Tachometer

[mcclelland]

Hi all,
the tacho on my S4 Elan is currently reading high by 200 r/m. Can anyone tell me can this be adjusted by me and if so, how. Or is it a send away job.
Many thanks in advance, regards George...

[UAB807F]

Yes, I think so although it's not as easy as you might think. It's not "give up and take it to a dealer" difficult, but it does need time & patience.

this thread a while back covered most of the pointers

elan-f15/tachometer-reading-too-high-europa-t21763.html

Brian

[ricarbo]

I have rebuilt several of these electrically/electronically, so have some experience and would say this:-
You can adjust the potentiometer to adjust the power the circuit applies to the meter. the effect of this will be that if you reduce the reading by 200 at a speed of 2000 you will reduce it by 400 at 4000rpm and 600 at 6000rpm. You are altering the 'gain' of the electronics.
You can alter the tension of the spring that pulls the movement back to zero. This is like the 'zero adjust' on an analogue multimeter. This can make it read 200 rpm lower at all speeds. There is a tension adjuster on the movement to facilitate this.
The movement is balanced by little coils of wire wrapped onto small legs. If this is wrong (and it was on every example i've worked on) the meter will read high or low depending on how the rev counter is orientated, i.e. different readings shown when installed in the dashboard with 6000 at the top contrasted with 3000 at the top. Think of it as though the tip of the needle was too heavy - it would read too low when the needle is pointing to the left, correct when it is straight up and too high when it is pointing right. You can slide the little coils along the legs to correct this.
I start by correcting the balance, then adjusting the circuit gain and lastly the zero adjust.
Richard

[69S4]

Hi all,
the tacho on my S4 Elan is currently reading high by 200 r/m. Can anyone tell me can this be adjusted by me and if so, how. Or is it a send away job.

No answers, but some sympathy. My tacho is about 200 rpm fast at tickover but based on rpm vs road speed seems to be reading correctly at 3000 and 4000 rpm. I spent ages (= decades!) trying to figure out why it idled at 1100rpm but sounded about right - checking for air leaks, ignition advance etc - until I finally was able to compare it with another external tacho. Atm I just live with it. It's easier to recalibrate my brain for the time being while I get on with some of the endless list of other jobs on the car.