ok i am replacing my parking brake cables so i have the center console removed and so my Dakota Digital Box is disconnected. and so eventualy after driving a while i got the check engine light for the VSS. but im wondering when the computer knows to give the trouble code for that? cause sometimes i will be driving for like 5 min before it comes on. and other times it seems i am driving 15-20 min before it comes on. also how does the computer know that im not just sitting there reving my engine or something for 20 min and therefore there is no VSS signal cause im not moving? im just curious..

 

Time running, temperature, engine under load (but no signal on VSS). Several variables.

 

i was thinking that it was mostly likley other variables\sensors. i was thinking maybe the TPS had alot to do with it. i was just wondering what exactly it gets its info from.

 

Mine does the same thing. It's a beat up (for now) 87 Z28 with the stock 305, and I've had it for a few weeks and still haven't gotten around to fixing the speedometer, it's completely inoperable.

I've noticed that city driving rarely makes the SES come on, but just a few minutes of highway speeds pops the SES with a code 24, no VSS input.

Another post hinted that this was because the ECM didn't start looking for VSS input until overdrive, but I'm not sure of the accuracy of this claim. I'll try to find the tech article and post that.

 

1988-1990 5.0 (VIN F) and 5.7 (VIN 8) - Camaro and Firebird

Code 24

Vehicle Speed Sensor (VSS) Circuit

Circuit Description:
The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the VSS buffer which alternately grounds CKT 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and the ECM will calculate vehicle speed based on the time between "pulses".
A Scantool reading should closely match with speedometer reading with drive wheels turning.

Code 24 will set if:

- CKT 437 voltage is constant

- Engine speed between 1400 and 3600 RPM

- Less than 2% throttle opening, about .10V (100mV) above closed throttle

- Low load condition (low airflow)

- Not in Park or Neutral

- All conditions must be met for 4 seconds

These conditions are met during a road load deceleration

This is part of the flowchart that you use to try and diagnose code 24. Unfortunately I have no way of scanning the whole thing. HTH.