Hall Sender - What is it? Is this related to 99.9MPG

There must be a logic circuit that takes the vacuum and hall sender signals and works out the MPG.

The vacuum will be directly relational to how far the air plate in the meter head is moved and hence how much juice is being squirted in to the engine. The hall sender will tell it how far its travelled and so it can calculate the MPG.

I've looked through the images and can easily make out the part number on the '88 cabrio I used to own, the sportline is more grainy but I'm 99% sure it's the same…. So…. Both have the 321 907 345B part number.

I can see Oldfart's point. Wouldn't the other mileage related readings be wrong if the hall sender was a bit duff?

I'm enjoying this  

Right - I'll try to brain-dump in a sensible order

• The connection to the hall sender in the back of the speedo isn't ideal - a tongue of plastic circuit board foil carrying three contact strips is slid under three sprung contact points, and held in place by a plastic pin that projects through a hole in the foil, as well as a clamp that's part of the sensor housing. Mine had no corrosion, but if you've had a leaking scuttle it could get damp.

I have taken photos - and once the film is developed……. (old skool rools)

• OldFart - interesting that your MFA gives sensible mileage readings but double MPG. My MFA gives both inaccurate mileage readings - and 99.9 MPG the moment I start rolling. Avg speed is also out (naturally). I think you're probably right that the MFA is using more than just mileage to work out MPG, but it's still not clear what.

Mileage is obviously vital - so I reckon eliminating the Speedo hall sensor as a fault is a good start. If this sorts out my mileage readings, we'll be in the same boat…. I hope you don't snore

Next question - does the MFA work out the PerGallon bit of MPG from Revs? Vacuum? or a combination of Revs and Vacuum?

On the back of my clocks the following are connected to the foil circuit board;

• rev-counter
• Housing for vacuum-activated analogue MPG needle

The up-shift light, and the switch on the gearbox that swaps between this light and the analogue MPG needle have been effectively eliminated from the picture, as they don't seem to feed any useful info to the MFA (both are activated by on-off switches, not a graduated/variable switch/sensor as you would expect to find).

I reckon that the solution will lie either within the plastic housing that is part of the analogue MPG meter (and might consist of a variable resistor operated by variable vacuum), or within the circuitry linking the rev counter to the MFA (if such a connection exists).

The truth is out there……

This page is really useful (as is the site it is part of).

4130 Products - Changing the color of the gauge cluster lights.

In particular - if you look at the back of the clocks you will see that there is a vacuum sensor, but the car does not have an MFA (I think) that measures MPG, as it is a Golf GL.

NB - the sensor is for the upshift light (which it does have) and looks exactly the same as the upshift vacuum sensor I have, which in my Golf GTI is sited in the engine compartment alongside the distributor.

Ali Cabrio

Here are some lovely pics for you all…

Dash pics from '88 Cab GTI





Dash from a '91 Sportline - notice the variation on the vacuum sensor…






As late cabrios still have the vacuum sensor but no MPG needle I guess the MFA MPG reading must be using this? No other function would, would it? RPM is electronic.

Just had to say Great pics…I love this site…makes me feel almost normal!!!
i don't know anybody in the world who would find this stuff interesting except you people and me!

thanks Drew!

We really should get out more

I checked through the Haynes manual wiring diagrams last night (Brown coloured manual).

The crucial bit is labelled G55 in the later (1984 on) cabrio diagrams, and is described as a "Vacuum Sensor". It has three electrical connections running to it.

Oldfart - your third picture shows this perfectly - the black plastic ribbed cylindrical housnig has three tracks from the circuit board foil running to it.

Interestingly it looks different from mine which does have the MPG needle. However - the mechanism for the MPG needle appears to be separate from the vacuum sensor.

In the Haynes diagrams the MPG needle (G51 I think ) is "associated" with the vacuum sensor by dotted lines - indicating I think a connection that is not electrical (i.e. the vacuum hose).

To complicate matters, BOTH the upshift indicator light AND the MFA are linked to the rev counter. So the list of things that could be up the swannee to give an MPG reading of 99.99 is growing by the day, and includes, so far;

• Speed sensor (known weak point)
• Vacuum lines (can perish)
• Vacuum sensor on back of clocks
• Rev counter
• MFA circuit boards

 
oooh I love these knotty problems


Ali Cabrio

AliCabrio said

We really should get out more

I checked through the Haynes manual wiring diagrams last night (Brown coloured manual).
Ali Cabrio

I felt so geeky when posting those pics, knew you'd all appreciate them though! I also had a look at that brown haynes manual too ops:  

Interestingly it looks different from mine which does have the MPG needle. However - the mechanism for the MPG needle appears to be separate from the vacuum sensor.

