltmeter' part of an original VW VDO tacho, so what I meant to say was that there's no guarantee that the voltage *output* from the circuit board in a £10 tacho is scaled to give the right needle deflection on a VW VDO tacho. Probably trimmable though, so may well be a quick way to make a VW VDO diesel tacho work with coil pulses in a petrol van, but where's the challenge in that?!

My diesel tacho arrived today, so I took some photos of the circuit board and traced the circuit.





Despite the board being a different shape, the circuit is almost identical to the diesel version I showed in the first post. The IC has a TI part number on it (F423XRX), and I can't find any trace of that online, but the pinouts appear to be the same as the sak215 in the fist post above, and those are readily available (e.g. here).

The board has various empty component locations, and it seemed likely that it's a common board which gets populated with different components depending on whether it's for petrol or diesel. I haven't yet traced the circuit for the alternative, possibly petrol, configuration. One problem would be knowing what component values to use, as the circuit evidently isn't exactly the same as the petrol schematic in my first post. There's a diode at Z12 for instance, which isn't in the earlier petrol schematic.

So I turned to Google, and I found a photo of the circuit board from a four cylinder petrol Golf, and sure enough it seems to correspond to the alternative component set on my board. So it should help me to identify all the components.



So then I'd have a choice - I could either use the existing diesel circuit board and reconfigure it to match the Golf version, or I could try to make a replacement PCB. And of course, having discovered how to have PCBs made cheaply, everything seems like a problem best solved with a new PCB at the moment . And I could leave the diesel version unmolested in case I give up and want to sell the complete diesel tacho on at a later date.

So I made a start by measuring and reproducing the basic PCB shape.


That pile of components next to it needs to be arranged in a similar fashion to the original PCB.

One good thing to come of this - I've been able to test the Dupont sockets on the tacho pins - they fit nice and tight so should be fine as a connection method for my dashboard PCB kit.

CJH wrote:The IC has a TI part number on it (F423XRX), and I can't find any trace of that online

Ok, searching on the wrong digits. What I thought was a serial number is evidently the part number: SN29736P1

Even better - a fully documented '88 Vanagon petrol circuit board in this Samba post. Home straight now

First attempt at laying out and routing the components for a petrol board. Still need to check specs/dimensions etc for the components, and check the trace widths - there are some very fat traces on the original board and I need to figure out if they need to be that fat. I'm toying with the idea of making the board a dual petrol/diesel configuration like the original. If not, then I think the petrol layout could be improved a bit, certainly compared to this first rough layout.

Good work CJH, watching with interest.
can you tell me if the fuel and temp gauges on the tacho for the diesel are the same as for the petrol?

Yep, they will be

Cheers.

Ah, I see now. I hadn' t realised you were gettng a VW Diesel Tacho with the integrated fuel and temp gauges. ,Who supplies those then?