Polo 6n2 dash tach shows half rpm

[SUB-LIME]

Hi all,

I have had an issue that I've wanted to sort for a while and I haven't found an answer.

I did a 20vT swap done on my 6n2 and had a PP xms5B fitted by Panic which has been a pleasure except the cars stock dash tach shows 50% of the cars actual rpm; So it shows an idle of 450/500rpm and works that was through the rev range.

The tacho works perfectly on the MP9 system but not on the PP XMS5B.

Can anyone shed some info on why this would be?

[PapaJo]

The 20vt has a single open window in disk giving you the rpm's from the cam. Polo 6n2 works with a 4 window, which gives you 2 pulses per full 360 revolution on the motor. That is why you read only half the rpm's.

What is the rpm's showing on PP software? Correct 900 rpm? Are you taking the timing trigger from the cam or 60-2 on crank? If you use the 60-2 from crank for trigger on management system and use the cam trigger for rpm indication only, you can replace the cam disk with a 4 window disk ( some work needed as the shaft sizes are different) and rpm will read correct.

[SUB-LIME]

Reading 60-2 only. And yes it shows correct rpm on PP. Thanks for the advice. Will ask Panic to look at your suggestion and implement.

[PapaJo]

Reading 60-2 only. And yes it shows correct rpm on PP. Thanks for the advice. Will ask Panic to look at your suggestion and implement.

Just to double check on the current setup and wiring:
Is your tacho driven from the PP or is it connected to the pulse unit on the head?

[THANAS]

You should be able to change settings on the PP to put out 2 pulses per rpm instead of 1. The CPS on the 20vt is single window and puts out one pulse per 2 rpm or 720 degrees.

[SUB-LIME]

@THANAS , changing the PP to put out 2 pulses, would that affect the cars tuning or performance?

[THANAS]

Not at all.

[PapaJo]

Is this sorted now?

[SUB-LIME]

Not yet, had to work on a few other things on the car first. Will get to it today

[PapaJo]

Is this sorted now?

[panic-mechanic]

you can't change the number of pulses PP puts out on the rev counter.
I could not find the direct wire for the rev counter and that polo picks the rpm from the hall generator output.
There is no hall as it runs from 60-2 so I just connected the usual hall in to the rpm output as that at least starts the pumps and shuts up the cluster.
Yes one can use the cam output but it has to be wired and a 4 trigger disc made or modified or whatever it takes.
Problem has been Time so far as it is their only transport so have not really prioritized something as a mundane as a rev counter reading half while it was here to get it running properly.
If he has the opportunity to leave it for a bit like was supposed to happen in december I can look at it.