I agree on this that it is separate in the circuit diagrams. I remember taking my '83 dash apart once to try n sort the issue and it was different to the '88 cabrio too. Didn't take any pics though :doh:  

In the Haynes diagrams the MPG needle (G51 I think ) is "associated" with the vacuum sensor by dotted lines - indicating I think a connection that is not electrical (i.e. the vacuum hose).

The dotted line is normally used in the diagrams to show that a part is optional. Hence most cabrios don?t have the MPG needle. The needle relies on the vacuum sensor for its electrical info.

The 3 connections with solid lines from the vacuum sensor connect to the MFA. The MFA also has a connection to the speed sensor so I guess in my mind the possibilities are…

- The MFA circuit board
- Vacuum Sensor
- Vacuum pipes
- Speed sensor

My gut feeling points towards the first three on the list.

yes, this is still looking good…I can't wait for each installment - who needs a social life anyway?

I'm inclined to think it's the vacuum sensor on mine - I'll try a substitute when I can get a scrap one ( and when I can be persuaded to take my dash to bits again…having just about got it all working and squeak free!)
I just hate to have something not working…and clearly I'm not alone! Also, the mpg bit is the most interesting bit of the MFA when you're twiddling about on a long journey..so it makes most sense to fix it! I've checked the vacuum lines, and can't find any leaks, and my average mph seems about right, so the sensor seems a likely candidate for faultiness!

Just had a look at the original campaign register site thats still out there!

Check out this link….

http://www.boards2go.c…=1042874383&user=campaign

The last post by Crasher, AKA Steve Cresswell from C&R seems to think it was the distance sensor that we and haynes call the speed sensor.

I can rememeber seeing this thread when I was a golf newbie but thought it had since got lost on another forum in between this one and that one.

Michael, AliCabrio - Have you got your's from VW yet?

Yep -mine's arrived.

I'd hoped to put it in this weekend, but the coolant pipes also need replacing and took priority for the moment.

I should be able to get the speed/distance sensor replaced this week(if the weather holds).

I'd hoped to buy a second MK1 dash to pull apart but got outbid  :roll:

I'll keep you posted

ALi Cabrio

Ross -

I have had my parts for about 10 days now.  I shall post pictures etc when I attack this.

I am waiting for an instrument cluster from Timski in order to "dry run" the solution and check it works before removing at 99.9% working instrument cluster from my vehicle.  To achieve this I'll run 12 volts to the spare cluster , generate a vacuum to the econo-meter using a vacuum pump and spin the speedo using a low speed setting via na electric drill

Once I have a cluster (of the correct designation - they changed regularly) I shall additionally go and see my Mark 1 specialist contact at VW for his advice - he has experience in bringing these units back to life as fully functional.

Patience!

The thread you directed us to on the old Campaign register is really useful.  We'll see what the problem is - either dirt and corrosion (lets hope) or a corrupted piece of software.

Unfortunately I am inclined to think it is the latter.  I remain unconvinced that the MFA takes any vacuum input - which raises the question as to how it calculates MPG.  I have not seen any suggestion, evidence or hint that there is any form of flow reading on the fuel line.  Result - the question in my mind is how does the MFA "know" that you have not done 20 miles at 5000 RPM as opposed to covering 20 miles at 1500rpm.   

I am guessing that the MFA has a simple piece of code embedded in it that estimates MGP given speed & distance but we'll see what Mervyn at VW says.

Best I can do at this stage.  I am prepared to be proven wrong!  It well could be that there is an input from the econo-meter to the MFA but how this functions is unclear (the Mrk 2's had two vacuum inputs)- and further  why does the MFA roll across to 99.9mpg when the circuits are open, the engine is OFF (ie zero vacuum), and the econo-meter / change indicator at the gearbox is disconnected.  The input to generate the default 99.9mpg figure must be an electrical input from the speedo as this is the only functioning instrument when conducting the "roll test".

We'll get there.  Either there is no repair possible or we'll execute a repair.  Either way, we'll know and with Alicabrio's help, I am sure i can crack this.

