The notorious MFA and 99.9 mpg

OK I am seeking dashboard nirvana - what a battle!  I had an intermittent problem of everything failing except the speedo.  Traced this back to a broken wire in the multi -point plug, ran a bi-pass - sorted.

Then lost the gear change indicatior - traced this back to a disconnected vaccuum hose coming off the inlet manifold - sorted

Then had outside temp reading -40C .  Thanks to a post on here I became aware this was the default reading - new sensor and mount - correctly installed on the bumper bar (I'll post photos of this another day) - sorted.

Flashing red light on the temp gauge - flushed system and replaced with 50/50 mix de-ionised water and G12, cleaned up the sensor connections and ensured these were and remained, dry  - sorted.

So now everything works except the mpg indicator.

It appears there is only one vaccuum hose running through the firewall and up to the dash.  The econo-meter (active only in 5th gear) works fine so we have vaccuum.  I can disconnect this hose at the engine end, suck on it and maintain a vaccuum by placing my tongue over the hose .

What I don't understand is the vaccuum connection to the MFA.  Is there one or does the MFA run its only MPG calculation using time spent driving, average speed and distance?  If there is a vaccuum connection to the MFA does this run off the same vaccuum hose that runs to the econo-meter?  If so, how?  If it runs of a T-intersection  and the connection to the MFA has been dislodged, how come I can mainatin a vaccuum in the hose?

Any help/assistance/photos/pointers/advice greatly appreciated.

Many thanks.


M

when I forgat to connect my vacum hose the mpg rading was very low around 18mpg not high like 99.9. I can't help further as my car is Cabriolet with not econometer like the earler GTI's

Not sure I can help - but a question - when you say your 5th gear econo gauge was not working - what did you mean - my orange light stays on in all gears (over 2000 RPM) and the econo gauge does not kick in  when 5th is selected…

Though the other day I steam cleaned the engine and it worked for a short time……anyone out there got any ideas

The gear change indicator light works off a vaccuum hose from the back of the inlet manifold and connects to a sensor at the front of the engine bay.  This sensor sends an electric signal to the gear change light.  Mine works on 1st through 4th gears and activates at about 2,000 rpm which (from reading the forum on here) is about correct.  There is an adjustment screw so you can set the rev range the indicator light displays but I do not know where this is.

The eco-needle is only active in 5th gear.  It is driven from an electrical signal taken from the top of the gearbox.  When you shift into 5th, the sensor at the top of the gearbox de-activates the signal to the gear change light and activates the eco-meter which in turn is driven by a vaccuum tube.

If your eco-needle became active after a steam clean this would indicate a poor connection at the gear box sensor - bit fiddly to get at unfortunately.

This is my understanding of the system ….

M

Let's see if this helps with the puzzle.  I can reset the MFA / MPG indicator to 0.0 but as soon as the can has covered 0.1 miles the indicator reverts to 99.9.

I am beginning to think this is not a vacuum tube issue but an electrical fault.

Has anyone ever cracked this?

M

I'm close to cracking it.

The LCD display and the econometer needle use slightly different sensors - here goes;

• The upshift light runs off vacuum ONLY - it is triggered by an electrical sensor in the vacuum line to the distributor. This sensor should look like a three-way connector with a vacuum hose connected on opposite sides - and an electrical connector off one side. Mine is right next to the dizzy. There is a grub-screw that sits in one of the faces of this connector - I reckon this is the one that can be adjusted to change when the shoft light activates.

• The econometer needle ALSO runs of vacuum only - taken from one of the outlets on the front of the inlet manifold.

Here's where it gets interesting.

• The digital MPG reading on the MFA uses (I think) the vacuum reading (not 100% on that) coupled with the speedometer reading - which is why it displays something sensible until you start moving. To do this it uses a sensor on the side of the speedo, coloured yellow/cream. You can only get at this sensor if you take your clocks out and disassemble them. I've found the following website which has pictures of the disassembled clocks, how-tos and lots of other helpful stuff;

4130 Products - Step-By-Step

My MFA still shows 99.99 MPG, and I'm planning to get at the sensor in the near future.

Hope this helps

Ali Cabrio

- The upshift light runs off vacuum ONLY

Yep - I agree - via the vacuum hose that runs from the firewall side of the inlet manifold to the distributor

- it is triggered by an electrical sensor in the vacuum line to the distributor.


