Tachometer adjustment

[Dobble]

Hi guys

Following on from this thread https://forum.club8090.co.uk/viewtopic. ... 5&start=30" onclick="window.open(this.href);return false; where it was established that my tachometer is reading incorrectly I have just bought one of these: https://www.ebay.co.uk/itm/LCD-Digital- ... SwW7heGCMc" onclick="window.open(this.href);return false;

Does anyone know how I adjust the tachometer as I can't seem to find any information on the subject.

Thanks
Ben

[Aidan]

Ben
I don't know how much adjustment is possible, the whole unit was vehicle and engine specific, which meant that the pulley sizes were already allowed for so the adjustment may only be by small amount to calibrate to factory signal, but on the back of the speedo unit in the dash there is a small hole to access a trim pot which can be adjusted with a small flat (I think) screwdriver, the hole is usually covered by a white paper label, again indicating that it was factory calibrated before being sent out by VDO to VW for installation ibn the vehicle
You may have to lift the dash pod out to get access to do the adjustment

[Dobble]

Thanks Aidan,
I checked the dash against my new digital Tachometer this evening and throughout the rev range the dash seems to read about 25% higher than it should. This seams about right as I entered all the info into a gear ratio calculator to give speed at a given RPM. I have a new clutched alternator pulley that has a larger diameter than the current solid pulley so I will be putting that on to see what difference that makes first then adjust the pot after for fine tuning.

I'll let you know how I get on.

Thanks
Ben

[Dobble]

Just to follow up on this. I changed my alternator pulley to a larger clutched one which made a 5% correction to the dash reading. I then found the potentiometer under the white sticker Aidan mentioned at the back of the tachometer. I just felt for the hole and poked the sticker through. I used a 0.4x2mm flat screwdriver which was perfect to adjust the pot ( this one https://uk.rs-online.com/web/p/screwdrivers/7947707/" onclick="window.open(this.href);return false; ). I found there is loads of adjustment. I started by adjusting the pot at idle to read the same as the digital gauge then compared readings at 2000 rpm and 3000 rpm which were both in check after the initial adjustment at idle.

Thanks to Aidan for letting me know about the potentiometer.

Cheers
Ben

[Aidan]

Tidy, something I know about in theory but have never actually done as all my vehicles have had stock engine and alternator set up, but I have a customer vehicle here that may need calibration so good to know there is some room for correction.