Converting a 5-cyl tacho to 4-cyl (or the other way around)

[silverbullet]

Long story cut short: I have an Audi 80 (typ 81/85) dash pod that I was going to put in my SA bus (5 cyl 2.6), but may now fit to the syncro (wbx) because the speedo is 1600 turns/mile and that's a match.
Can I simply swap the D/A chip to change the tacho in/out values or am I better off tracking down a 4-cyl Audi 80 tacho?
Ta in advance, as ever

[syncroandy]

Hi Ian, TBH I've not spent much time looking at the functioning of the '80's style tachos, but I think they're more likely to have a frequency-to-voltage chip than a digital-to-analogue. A tacho of this type is really a fully 'analogue' circuit, not digital. The input waveform will typically be quite messy and have a circuit to filter and clean it up. Have you tried to adjust the 'sensitivity' using the small pot ? (I'm assuming its a typical VDO unit of the period) It's possible the 20% change you need is within the range of adjustment. If not, I'd suggest working out the circuit diagram, identify the component values in the region of the adjustment pot, and make an appropriate change to bring it's function to what you need.

eg. found this scematic on the web, it's not VDO, but the VDO one would I imagine be similar in principle:

[silverbullet]

Thanks Andy. Yes its a VDO instrument pack, contemporary with the T3, mid 80-s complete with two oil pressure switch feeds etc. I will try and identify the calibration pot first, unless I can find a 4-cyl Audi 80 tacho on German ebay!

[silverbullet]

This dash unit https://club8090.co.uk/forum/viewtopic.p ... 5#p7718965" onclick="window.open(this.href);return false;

[syncroandy]

The part of interest is the round PCB inside the back of the tacho..