MPG guage not working.Any ideas very welcome!

Hi can anyone help with the problem of my mpg guage doesnt work on my 1983 1,8 golf gti.The yellow light still works fine but the needle on the guage doesnt move although it can move freely as i had the clock and rev counter pod out to have a look and all seems to look ok?Where is it wired to or does it take its info from?
Thanks
Dombash

It's a vacuum guage so there should be a vacuum pipe.

TT

Thanks.Dashbourd to bits again then!I'll check to see if the pipe is sucking.Do you know where it goes to in the engine bay by chance?

It only works in 5th gear and it knows its in fifth via the combined MFA switch/reverse light switch on top of the gear box. You can check the wiring and guage are working by disconnecting the loom at the switch and shorting out the contacts. If the reverse lights come on you've done the wrong pair

No need to take dash out again immediatlely.

The problem (more than likely) will be in the engine bay -

The vacuum for the gauge is derived from the inlet manifold - this vacuum hose connects to the distributor - see below:


The gauge-needle is designed to only work in fifth gear (as detailed in posts above).

Inside the gauge there is a electro-magnetic switch - no power - switch open - with power the magnets pull a contact plate forward to close the switch thus opening a circuit to a second electo-magnet - same as in a voltage regulator.

Power to the switch comes from the distributor.  This circuit is in turn opened /closed  by a switch on the top of the gearbox - detailed below:


You need to trace the (brown?? - thinks its brown) brown wire from the top of the gearbox to a connection block that joins the circuit frm the gearbox to the circuit from the distributor - shown below:



Contrary to popular belief the gauge itself is not driven by vacuum - it is driven by variations in the electrical output coming from the distributor.  These variatoons are transferred to an electro-magnet within the gauge.  As the power output to the magnet increases / decreases, the more / less poweful the magnetic force, causing a metal plate to rise / fall.  This metal plate is linked to a lever that causes a cog to rotate thus causing  the needle to swing to and fro.  Shift out of 5th gear - the electreo-magnetic switch opens, breaking the circuit - the metal plate jumps to its (natural) un-influenced position and the needle "snaps" out of sight.

Quite cool really.  The gauge is nicely made & involves a great deal of  engineering - I have a spare if you need it.

Hope this helps,

MP

Thanks a million for all the detailed help you've provided.I will give it a bash at the weekend and see if i can sort it.Thanks again really appreciate it

Let us all know how you get on.

MP

AHA

Michael I think you just answered my question i just posted on the end of the MFA poll your running. Only just noticed i have no reversing lights, as well as the consumption gauge!

Will be checking the switch on the gearbox anyone know how much these are and if they are main dealer only?

main dealer only and about ?50 8O

Just got one from GSF for ?20, only thing is that when i took the old one out i had 5-6 push pins in the switch, the new one only had three????

So i thought this aint going to work, but it does fine.

Now the problem i had was that the needle and gear change light just stopped working, it was at this point i started to investigate everything and found that my reversing lights were not working.
Although the needle used to come on in every gear and whilst on tick over????
I noticed that there was a broken brown wire on a 2 point multi-plug that joins the brown from the gear box switch to the brown of the vacuum switch on the distributer, joined this up to an earth (as in the wiring dia).
The gauge needle came back on in every gear, but still no reverse lights.

So installed the new switch, and hey presto gauge only works in 5th and got reversing lights again so switch was naff.

BUUUUTTTT, i stil do not have a gear change light, anyone know how this works and where it gets it feed from??

see above

Sorry if i am being thick and have missed it from above, :dontknow:  but i cant see where it mentions the gear change light? I can see clearly from your very good description how the needle on the gauge works only in fifth. But the yellow change LED should come on in every gear???
I'm asuming that it gets its signal from the vacum switch on the distributer as per the gauge needle, then this signal goes through the control unit for shift and consumption indicator (relay part number 171 919 091 C from ETKA)?????

If this is correct and my needle is working i have now two choices for an error,

1. the LED is blown (thought they lasted for everish???)
2. the control unit/relay is knackerd

Whats your thoughts??