1980's Toyota VSS

[krohelm]

1987 MR2 generates a pretty weak VSS signal from the speedometer reluctor.  Is there any point in even attempting to clean this up?  Is it time to go digi-dash?

The images cover a 200ms duration.  For the 60mph one:

1026 revolutions per mile / 60 = 
  17.1 revolutions per second * 0.2 second window = 
  3.42 revolutions of the reluctor in the window

It just looks like a mess, more useful for "is the car moving or not" than actual speed.  Additionally, the signal amplitude rarely exceeds 500mv, and the Monsoon digital inputs are reported to require 1v to derive a signal.  This doesn't even hit 1v on a peak-to-peak sweep...

Edited September 13, 2017 by krohelm
Added images

[Simon]

If the signal is that week then we will certainly have issue picking it up. Being a VR sensor closing the gap up might help.

Or swapping the sensor out for a different one might also be an option. On the plugins we have not had an issue with the speed signal but I'm not sure if it fed direct to the ECU or via the dash and is then a cleaned up signal.

 

[Brad Burnett]

based on the profile pic, I'm assuming that the car is an AW11 mr2.  IF that is the case I'm fairly certain that it is a mechanical speedo to the dash and there is an electronic converter in the cluster.

 

[Adamw]

Hi Kenny, I googled "AW11 VSS" last night to see what I could learn.  These seem pretty odd - as Brad say a mechanical cable drives the speedo, then the speedo generates a "VSS" signal to send back to the ECU?  My logic and the little info I could find online suggests this would be a 0-5v square wave type signal coming from the speedo.  From your scope trace above I suspect either there is a pullup missing from the circuit or there is something wrong with that speedo.  I know you said earlier with the ECU pullup turned on the speed still didnt work but can you try another scope capture with the pull up turned on so we can see if we get anything different looking. 

[krohelm]

Yes, this is an AW11 mr2.  The speedometer itself generates the signal.  Genius engineering, carry the mechanical cable up to the front, then the low voltage signal wire all the way to the trunk...  I'm measuring with the probes on the speedometer terminals itself, with a spare speedometer I have on the bench.

I'll measure at the ECU next, with & without pullup.  It'll be a couple days before I'm able to, but the captures should be helpful.

[JMP]

the AW11, AE82, AE92's all have the same sort of VSS built into the speedo, it's a reed switch that outputs 4 ground pulses per revolution.  You can fit a later electronic speed sender from an AE101, 111, 112 to the AW's C series gearbox if you want to change to an electronic speedometer.

Edited September 14, 2017 by JMP

[Brad Burnett]

the AW11, AE82, AE92's all have the same sort of VSS built into the speedo, it's a reed switch that outputs 4 ground pulses per revolution.  You can fit a later electronic speed sender from an AE101, 111, 112 to the AW's C series gearbox if you want to change to an electronic speedometer.

This is what i was thinking.  I believe you can even use an electronic sender from later "W" series gear boxes.

[Matt Dunn]

There is a reed switch in the speedo head, and it switches to earth.  You do need an external pullup resistor which is usually inside the Factory ECU.

There may be a few other things connected to the wire which may be giving you some sort of a signal that you are seeing,

but I would guess that adding a decent pullup resistor to it will fix the issue.

Usually they give a very good clean signal, and you only seem to be getting very low voltage, less than 500mv.

Usually will be a 5V or 12V square wave.

[ACRAZYJOHN]

this is what i did in order to get a s2k cluster in my 1980 corolla. note i have a w58 that was cable driven. put that puppy on there.

link will be provided

https://www.marlincrawler.com/transfer-case/parts-upgrade/misc-hardware/vehicle-speed-sensor-mci

[krohelm]

this is what i did in order to get a s2k cluster in my 1980 corolla. note i have a w58 that was cable driven. put that puppy on there.

link will be provided

https://www.marlincrawler.com/transfer-case/parts-upgrade/misc-hardware/vehicle-speed-sensor-mci

This is brilliant, I'll keep it tucked away somewhere safe.  I have not found even the hour necessary during the day to take the measurements.  I may just plan on the CAN dash since I'm also planning out a supercharger once the release cycle at work settles down...  but I still owe the scope measurements!