[Supras] Non-linear speedometer+AFM sensors

[berniek at technicaldevelop.com]

Dave:


    Glad to hear your speedometer worked out.  The instrument is about 
as sensitive to balance from a hairspring standpoint as a pocket watch 
of old.


    The oscilloscope traces I was going to do had to do with the stock 
and Lexus AFM bodies connected in series, with a big shop vac doing the 
sucking.  The objective was to determine actual vortex frequency scale 
factor difference between the two.  However, I later read the 
information someplace, and never did the test.  I did, however, test at 
least the Lexus sensor output with a 'scope and found that anything 
which disturbs straight-in airflow will cause intermittent drop-out of 
the frequency signal.  This holds true despite the straightening 
honeycomb within the AFM body.  The filter with the best straightening 
capability that I've tried is the HKS "dome foam" type.  The baseplate 
is very good from fluid dynamics standpoint, although the filter element 
is awful ("get an overbore while you drive" mentality).  An Apexi filter 
I have here has a radiused inlet which looks good, but the distal end is 
closed right in front of the radiator bulkhead opening (where the stock 
filter was, which I opened up (I still need to locate the Apexi in front 
of that bulkhead with extension hose).  In any event, when behind the 
bulkhead the engine feels ratty and somewhat down on power over 5K RPM. 


    From what I've observed, all of the sensors are identical in 
performance, having about 4-5 of them.  There are some unit to unit 
variations in mass flow correction thermistor resistance, but that is 
expected with thermistors, and is well within the trim capability of the 
ECU to correct for.  I've opened up a sensor, and found a flex circuit 
board which appears to excite the led which illuminates the silvered 
polyester foil in the vortex sensor, and a simple amplifier circuit to 
square up the phototransistor signal at 5V peak.  There did not appear 
to be any frequency division or other processing occuring.  The 
thermistor is independently wired to a pair of connector terminals which 
connect to the ECU. 


    Hope this helps, but from what you have already written, apparently 
you have already found the answer.  Hope you had a good Resurrection Day.


    BernieK


'91T 5 speed, refreshed JDM, '98 MkIV wheels, Lexus AFM, 550cc ND 
injectors, Walbro, modified stock adjustable FPR, 57 trim CT26 (no 
turbine clip, but wastegate passage bored out), deflashed valve bowls, 
'98 mounts (strong!), Treadstone IC, hard piping, Lipp, 3" Random Tech 
downpipe and single cat, 80 mm HKS LET-T16 exhaust, Dawes ball and 
spring boost controller (for now, set to 17 PSI), modified Raptor 
recirculated BOV, 2 mm Cometic HG, ARP head studs, helicoiled exhaust 
stud tappings, chrome moly lightweight flywheel, Exedy organic clutch 
(street use).


dgalyardt at cox.net wrote:
> Bernie
>
> I wanted to (belatedly) thank you for this.  I was unaware how sensitive the hairspring movement was.  I had a needle from a different speedo on it and at an indicated 80 MPH, I was doing about 67.  I put back in the original speedo and needle and it is right on.
>
> On a different subject, a couple of years ago you posted something about planning to do a test on the 42010 and 42011 AFMs with an oscilloscope.  Did you ever do this and if so, what were the results?  I have put a 42011 on my 87 and it seems to work OK.
>
> Thanks again for your detailed technical contributions!
>
> Dave
> 87T
> 89NA-T
> ---- berniek at technicaldevelop.com wrote: 
>   
>> The indicating needle is balanced, usually with a very small counterweigh
>> that is micro-spot welded or glued on.  Fundamentally, the speedometer is
>> comprised of a magnet rotating within a "drag cup", an aluminum cup which
>> tends to rotate with the magnet due to eddy current induction.  However it
>> is restrained by a hairspring, so the equilibrium position of the drag cup
>> and the small shaft which drives the needle are linear with respect to
>> speed.  So if the needle is out of balance, that may be the source of
>> error.  There are small pullers meant for removing pointers from pressure
>> gauges for recalibration, or you might be able to pull the needle off with
>> needle nose pliers on the shank, with a small piece of wood as a fulcrum
>> at one of the faceplate supports.  Either use the old needle, or rebalance
>> the new one by allowing it to seek its heavy spot by supporting it on thin
>> wire, and adding a few fibers of wood or things like #2 lockwashers on the
>> rear of the pointer.
>>
>>
>> If your odometer behaves the same way, the problem is elsewhere, perhaps
>> in sizing and skipping of the driven gear in the transmission.
>>
>> Hope this helps.
>>
>> BernieK
>>
>>     
>
>
>
>