Hmmm - not "in the vacuum line" but at its end.  The vacuum hose connects to a domed shaped disc which is part of the distributor assembly.  This has a number of electrical cooenctions coming off it

This sensor should look like a three-way connector with a vacuum hose connected on opposite sides - and an electrical connector off one side. Mine is right next to the dizzy. There is a grub-screw that sits in one of the faces of this connector - I reckon this is the one that can be adjusted to change when the shoft light activates.

That sounds correct.  I am not sure I know what a "dizzy" is - i guess it is the distributor cap - or that's what we call it in Australia (where i am from)

- The econometer needle ALSO runs of vacuum only - taken from one of the outlets on the front of the inlet manifold.

Yep - with the eco-gauge being connected to the electrical 5th gear sensor on the top of the gearbox

Here's where it gets interesting.

- The digital MPG reading on the MFA uses (I think) the vacuum reading

I am not convinced it does.  I think the MFA runs off electrical impulses only. It could take a sweep reading from a rheostat as the eco-meter needle fluctuates but this is a huge amount of engineering to produce MPG figures.  I think the MFA runs a calculation using time, speed, distance, average speed and average RPM - now I have a problem

(not 100% on that) coupled with the speedometer reading

Yep - I agree - it takes a reading from the speedo and  also the tacho - you could do 5 miles at 5,000 rpm in 2nd gear and 5 miles at 1,500 rpm in 5th gear - and you will get very different mpg's


which is why it displays something sensible until you start moving.  To do this it uses a sensor on the side of the speedo, coloured yellow/cream.

Yeah -but the MFA/MPG will click to 99.9 mpg even when the vehicle is moving in reverse….

Now to the problem.  The MFA / MPG indicator will set correctly with the engine running - which means the tacho is generating an impulse to the MFA:  and the MFA correctly calculates average speed and distance covered - which means it it either taking an impulse from the speedo or the gearbox - or both.  If it is from the speedo, why does the MFA / odometer work correctly and average speed (which is ditstance/time - but it has to get distance) also work but not MPG? If this info is taken from the gearbox should not the MFA only give you accurate distance, and thus accurate av speed when 5th gear in engaged

You can only get at this sensor if you take your clocks out and disassemble them. I've found the following website which has pictures of the disassembled clocks, how-tos and lots of other helpful stuff

I'll have a look



Really useful post AliC - loads useful

Hmmmm - sounds like your MFA is going something slightly different from mine.

My MPG figures show zero at standstill - and then go to 99.99 on moving (like yours)*.

BUT the readings I get for miles/avg speed are out - by a little - which makes me think that the sensors on the speedo are reading something - but not enough to give a sensible MPG figure.

I guess if the speedo sensor is OK - you will get avg speed/distance no problem, but MPG will be up the swanee. If the speedo sensor is duff, but the rev counter sensor OK - everything will be Kaput.

Hmmm - into the rabbit hole we go…..

Dizzy means distributor - apols for the abbrev's

There is a metal, flying saucer shaped disk on the side of the distributor - which operates the advance/retard system for the spark plug.

The sensor I'm referring to on my cabby is different from this - it's about 1.5" across (maybe smaller), and made from black plastic. I reckon it could be placed anywhere in the vacuum line from the intake manifold to the distributor. I don't know if it could be placed in one of the other vacuum lines from the intake, but I guess this is possible.

Ali Cabrio

* [if you reverse at speed does your speedometer go forwards or backwards? I reckon that it's geared to run forwards whatever gear your in, otherwise the end stop would get knocked off the speedo dial every time you parallel parked a bit too enthusiastically!]

Hmmmm - sounds like your MFA is going something slightly different from mine.

Don't think so - exactly the same me thinks
My MPG figures show zero at standstill - and then go to 99.99 on moving (like yours)*.

BUT the readings I get for miles/avg speed are out - by a little - which makes me think that the sensors on the speedo are reading something - but not enough to give a sensible MPG figure.

"Same  same" .  I don't think this is in the speedo - which is cogs and wheels driven by a cable and a worm drive.  I think it is because the MFA and the speedo work from different data sets - the speedo via its cable and the MFA via an electronic impulse generated by a rotaing shaft in the gearbox.  Different inputs - slightly different outputs.

We need to eliminate the assumption that the fault is in the speedo before ripping out the dash.  I am going to try a "roll test".  Roll down a hill with all the circuits closed and see if the MFA rolls over to 99.9.  Repeat with the circuits open but engine off (no input from the tacho) and see what happens (input from speedo & gearbox or only one of the above) - remove MFA plug from gearbox and repeat (speedo only) - then we'll know.
I am coming to the conclusion that the problem is a leak to earth in the under-bonnnet wiring caused by moisture & dirt - but this is only a guess at this stage.

