Shift light stays on in deceleration, stumbe (Page 1/2)
PatrickTRoof MAR 18, 02:46 PM
My 1985 GT 4 speed has developed an issue now where the shift indicator light will stay on even when I let off the gas. Normally, if the car reaches the right speed to trigger the shift light, and you let off the gas, the light will go out immediately. Now, it will stay on when decelerating between 3-2K. This is a sporadic problem, meaning it will do it sometimes and then stop. This issue is accompanied by a stumble at around 2.5K at low throttle input. I'm not sure if the stumble is from RPM or throttle position. I can't help but think the two issues are connected.

I just installed a brand new distributor less than a week ago, and the car ran great for a few days before starting in with this. I made sure the timing was set the right way. IAC is set correctly. I just did a sweep test of the TPS with a multimeter, and from what I could tell it looked good, though I've ordered a graphing meter to get a more accurate reading of it. When that arrives I'll test the MAP sensor too.

If anyone has any other ideas, I'm all ears, please!!!

[This message has been edited by PatrickTRoof (edited 03-18-2021).]

Patrick MAR 18, 09:18 PM

quote
Originally posted by PatrickTRoof:

Normally, if the car reaches the right speed to trigger the shift light, and you let off the gas, the light will go out immediately. Now, it will stay on when decelerating between 3-2K. This is a sporadic problem, meaning it will do it sometimes and then stop. This issue is accompanied by a stumble at around 2.5K at low throttle input.



The shift light is something that a lot of us, after the novelty has (quickly) worn off, disconnect. I know I sure as heck didn't want a red light continually flashing and potentially distracting me from noticing when a real issue was occurring.

Having said that, it is rather odd that the shift light would sporadically stay on when the gas is let off.

[EDIT] Heh heh... I found an earlier post of mine regarding shift lights.


quote
Originally posted by Patrick Here:

As has already been suggested, the best thing to do in regards to the "SHIFT" light is to remove the bulb. In case you're not aware, the "SHIFT" light is to inform the driver of when to shift gears to help maintain the best fuel economy. Is that what anyone really wants, some incessant nanny telling you when to shift... over and over and over and over again?

I found it nothing more than a constant distraction. If a red light illuminates on my instrument panel, I want it to be indicating something important.



[This message has been edited by Patrick (edited 03-18-2021).]

olejoedad MAR 18, 09:25 PM
Shift light is controlled by ECU.

Do you have a spare ECU that you could install?
PatrickTRoof MAR 19, 04:31 AM
Patrick, I agree that the shift indicator light, while a cool feature, is a bit distracting, especially now in my case with its hyperactive behavior. However, right now, it seems to be indicating that I have a problem and I'd like to find it, haha!

Olejoedad, I believe I do have a spare or three kicking around. Do you happen to know if they're all the same except for the prom chips?

[This message has been edited by PatrickTRoof (edited 03-19-2021).]

olejoedad MAR 19, 07:02 AM
They are not all the same.
However, any V6 manual ecu will work as long as it has the prom that came with it.
I suspect the shift light programming is contained on the prom.

And any advice I share is worth exactly what you paid for it.

Ryan at Sinister is a good resource for you on this.....
PatrickTRoof MAR 19, 08:48 AM
I think the prom would tell the ecu whether it has a 4 or 5 speed, but I think the actual switching is done by the ecu. I'll have to do some research. I know the ecu uses engine speed and vehicle speed to tell what gear the car is in, then uses vacuum from the MAP sensor to determine when to light the indicator. I think it also uses tps input, which is why the light *normally* goes out when the throttle is closed.
olejoedad MAR 19, 09:57 AM
The ECU doesn't know if it's a 4 or 5 speed, it doesn't need to know.

The shift light is a manual only feature, most likely carried on the prom.

I have attached a photo that may help you.
PatrickTRoof MAR 19, 01:00 PM
It does need to know if it's a 4 or 5 speed. If you have a 4 speed and you accidentally put an ecu from a 5 speed in there, you're going to be riding around in 4th gear with the shift light on, because it thinks it has one more gear. Thanks for the photo though, that is helpful.
PatrickTRoof MAR 20, 10:53 AM
I fixed it. When I came out from work last night, the car started, ran for one second and then died, and I knew exactly what that meant because it happened one time before. Pin C of the 4 pin connector on the ICM had come loose. Pin C provides RPM reference signal to the ECU. I pulled the pin out of the connector and bent the tab out so it would fit tighter, and plugged it back in. The car fired right up, ran great with no stumbles, and the shift indicator light worked exactly as it should.

Say what you want about the little orange arrow, but in this case it did a good job of telling me something was wrong! LOL

Thanks everyone for the replies.
reinhart MAR 22, 06:21 PM
Looking at that ECM PROM chart, are the Federal and California PROMs actually different as far as programming? My understanding was Ca vs Fed cars were all the same. GM just wanted to charge extra for CA cars.