What trips the shift light? What kind of conditions have to be present for the shift light to come on?

 

as far as I know, you run a wire from the one you tach gets the signal from. The light has "pills" that will tell it to light up when it sees the certin RPM you want. Differnt pills will make it light up at differnt RPMS

 

As covered before (you may wish to use the search function found in the upper right hand corner of the page next time):

The shift indicator is only an aid to obtain better fuel economy. It shows when ever it is necessary to up-shift for better economy at any acceleration (except for in 5 gear, neutral, or reverse).

The ECU monitors transmission gear, Load (TPS position) and vacuum to determin when it comes on.

 

that would be wrong for the S4 FC, and only used on aftermarket tachs

 

ive searched and found little. I was just wondering, because my shift light comes on a 4 grand, and will also flicker when in 5th when at a steady high speed. usally over 65 mph. I thought if I knew what trips the light it may lead me to the problem.

 

bump

 

As Icemark described above, a combination of several inputs to the ECU "tell" it when to activate the light. One such input is from your 5th gear switch on the tranny (which tells the ECU to disable the shift-up light function). Check your 5th switch at the ECU (use the FSM FUEL section for ECU pinouts) to ground (on S4 NA's, it's pin1I, large plug, 5th upper pin from the right). You should get 12v in 5th gear, 0v in others...

 

The shift light is for fuel economy so most of us here don't have much interest in it. I don't even think it ever worked well. I've seen my go on and off at ALL sorts of times. I just learned to ignore it.

After a few years of driving you don't even notice when it is on or not. It's only when someone else drives my car and asks..."what's that light for" that I remember it is there and think.."I need to remove the bulb or pull the wire on that". Of course I forget because after years of driving it I don't even notice it exists.

 

So I guess S5 N/a don't have this stupid little light then either that or mine doesn't work.

 

Only found on manual tranny 86-88 models.