I guess if the speedo sensor is OK - you will get avg speed/distance no problem, but MPG will be up the swanee. If the speedo sensor is duff, but the rev counter sensor OK - everything will be Kaput.

Hmmm - into the rabbit hole we go…..

Dizzy means distributor - apols for the abbrev's

There is a metal, flying saucer shaped disk on the side of the distributor - which operates the advance/retard system for the spark plug.

The sensor I'm referring to on my cabby is different from this - it's about 1.5" across (maybe smaller), and made from black plastic. I reckon it could be placed anywhere in the vacuum line from the intake manifold to the distributor. I don't know if it could be placed in one of the other vacuum lines from the intake, but I guess this is possible

Yep got one of those.  My apologies.  I should have looked more carefully.

M

Hey AliC - see if this helps.  Where is the cream and yellow sensor on the speedo to which you refer?

Blimey, this is like GCSE physics all over again.  :roll:

I think that the sensor is hidden away under the black speedo outer housing.

You have to undo the bolts holding the black outer housing on, remove the flimsy blue coloured circuit board VERY carefully, then remove the housing and you should be able to see the gubbins inside.

Regarding sensors, I reckon that there's a sensor on the speedo to tell the MFA what speed your doing - based on a magnet/pickup on the speedo gearing that's hidden away inside the clocks.

The switch on the gearbox is just that - a switch. It doesn't provide any speed data at all, it just switches from the gearchange light to the econometer needle in fifth gear.

I reckon the MFA uses vacuum plus speedo data only to calculate MPG,and I do reckon there's a sensor linked to the vacuum into the back of the clocks to do this.

Does your vacuum pipe connection link to a snail shell shaped plastic moulding bolted into the back of the clocks?

Ali Cabrio

PS - I should really log on to this site in the morning so you have a chance of getting an answer at a reaonable time in Oz.

This is not my instrument cluster.  I nicked the photo from the website you referred me to.

I try the "speedo isolation" solution over the next couple of days and let you know the results - then we'll know.

You could be right.  The plug on the gearbox is just a dumb switch .  Don't know about the "snail fitting" behind the cluster - but if I cannot solve this by getting at the problem without taking out the cluster I am going to live with the problem and do what most members do - just use the MFA for clock / trip / average speed / distance / and (most importantly) oil temp purposes  

BTW - I am in Norfolk UK - not OZ - but Australian by birth

M

Ah ha. Well I'm not far down the road in South Cambridgeshire.

We should meet up and shout at our respective dashboards together…..  

My Golf's off the road at the mo with a leaking coolant pipe, and I'm too skint/busy to fix it at the moment. But that may give me a chance to pull the dash out and have another look at it.

I'll keep you posted


Ali

Results of the "Roll Test"

Test 1:  Set MFA/ MPG indicator to 0.0

Roll vehicle to 20mph with switches off

MFA/ MPG indicator remained set at 0.0.  Speedo reads as normal

Test 2: Set MFA / MPG to 0.0

Roll vehicle to 20 mph with switches open but engine OFF

MFA /MPG indicator failed - rolled over to 99.9 MPG

Test 3: Set MFA /MPG to 0.0 plus disconnect MFA plug from gearbox

Roll vehicle to 20 mph with switches open but engine off

MFA /MPG indicator failed - rolled over to 99.9 MPG

Test 4: Drive vehicle normally with MFA plug disconnected at gearbox

MFA performed normally ( distance / average speed / oil temp / clock / outside temp but not MPG indicator)

Results:

1.  MFA / MPG indicator is not driven by engine generated vacuum
2. All MFA input is electrical
3. MFA gearbox output is a dumb switch: does not provide input data to MFA
4. All distance / speed / MPG input data taken from speedo
5. Inputs in (4) only available when speedo has power

So AliC - you were correct.  Looks like dash out and speedo rebuild to address problem. Will either be a bad connection or a leak to earth.

I think I will live with it.

M

Supplementary Result:

Gear change indicator and eco-meter non-functional when MFA switch disconnected from gearbox.  Functions returned to normal when connection re-establshed.

M

Hi AliC -

See if this helps:

Nearly cracked this - pick up the thread at

https://vwgolfmk1.org.uk/forum/index.php?page=topicview&id=electrics_2/mfa-99-9mpg-bad#post_710587
If you are having the same problem! M