Tacho options for a petrol dash?

An alchemy of sparks, copper wire and earth

Moderators: User administrators, Moderators

Tacho options for a petrol dash?

Postby CJH » 26 Jun 2018, 12:14

I've been looking for an OEM petrol rev counter to replace the time clock in my early dash. I know the loom/PCB is different, but now that I've replaced the foil PCB it's straightforward to wire up a tacho. I've previously run a wire from the coil in preparation for this.

Petrol rev counters seem like the proverbial hen's teeth and are correspondingly expensive. But diesel rev counters seem to be available new, and while not cheap that does at least keep a lid on the secondhand prices. So I began investigating what the differences are, and whether it would be possible to run a diesel tacho in a petrol dash. I think there are two possibilities:

1) Does my alternator have a 'W' terminal? I have no idea at the moment without checking - anyone know if that's likely? If so, then I could run a diesel tacho directly, although I may have to tune it to suit the rotation rate of my alternator.

2) Can the circuit board on a diesel tacho be adapted to suit a coil pulse? I found various links about going the other way - running a petrol tacho on a diesel engine. In fact this really useful link shows the two circuit board schematics:

Diesel:
Image

Petrol:
Image

And photos of both sides of the petrol version PCB:
Image

The only missing piece of information for the petrol version seems to be R5, with a value of 'dunno' - and the photo isn't clear enough to read the stripes. But other than that it seems feasible to convert the diesel board to petrol, if the physical board layout has places for all the missing components. If it doesn't, well there's enough information there to make a whole new circuit board to make the diesel tacho hardware work with a coil pulse.

Am I wasting my time - is there an easier conversion, or perhaps a ready source of petrol tachos that I've missed?
"I'm a man of means, by no means....King of the Road!"

1983 Viking Xplorer, 2.1DJ
User avatar
CJH
 
Posts: 2960
Joined: 15 Jul 2013, 06:51
Location: Nottingham
80-90 Mem No: 12576

Re: Tacho options for a petrol dash?

Postby CovKid » 26 Jun 2018, 12:23

If I remember rightly, there was something on the SAMBA on converting one to the other. No you won't have a W terminal.
Roller paint your camper at home: https://tinyurl.com/y7723h29 for MP4 download.
User avatar
CovKid
Trader
 
Posts: 8364
Joined: 30 Apr 2006, 13:19
Location: Ralph - Nuneaton
80-90 Mem No: 3529

Re: Tacho options for a petrol dash?

Postby bigbadbob76 » 26 Jun 2018, 15:41

Watching with interest.
'86 1.9 DG, 4 speed, camper.
'Q 2cv6 trike with diy EFI.
'03 Furred ranger.
'95 Suzuki bandit 750.
User avatar
bigbadbob76
 
Posts: 1538
Joined: 07 Nov 2016, 14:41
Location: Isle of Skye
80-90 Mem No: 15707

Re: Tacho options for a petrol dash?

Postby CJH » 26 Jun 2018, 16:00

First attempt at translating those images into a PCB design. No idea of the correct dimensions, or how much scope there is to move components around, and I haven't bothered to match component sizes and specs properly yet - it's just to make a start, and perhaps stimulate some discussion. I'm not even sure that those schematics and photos correspond to the OEM tacho in the T25. I think I may have to invest in a diesel tacho if I'm going to take this any further.

Image
"I'm a man of means, by no means....King of the Road!"

1983 Viking Xplorer, 2.1DJ
User avatar
CJH
 
Posts: 2960
Joined: 15 Jul 2013, 06:51
Location: Nottingham
80-90 Mem No: 12576

Re: Tacho options for a petrol dash?

Postby itchyfeet » 26 Jun 2018, 18:24

Wow you really like a challenge :D

52mm pod mounted and go camping with the time you saved :lol:
1988 DG WBX LPG Tin Top
1989 DJ digijet WBX Holdsworth Villa 3 Pop Top
itchylinks
User avatar
itchyfeet
 
Posts: 10250
Joined: 23 Jul 2007, 17:24
Location: South Hampshire
80-90 Mem No: 12733

Re: Tacho options for a petrol dash?

Postby CJH » 26 Jun 2018, 18:39

itchyfeet wrote:Wow you really like a challenge :D

You've rumbled me :D

itchyfeet wrote:52mm pod mounted...

Yeah, there is that option, but I already have the ashtray-mounted oil pressure and temperature gauges, and besides, there's no challenge in that.

itchyfeet wrote:... and go camping with the time you saved :lol:

I can't argue with that.
"I'm a man of means, by no means....King of the Road!"

1983 Viking Xplorer, 2.1DJ
User avatar
CJH
 
Posts: 2960
Joined: 15 Jul 2013, 06:51
Location: Nottingham
80-90 Mem No: 12576

Re: Tacho options for a petrol dash?

Postby itchyfeet » 26 Jun 2018, 18:47

CJH wrote:
itchyfeet wrote:52mm pod mounted...

Yeah, there is that option, but I already have the ashtray-mounted oil pressure and temperature gauges, and besides, there's no challenge in that.



cut another ashtray sized hole and then you can fit an airfuel gauge too?
1988 DG WBX LPG Tin Top
1989 DJ digijet WBX Holdsworth Villa 3 Pop Top
itchylinks
User avatar
itchyfeet
 
Posts: 10250
Joined: 23 Jul 2007, 17:24
Location: South Hampshire
80-90 Mem No: 12733

Re: Tacho options for a petrol dash?

