My car is a 90RS, that came with a 305TBI automatic. It now is carbed with no ECM at all, and has been converted to a T5.

Now, some time ago when I had the dash apart, I mistakingly put a bulb in the shift light socket (it was previously empty). The shift light comes on and goes off..seemingly at random. What is causing it? Just curious

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Since you removed the ECM, I have no idea. The ECM uses the same connection for the upshift light and the torque converter, so the light would come on when the computer tells the converter to lock up.

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1991 Camaro Z28
5.7L 5-Speed (originally 305)
317 RWHP, 418 RWTQ
13.23 @ 107.62 MPH (2.10 60')
Southern California
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-=ICON Motorsports=-

 

I thought I read somewhere that the output to the shift light had something to do with "shift now for maximum fuel economy". acually, gimme a minute.....

Ok, this may or may not apply, but:

"Shift Indicator Light

If your car has a manual transmission, there may be a "SHIFT" light on the insturment panel. This light will show you when to upshift for best fuel economy at any acceleration.
When this light is on, shift your transmission to the next higher gear if weather, road and traffic conditions permit. For maximum fuel economy, accelerate slowly and shift when the light goes on. When more performance is needed, accelerate as desired and shift when the light goes on.
Downshifting one or more gears may be required to keep the engine running smoothly or to maintain satisfactory performance. When downshifting to a lower gear, the light may come on for a moment if the accelerator pedal is not released completely. Disregard this light during a downshift.
The onboard computer makes the "SHIFT" light work. This computer knows how fast the engine is going and how hard it is working, how fast the car is going, and how far the accelerator is pressed down. It uses this information 10 times every second to decide if you could get better fuel economy by shifting to the next higher gear.
When the accelerator is released, it is normal for the light to be off.
While accelerating, it is normal for the light to go on, off, and on again if you quickly change the position of the accelerator. In essence, if you change your mind, it will too and it will respond very quickly."

That's out of the owner's manual for my 87 Trans Am with 5 speed.

 

That's exactly how the ecm unlocks the TCC. It looks at TPS changes over time, looks at speed and engine rpm. All those factor into the unlocking of the tcc except there is an extra switch on brake that unlocks the tcc right away.

 

BTT

I dunno..I'm thinking it must just be a loose wire somewhere

 

I'd say it's probably confused by your mods... All it's thinking is "what the hell happened to the transmission?" Kinda like when I switched to an Edelbrock carb... my car was always yellin' at me "Hey pull over, you lost your engine" I would think there is some small tweak you need to make to get the light working properly... I'd just say take out the bulb and leave it at that.

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1984 Firebird - Daily driver, 305 LG4, 3.23 rear, T5 - Hurst short shifter, Edelbrock 600 CFM 1406 carb, Performer intake - Edelbrock Pro-Flow air filter, gutted cat, IROC 16x8 5 Spoke Wheels, AIWA bargain basement (from Sears of course) CD player, Eight-ball shift ****
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I'm not sure exactly what triggers or untriggers the Shift light, but I can tell you how it operates in my car, a '92 305TBI 5-speed.

If you slowly accelerate from a stop, it tells you to upshift pretty early; I don't know what RPM though. BUT, if you step hard on the gas, it won't tell you to shift...the shift light only comes on after you stop accelerating.

Also, if you let up on the gas, like to use engine braking when going downhill, the Shift light doesn't come on then either.

Jester, if the light is controlled by the ECM as these guys said, then yes, something odd is happening, maybe a loose wire. But next time the light comes on, try stepping harder on the gas, or letting up on it, see what happens.