MP

The vacuum is linked to how much petrol is being injected. More vacuum means more petrol as the air plate in the meterhead is deflected more. There is a direct link between the two. I think this is used by the cluster along with distance traveled to work out MPG. :dontknow:  :dontknow:

is this helpful?
http://www.boards2go.com/boards/board.cgi?action=read&id=1043263342&user=campaign

Michael said

Unfortunately I am inclined to think it is the latter.  I remain unconvinced that the MFA takes any vacuum input

I have a later cab and when you leave the vaccum off the MPG Reading is completely inaccurate.

Hope this helps

Rob

early -1800: yep understand how the vacuum and fuel useage inter-relate.  This is detailed in the thread.  The question is how the vacuum reading is communicated to the MFA, if at all.  This is the functionality that remains to be explained.  As I stated earlier, it could be a rheostat reading from the econo-meter as the needle swings to and fro but this seems (to my mind) to be a vast amount of engineering to produce a single digital output -  but we'll see.  

cabriaulait: yep - clocked it - thanks

Robb: interesting observation in two regards (1) indicates the vacuum inut and the MFA do communicate - but begs the question - why is it, if the vacuum is disconnected, the MFA doesn't default to 99.9mpg …..

At this stage gentlemen, we don't have sufficient datapoints.  More work required.  The test cluster should yield some results.  Using a mutimeter I am hoping to be able to isloate and track each input to the MFA.

Give me a bit more time to work on this - hopefully we'll have a result by Christmas.

MP

Just to clarify, my reading just read 15mpg and didn't move, I would assume that a 99.9 would be down to a failure of a sensor (which one i don't know) high or low resistance. Didn't i read eairler that it clocked upto 99.9 then stopped?

Rob

AliCabrio said

We really should get out more

I checked through the Haynes manual wiring diagrams last night (Brown coloured manual).

The crucial bit is labelled G55 in the later (1984 on) cabrio diagrams, and is described as a "Vacuum Sensor". It has three electrical connections running to it.

Oldfart - your third picture shows this perfectly - the black plastic ribbed cylindrical housnig has three tracks from the circuit board foil running to it.

Interestinly it looks different from mine which does have the MPG needle. However - the mechanism for the MPG needle appears to be separate from the vacuum sensor.

In the Haynes diagrams the MPG needle (G51 I think ) is "associated" with the vacuum sensor by dotted lines - indicating I think a connection that is not electrical (i.e. the vacuum hose).

To complicate matters, BOTH the upshift indicator light AND the MFA are linked to the rev counter. So the list of things that could be up the swannee to give an MPG reading of 99.99 is growing by the day, and includes, so far;

• Speed sensor (known weak point)
• Vacuum lines (can perish)
• Vacuum sensor on back of clocks
• Rev counter
• MFA circuit boards

 
oooh I love these knotty problems


Ali Cabrio

Theres your answer to whether the vacuum is measured, G55, Its also listed against the 1983 GTI and is connect as shown in the diagram. the vacum sensor is a transistor which varies with pressure, much the same as the hall sender is a affected by magnetic fields (Hall effect, which I would explain further but its along time since I did electro magnetism!!!)



The diagram above shows a sensor in one side and the gauge at the other, I would suggest that this vaccum cavity was changed with the later cabs. and on the outside like ross's pictures.

Both these sensors are Trasistors based and liable to fail or go out of spec, so the problem could be either. Transistors have become considerably more reliable since the early 80's.

Rob  

Alicabrio,

Robb might have hit on something here.  The transfer of the variations in the pressure generated by the vacuum is converted to an electrical signal via a transitorised circuit and communicated to the MFA.

 
I must have a different Haynes to you as I don't have wiring diagrams for cabriolets - in fact I don't have a diagram showing component G55 or G51 - but no matter.

I am going to try and get replacement component show above (G55 in Robbs diagram) - this will give complete replacement  of all interconnecting components - ie  (1) Hall Sender (19) Voltage Regulator and (2) vacuum transfer unit - mumble- whatever it is called.

I am still of the view the fault is in the Hall Sender - as the econon-meter was isloated in the roll test.

Note  & as you are aware  - there was a redesign of the instrument cluster 1983/84 .

MP

Hi Micheal,

Glad to help. The part your refering to is listed as discontinued on my ETOS cd, so not sure if you can get a part direct.

My haynes manual has a suplimental section 12, but is brown. do you have that section in yours, the diagram is part of that. (Not sure when they changed but it covers the later cabby as well) It breaks down the circuit on the back of the console so helps allot.

Rob