4+3 Overdrive weird issues
Thread Starter
5th Gear
Joined: Jun 2021
Posts: 5
Likes: 0
4+3 Overdrive weird issues
Hi,
I am a new owner of an 85 Z51 with the 4+3 (got it in February, on the road in May.).
I've seen a few issues with it lately, wondering if anyone can help troubleshoot it. I've changed the fluid in it, and have a filter on order.
Issues I've seen:
Wondering if this is somehow ECU related? I could see a wiring issue, or relay issue with the overdrive by itself, but that doesn't explain the fuel economy readouts happening at the same time.
Thanks
I am a new owner of an 85 Z51 with the 4+3 (got it in February, on the road in May.).
I've seen a few issues with it lately, wondering if anyone can help troubleshoot it. I've changed the fluid in it, and have a filter on order.
Issues I've seen:
- Press the button on the shifter, light on dash turns on, does not shift
- Press the button on the shifter, light does not turn on, no shift
- Light is on, car is driving, normally, no extra load (or sometimes decelerating), light turns off, shifts back to 1:1. Cannot get it to turn on again.
Wondering if this is somehow ECU related? I could see a wiring issue, or relay issue with the overdrive by itself, but that doesn't explain the fuel economy readouts happening at the same time.
Thanks
Cruising
Joined: Mar 2021
Posts: 14
Likes: 5
From: Atlanta, GA
https://www.5speeds.com/43table.htm
Here's a link that describes the logic of the shifting. The ECM gets the vehicle speed from the dash cluster, so it could have been losing the speed signal and not allowing it to engage. The ECM controls the light on the display for the engagement. If the light is on the ECM thinks it put it into overdrive. So it could be a wiring issue that is keeping it from engaging when the light is on.
Here's a link that describes the logic of the shifting. The ECM gets the vehicle speed from the dash cluster, so it could have been losing the speed signal and not allowing it to engage. The ECM controls the light on the display for the engagement. If the light is on the ECM thinks it put it into overdrive. So it could be a wiring issue that is keeping it from engaging when the light is on.
Racer
Joined: Feb 2020
Posts: 483
Likes: 109
From: Fairfax, Virginia
Since you have an 85 with the button in the shifter you can remove the ECU from the control loop entirely by connecting two wires. You do lose out on the auto up and down shifts, but you have more control over the OD and can use the clutch without having to guess when it's gonna drop out from underneath you to help it live longer.
Thread Starter
5th Gear
Joined: Jun 2021
Posts: 5
Likes: 0
Since you have an 85 with the button in the shifter you can remove the ECU from the control loop entirely by connecting two wires. You do lose out on the auto up and down shifts, but you have more control over the OD and can use the clutch without having to guess when it's gonna drop out from underneath you to help it live longer.
Melting Slicks
Joined: Jun 2011
Posts: 2,383
Likes: 385
From: Leland , North Carolina
Thread Starter
5th Gear
Joined: Jun 2021
Posts: 5
Likes: 0
So at this point I've narrowed it down to either:
Ran a wire from B to a switch, and from the switch to ground, so I have a temporary way to activate the overdrive.
Next step I think would be to check on the shifter switch. Is there an easy disconnect for that I can get to, that doesn't involve taking the entire center console apart to get to that switch?
If that switch is OK, I'd be fine with wiring the switch in the shifter to that B wire, and using it that way, bypass the ECM entirely (really only lose out on kickdown, oh well).
- The switch in the shifter
- The wiring from the shifter switch to the ECM
- ECM (looking likely since the output for the mileage isn't working either)
- Wiring from the ECM to the relay
Ran a wire from B to a switch, and from the switch to ground, so I have a temporary way to activate the overdrive.
Next step I think would be to check on the shifter switch. Is there an easy disconnect for that I can get to, that doesn't involve taking the entire center console apart to get to that switch?
If that switch is OK, I'd be fine with wiring the switch in the shifter to that B wire, and using it that way, bypass the ECM entirely (really only lose out on kickdown, oh well).
Instructor
Joined: Jan 2021
Posts: 146
Likes: 37
Have to take the console out. It isn't as hard as it looks, undo the top of the shifter and then remove all the screws in the console. If you remove the bottom screws on the center dash faceplate you can usually wiggle the console out by being careful at that point.
While you're at it you can replace the switch with a toggle kind instead of an intermittent switch. Then it functions as an on/off switch instead of needing to hold it down to disengage the OD.
