Vehicle Speed Sensor Question

[deez]

I am trying to find an idle issue, and I was checking the VSS and it bounces around non stop, even when the car is stopped. Now, if I recall, its a reed switch that is triggered by magnets in the speedo, which opens and closes the switch and gives it that reading of it bouncing around. Now at a stop, shouldnt it just read 0?

 

[belize1334]

0 or 1, depending on position, but it shouldn't change. It is possible though that it's sitting in transition and it's just your multimeter which is bouncing around. I.e. if the reed switch is 'between' open and closed then the multimeter will get confused. Where are you connecting the multimeter? Are you readin the reed switch directly or at the ECU? Try disconnecting the speedo cable from the transission and rotating it by hand. This will simulate vehicle speed and should cause the reed switch to cycle through open/closed/open etc.

 

[misterfixit]

change the battery in the multimetrer. my fluke goes wappy when the battery is about to die.
Rich

 

[deez]

I should have specified I am using my logger for readings, not a multimeter. It bounces around between 0 and 70'ish

 

[belize1334]

Ah. Well, you could have a flaky reed switch which would be jumping sporatically between 0 and 1 and thus making the ECU think the speedo cable is moving. You could also have something interfering with the signal.

In my situation I determined that there was a short in one of the circuits between the reed switch and the ECU which was grounding the wire so that the ECU always saw 0. In your case, it's possible that something is flaking out and putting weird noise in the line. Things that use the VSS are the variable speed wipers, the cruise control, not sure what else.

My fix was to take the instrument panel out and run my own line directly from the reed switch to the ECU after disconnecting the factory wire from both ends (switch and ECU). It's a bit ghetto but it works.

 

What are you looking at on a logger to determine this? I have found that the "VSS" address that many of them log doesn't work properly. The only reliable way to see if the reed switch is registering correctly is by inputting address C6 into one of the MMcD versions that allows custom address logging. It should be at ~228 when the car is stopped, and quickly drop as you start moving.