Postby CJH » 26 Jun 2018, 19:02

You had about 5 minutes, between me finding out that the hardware might be common and the idea taking hold. You missed it - it's out of my control now. I've ordered a diesel tacho - if the idea turns out to be a dead end I'll just sell it on.
"I'm a man of means, by no means....King of the Road!"

1983 Viking Xplorer, 2.1DJ
User avatar
CJH
 
Posts: 2960
Joined: 15 Jul 2013, 06:51
Location: Nottingham
80-90 Mem No: 12576

Re: Tacho options for a petrol dash?

Postby syncroandy » 26 Jun 2018, 19:46

My thoughts on this FWIW, though clearly I am not that bothered as I have a far better option available than bodging tired old 1970's/80's components :)

Golfs of the same era have similar tacho units that can be fitted into the T3 binnacle, these may be a useful source of the basic tach unit. There are variants with higher max rev limits (eg. 7k).

Petrol tacho's use an input frequency of 2 pulses per rev, whether coil-driven, or ECU-driven. Subaru uses the same convention here as VW, for those with 'impure' wasserboxers fitted. Coil driven tacho units have an input filter circuit to cope with the ~150v spikes coming from terminal 1 (coil neg). The VDO tach can be modified by changing some resistors to work with a ~12v signal which ECU's typically produce.

Diesel tacho's designed to work form the W terminal use a much higher input frequency of the order of 10x the petrol tacho frequency.
Syncrosport
Volkswagen Transporter, reloaded.

252 GC5 EJ25 AAN L90D
246 097 AFN AVL+ L90D
syncroandy
Trader
 
Posts: 1592
Joined: 18 Oct 2005, 18:15
Location: North Lancs. UK
80-90 Mem No: 851

Re: Tacho options for a petrol dash?

Postby CJH » 26 Jun 2018, 20:04

Thank you - your thoughts are worth a lot, and there's no question that yours is the superior solution. I saw your setup at Busfest last year, and was highly impressed, and given the prices some are are asking for 'tired old 1970s/80s components' I'd say your solution is cracking value.

I looked at some contemporary VW tachos, and figured that some of them might be interchangeable, but I didn't find a unit that has the fuel and temperature gauges, and I need to retain those obviously. Are you suggesting that the basic tacho unit from a golf would install behind the faceplate of a T25 tacho/gauges combo?

I don't have the electronics knowledge to adapt the circuits, but I can implement someone else's schematic. I've read that the different pulse rates result in different voltages out of the capacitors, and the modifications are aimed at scaling that output voltage, then trimming it, to produce the correct needle deflection. I'm hoping I can tell what I need from the diesel tacho that I've ordered 'non-destructively', and if the circuit looks similar to those schematics I'll have a go at converting it to coil pulses.
"I'm a man of means, by no means....King of the Road!"

1983 Viking Xplorer, 2.1DJ
User avatar
CJH
 
Posts: 2960
Joined: 15 Jul 2013, 06:51
Location: Nottingham
80-90 Mem No: 12576

Re: Tacho options for a petrol dash?

Postby itchyfeet » 26 Jun 2018, 20:08

any reason why you can't fit a diesel alternator?
1988 DG WBX LPG Tin Top
1989 DJ digijet WBX Holdsworth Villa 3 Pop Top
itchylinks
User avatar
itchyfeet
 
Posts: 10250
Joined: 23 Jul 2007, 17:24
Location: South Hampshire
80-90 Mem No: 12733

Re: Tacho options for a petrol dash?

Postby CJH » 26 Jun 2018, 20:11

itchyfeet wrote:any reason why you can't fit a diesel alternator?


Good question - I haven't looked into why there's even a difference.
"I'm a man of means, by no means....King of the Road!"

1983 Viking Xplorer, 2.1DJ
User avatar
CJH
 
Posts: 2960
Joined: 15 Jul 2013, 06:51
Location: Nottingham
80-90 Mem No: 12576

Re: Tacho options for a petrol dash?

Postby CovKid » 26 Jun 2018, 20:22

Can't see that there would be - other than the W connection.
Roller paint your camper at home: https://tinyurl.com/y7723h29 for MP4 download.
User avatar
CovKid
Trader
 
Posts: 8364
Joined: 30 Apr 2006, 13:19
Location: Ralph - Nuneaton
80-90 Mem No: 3529

Re: Tacho options for a petrol dash?

Postby AngeloEvs » 26 Jun 2018, 20:29

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

I fitted a full size petrol Tacho in the analogue clock housing but the fuel and Temp gauge had to be reloctated. I designed the dash to suit a smaller steering wheel.

Alternatively, I fitted a 52 mm Tacho in my spare Dash in the space under the LED display but I think your PCB board sits over that area. I

Imagedashwp3 by Angelo Evans, on Flickr
1987 DG Karisma LPG with remodelled interior
User avatar
AngeloEvs
 
Posts: 873
Joined: 22 Nov 2007, 19:22
Location: Upwell, Norfolk
80-90 Mem No: 4709

Re: Tacho options for a petrol dash?

Postby 937carrera » 26 Jun 2018, 20:37

Looks like you can add a W terminal to an alternator

http://www.ybw.com/forums/showthread.php?294596-Adding-a-W-terminal-to-alternator

and it looks like the output is a voltage with variable frequency from one of the phases of the alternator, the frequency being used to drive the tacho

http://www.ybw.com/forums/showthread.php?116230-Alternator-W-terminal

Better putting a W terminal on a petrol alternator as they have /need higher output ?
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine
User avatar
937carrera
 
Posts: 1701
Joined: 05 Apr 2015, 19:29
Location: N Yorks.
80-90 Mem No: 16333

Next

Return to Electrical & Instrumentation

Who is online

Users browsing this forum: No registered users and 10 guests