While you're at it you can replace the switch with a toggle kind instead of an intermittent switch. Then it functions as an on/off switch instead of needing to hold it down to disengage the OD.
Racer
Joined: Feb 2020
Posts: 483
Likes: 109
From: Fairfax, Virginia
https://www.corvetteforum.com/forums...-on-4-3-a.html
here's a link to the guide on the bypass, the 85 stock switch will work without replacement. Starts at post 2, I'd jump the 2 pins with a paperclip to see if that makes it work on demand.
here's a link to the guide on the bypass, the 85 stock switch will work without replacement. Starts at post 2, I'd jump the 2 pins with a paperclip to see if that makes it work on demand.
Last edited by ThatOneKid; Jun 9, 2021 at 02:15 PM.
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Thread Starter
5th Gear
Joined: Jun 2021
Posts: 5
Likes: 0
Thanks, that's what I was gathering from the wiring diagram in the shop manual as well. Just confirming though, are we connecting A7 to C8, or C7?
C8 should just be the feed from the 1st gear switch, connecting that to OD output to relay shouldn't have any bearing on whether or not the shifter switch does anything.
C8 should just be the feed from the 1st gear switch, connecting that to OD output to relay shouldn't have any bearing on whether or not the shifter switch does anything.
Racer
Joined: Feb 2020
Posts: 483
Likes: 109
From: Fairfax, Virginia
C7, the first gear switch is basically useless to the ECM after this since I believe it is only used for the first gear inhibit on the OD. The OD will remain inactive until an internal pressure switch is triggered, still not a good idea to have the OD on in first though, the pump is driven off the output shaft so it takes a little bit of speed to get enough line pressure to actuate the clutch without slippage and heat buildup, especially on a high mile unit.
Thread Starter
5th Gear
Joined: Jun 2021
Posts: 5
Likes: 0
Thanks for the help, managed to get the connection made and it seems to be working good. Looks like I'll need to either fix this ECM or replace it.
For 85, does anyone know if they did different ECMs for 4 spd vs automatic? Z51 vs non-Z51? Cruise vs non-cruise?
For 85, does anyone know if they did different ECMs for 4 spd vs automatic? Z51 vs non-Z51? Cruise vs non-cruise?
Racer
Joined: Feb 2020
Posts: 483
Likes: 109
From: Fairfax, Virginia
I doubt it is the ECU it could be a failed first gear switch preventing the ECU from enabling the OD.
Last edited by ThatOneKid; Jun 11, 2021 at 05:43 PM.
Racer
Joined: Jan 2015
Posts: 499
Likes: 63
From: Arkansas
4+3 OD malfunction
Hi,
I am a new owner of an 85 Z51 with the 4+3 (got it in February, on the road in May.).
I've seen a few issues with it lately, wondering if anyone can help troubleshoot it. I've changed the fluid in it, and have a filter on order.
Issues I've seen:
Wondering if this is somehow ECU related? I could see a wiring issue, or relay issue with the overdrive by itself, but that doesn't explain the fuel economy readouts happening at the same time.
Thanks
I am a new owner of an 85 Z51 with the 4+3 (got it in February, on the road in May.).
I've seen a few issues with it lately, wondering if anyone can help troubleshoot it. I've changed the fluid in it, and have a filter on order.
Issues I've seen:
- Press the button on the shifter, light on dash turns on, does not shift
- Press the button on the shifter, light does not turn on, no shift
- Light is on, car is driving, normally, no extra load (or sometimes decelerating), light turns off, shifts back to 1:1. Cannot get it to turn on again.
Wondering if this is somehow ECU related? I could see a wiring issue, or relay issue with the overdrive by itself, but that doesn't explain the fuel economy readouts happening at the same time.
Thanks
If your not sure on how a relay operates, search for Relay Fundamentals with diagrams.
The OD relay is on the drivers side mounted on the firewall, see diagram.
The heat from the exhaust does damage the relay and wire connectors, many years operating in a hot environment., check them carefully. Be aware the wires may be frayed and can fracture easily .
Spoiler
The relay contacts could be corroded, weak connection or pitted. Carefully pry up the latching tabs, remove the black plastic cover and inspect the contacts. If corroded it's best to replace the relay. You sould check the condition of the Fuel pump relay while your under the hood.
The wiring,connections, pressure switch on the Transmission could be corroded, damaged.and saturated with oil.
I've never had the trans pressure switch or internal solenoid fail or become intermittent.
I've had two Micro Switches on the OD Shift Arm fail.
Good Luck,
Michael.
problem.