Just curious if anyone knows what the factory shift light is based on... What controls the actual times it comes on?
Does it try to make you shift for optimal efficency or performance or a combination? Can the shift light points be changed?

Bunch of questions, any thoughts?
scular

 

I think it encourages you to shift for maximum mileage. Lots of VW's and Audis of that era had them as well. I believe the upshift light controler resides completely inside the cluster - it determines the shift points from the signals that the cluster is already getting, there's no extra sensors or anything for the upshift light. I say this because my Euro 944, which did not have an upshift light from the factory, had a US cluster in it when I bought it and the upshift light worked perfectly. Needless to say, this was one of the big factors in putting the Euro cluster back in - the Euro tach doesn't have that goofy upshift light.

I've got no idea how to reprogram it, other than to "reprogram" it with a small piece of black tape.

Bryan

 

I believe its based on engine rpm unless in 5th gear where it is inoperable. if you hold the clutch in and rev to like 4200 and hold it there in 4th gear while moving, the light will come on, even tho the gear is not engaged.

 

IIRC the VW/Audi lights used vacuum sensors, hence when the light is on and you drop the pedal to the floor the light goes out. Porsche may or may not, use the same thing.

 

It's for compliance with the CAFE (Corporate Average Fuel Economy)regs in the US (EPA? DOE?). I'm not clear if this was an average of _all_ models of all cars made by a brand, or just all cars in a model line.

But the thinking was along the lines of "If the driver upshifts when the light says to, the car will meet the required mileage spec for that year." Thus avoiding gas guzzler surcharges on the sale price, and returning ~maximum MPG if you drive per the light.

Not sure, but I _think_ it's run as an output from the DME. I think (?) the VW/Audi's that used the vacuum system had engine management systems that were less sophisticated...

Jim, "Does that mean that they didn't wear heels, stockings, or makeup?"

 

Yeah, I concur with the CAFE requirements post. The 944 made up about 50% of Porsche's production in the mid to late eighties, so incremental mileage improvement in this model made a big difference over the mileage profile for the entire product line. The 928, for example, did not have an upshift light. But it was the lowest production Porsche unless you count the 911 turbo as a separate model from the 911.

VW and Audi just LOVED Bosch CIS and therefore have vacuum powered everything. My old quattro had vacuum diff locks, vacuum door locks, vacuum cruise, and it did half a dozen things with the FI as well. So the intent of the upshift light was the same in the VW/Audi line, and it behaved the same, but got there via vacuum instead of electric signals.

Does bring up a good question though. How does the upshift light know it's in 5th gear? There's no sensor on the transmission to tell it, but as Lamah posted, it doesn't come on in 5th gear ever. How does it know? Engine RPM versus road speed via the speedometer signal maybe? Interesting question but not one that keeps me awake at night. The upshift light went the way of the dodo along with the 85 mph speedometer not a moment too soon.

Bryan

 

It can't be based purely on RPM and gear, because it will go out if the load on the engine is heigh.

Jim, Porsche had to put a MPG gauge OR a shift light in, they chose to do both. -Just interesting.

 

Correct me if I'm wrong, but I thought I saw the light come on a few times while redlining. Uncertain though cause you dont exactly have time to look around the cluster in those types of driving conditions if you know what I mean. Come to think of it, the light doesnt bother me much, passengers seem to think its pretty cool.

 

At that point it's called a "F*#@IN' SHIFT ALREADY DAMMIT light"

 

I am going to wire it to the "Check Engine" light circuit when I install my 968 engine.

 

The 928 also has the shift light for a few years. I have an 85 928S 5-spd in my driveway that has the "F*#@IN' SHIFT ALREADY DAMMIT light" on the tach.

The shop manual says the DME monitors vacuum from intake. Don't recall which pin. I looked into disconnecting it. It's very annoying cruising around town.

 

Was this light an option? I don't have it (or really want it) on my '86 951. Just wondering.


Rick

 

Tabor, my '88 only had the upshift light... orange (?) arrow near the top of the tach... no MPG gauge.

My '89 Jetta GLI had an MPG calculator built into the trip computer. Mildly entertaining to coast down a long mountain and watch it go nuts; I once got it to 99.9 MPG...

Jim, sometimes 'way too easily amused....

 

Jim, that is interesting. I am only repeating what I have read. My '87 sure has both.

Tabor... Needs a rim.

 

The automakers recieved additional fuel economy "credits" (CAFE) for equipping their vehicles with items like the idiot shift lights. The credits allow an automaker to not actually meet its CAFE required numbers but still avoid paying fines