Speedo help please

[Barnster]

Hi All,

I had an SVA retest this afternoon - failed miserably once again on the speedo.

I tested with GPS on the way to the test - all indicated speeds appeared to be about 3% fast and all looked fine.

Trouble is, when tested on the rollers the speedo appears to be 3% fast but then it "settles" a fair bit lower.

Results:

30mph indicated - 46 actual

40mph indicated - 56 actual

50mph indicated - 69 actual

test stopped.

I tried changing increasing the pulse count 1000 and decreasing 1000 but this made no difference to the display speed or the dipping.

I tried two sensors and replacement speedo and also moved the sensor location but no difference.

I didn't notice the dip when testing on the way to SVA but on the way home noticed it's true!

Anyone experienced this and managed to fix it?

B

[OB1]

if it's fine on the way there and the way back and with gps - then either the sva rollers / speed is out (unlikely) or for some reason, the "mass" of the rollers is upsetting the pickup sensor. I assume it's a proximity sensor (i.e it senses metal in the form of the lobro bolt heads) - how close is the sensor head to the bolts - should be about 1-2mm - is there anything else at all near it - can you change the speedo settings to pick up a read switch instead? - if so, maplins do a security read switch and some magnets for abot £6 which IMHO work far better - you could even put them on a front wheel which will confuse the sva man and save all the muck at the diff end - certainly worth trying rather than another fail

not very clear picture here (no, the discs are not rusty - that's the flash) here  

[Barnster]

Thanks for the tip

May review in the morning...

Not in a very positive mood - just finished a 2x75cl bottle of Leffe Tripel before my tea and ready for a fight!!

Booked in to Westfield in a couple of weeks as they and I can't logically figure out what's wrong.

May try running cable directly from sensor to speedo outside the loom to eliminate any possible interference.

Have tried gaps from 1 to 5mm and no difference. Also change to pulse value in the region of 2000 makes no difference.      

Nothing metallic except the bolts so should work OK - everything replaced also (speedo and sensor).

As for changes - it's a VDO gauge so not sure what can be changed......

May ask them to hand back SVAd as it seems reasonable giving that the manual has been followed to the letter.....  

B

[OB1]

I have seen other entries on the boardroom that say not to use the supplied wires in the loom for the very reason of interference - also about having a clean and probably separate power supply to the speedo and sensor - another plus point for the reed switch approach - it's just on/off as opposed to sensing something (or nothing, or lots of them, or maybe every now and again on a wednesday) .. where's that beer..

know any local MOT garages where you could put it on their rollers and test first ??

[neilwillis]

Mine was out too, just like you described.   The answer was to move it closer to the lobro bolts.   Ignore the 1-2mm, and just edge it closer till it's working OK.   Also, make sure the bolts pass along the centre line of the sensor.

Mine still tends to play up when it's damp, so I'd suggest you cover the terminals and keep the moisture out too.

HTH

[OB1]

it looks like the VDO will accept a reed switch input - manual here

I found it a real pain trying to get the right clearance - you'd set it to work with the car jacked up in the garage, works ok on a test drive, then a couple of hours later it all starts going wonky    and it really is in the most awkward place !!

[Barnster]

Forgive my ignorance, what is a Reed switch, where do I get one, how do I fit it (to 2005 modular kit) and how do I calibrate?

I have a chance of SVA pretty much any day of my choice and if I can get working for a modest sum before my Westfield date, I am willing to try.

BS

[nikpro]

you haven't got an LSD by any chance as this plays havoc with the speedo on SVA rollers.

[Barnster]

No LSD, just a std 3.92.

Trouble is without access to a Rolling Road, it's difficult to test any changes I make.

I can only get up to 40 in the streets around my house and I don't want to risk taking out onto motorway 5m miles away with no plates/unroadworthy.

[OB1]

A reed switch is a small metal "reed" encased in little glass or plastic capsule. When a magnet passes over the reed, it pulls it onto the fixed portion to make contact - simple - buy one from a motosport dealer and you're looking at £30 - try

this

At £1.99 it's worth a punt although I don't know it's designed life span (I haven't worked out how many times a wheel with 4 magnets will rotate in a season as opposed to opening and closing a front door) - you'll also need some magnets - try these - YA29G 10x5 (you'll need 4)

I made a small alloy bracket fitted to the empty backplate tapped hole on the front o/side upright - magnets sikaflexed (or silicone would do) to brake disc boltheads - then re-calibrate your speedo for 4 pickups per revolution and the fact that it's a "reed" sensor and bobs your wotsit -