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).]
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.
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).]
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).]
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.....
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.
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.
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
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.
Update: The issue kept returning, no matter how I fiddled with the connector. I think the connector got messed up from changing the ICM a whole bunch of times thanks to the distributor making me think the ICM was bad (see my other post: https://www.fiero.nl/forum/Forum2/HTML/144767.html ). This was annoying, as I had only recently replaced the original connector with a replacement.
Luckily I had ordered an extra connector a while ago. I cut out the other one, spliced this one in, and haven't had the issue since (knocks on wood, kisses the rabbit's foot and throws salt over the shoulder).
Patrick, there is no feedback from the drivetrain to the ECU relative to what gear a manual transmission is in. None at all.
Absolutely, I believe the ECM uses a comparison between speed and RPM's to energize the shift light. In view of this there may be different prom chips that define those speed v RPM points relative to the FDR's. Last I looked the shift light can be turned on and off by unchecking the feature flag in the program.
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Shift light actuation is most likely due to MAP sensor vacuum signal. I'm on vacation and away from my factory service manuals, so that is conjecture at this point. There is probably and answer in the Drivability section.
Welp, I thought I fixed it. I drove the car to work today, and it started in with it again. I got to work, unplugged the 4 pin connector, plugged it back in and drove it, no problem.
On my break I went for a drive. It was fine for a few minutes, then started acting up again. I'm out of ideas.
