Help! Over Drive in 4+3 Transmission

[RiverFalls812]

I purchased my first Corvette three weeks ago; 1986 Yellow Convertible! Great Looking car. Over the last two weeks, the OD Icon has been lit on the dash, the arrow illuminates telling me to shift, but the tranny will not shift into OD. First concern, is this harmful to the engine, is this a OD Tranny fuild issue? I can't locate any yet to talk "Covette" with me. More questions to come I am sure.
Thanks,

 

[vetteoz]

First up, the computer will automatically engage the O/d when certain conditions are meet; speed , engine temp , etc. By default O/d is always on once above conditions are met; you have to manually disengage it. How do you know it is not in O/d?
The fact that the O/d light is on means the computer is at least trying to operate O/d by sending power to the relay.
The button allows you to manually over ride control of the O/d.It is common for the button (switch ) to fail with age not allowing you to control the O/d.

First check would be to see if the wire is still connected to O/d unit .Then check the O/d relay on firewall to see if it has power to it and is operating correctly.Can swap with fuel pump relay as check .The early style relay connectors have problems with insulation stripping back allowing bad connections and shorts.
Need to check wiring first before looking at other problems; sticking solenoid in O/d , etc.
If you do not know the history of car well worth dropping the pan on O/d , replacing filter and new trans fluid.

 

[RiverFalls812]

4+3 Tranny Issue

VetteOZ - thanks for the info. Great place to start. I'll begin that this weekend.

 

[vetteoz]

Some back ground reading for new owners
Ken Stapel's Corvette Obsession - Doug Nash Information

Also note that the button that over rides the computer is connected to the computer, not the O/d itself.
It is a "momentary" contact switch ( not a off / on ) that sends a signal (pulse ) to the computer removing computer control.

Once you have confirmed the computer is operating the O/d correctly you may have to replace the switch ($5 from Radio Shack ) to get manual control They commonly go bad with age .

 

[RiverFalls812]

CorvetteOz - So would start by taking apart the shift nob and examine/replace the switch and then work your way thru the firewall and then to the OD unit itself? Since the car is 23 years old, once I take it apart, it doesn't seem to make sense to but a 23 year old part back in place when for 5 bucks I can not have to worry about that piece again.
Thoughts?

 

[vetteoz]

So would start by taking apart the shift nob and examine/replace the switch and then work your way thru the firewall and then to the OD unit itself?

No The O/d should operate automatically by itself. You should check the wiring at relay on firewall , the relay and wiring to the O/d connector on the trans first. The button only gives manual control.If any of the other components are broken / not working then the the new button will do squat. If the O/d light is "ON" showing that the ECM is commanding the O/d turn on; how do you know you are not already in O/d?

 

[RiverFalls812]

No The O/d should operate automatically by itself. You should check the wiring at relay on firewall , the relay and wiring to the O/d connector on the trans first. The button only gives manual control.If any of the other components are broken / not working then the the new button will do squat. If the O/d light is "ON" showing that the ECM is commanding the O/d turn on; how do you know you are not already in O/d?

Previously when in OD I was at about 70 mpg at 2000 rpm; now I am closer to 2800 rpm. So it appears that I am still in 4th without the OD engaging; plus the "up arrow" on the dash is still illuminated.

I know this is very basic; but I have not worked on cars since I was a kid in the original muscle car days. Will the relay by marked by something specific on the firewall?

 

[vetteoz]

Will the relay by marked by something specific on the firewall?

Recall there are 2 relays on firewall , one closest to dist is the fuel pump relay.The relays interchange so if you were to swap them and your fuel pump no longer ran then it would indicate a bad relay.
Check the relay connector; lots of problems there with bad insulation
Check that you have 12V power on the thick red wire (from battery )

 

[larry bud]

If his OD light is turned on on the dash, then I'm almost positive the OD relay is working. Pretty sure they're in parallel to one another.

At that point, you need to look at the OD itself. Fluid level, OD solenoid (located in the OD valve body), sticking valves in the valve body.

After that, it's the OD itself.

BTW, Yellow wasn't a factor color for '86.

 

[RiverFalls812]

Larry, So low fluid in the OD unit will cause it not to engage?

 

[vetteoz]

Larry, So low fluid in the OD unit will cause it not to engage?

Yes
just like a regular auto trans.

 

[larry bud]

Absolutely. Not enough pressure to push one or more of the valves in the valve body.

 

[RiverFalls812]

4+3 Transmission

Absolutely. Not enough pressure to push one or more of the valves in the valve body.

I have confirmed all of the electricals and computer systems are good. That leaves me with the OD unit itself. It was only slightly low on fluid, but did have a faint burned odor. I am thinking in need to find a Corvette Specialist to look at the unit itself.

 

[vetteoz]

I am thinking in need to find a Corvette Specialist to look at the unit itself.

Need to talk to these people
DARRELL

SK Speed - Peformance Auto Parts and Racing Accessories

Read
Corvette 4 + 3 Overdrives

May want to contact this guy.He rebuilt his own with parts from above

86 vette switching an auto trans to a manual transmission [Forums - C4 Technical] - Corvette-Guru.com

 

[Hot Rod]

Hi.

I had a 4+3 Vette some years ago with some leakage from the OD, never fixed it so I added some fluid a couple of times/year. Slightly low on fluid made it behave a little bit strange and sometimes not engaging.

I would start with a new filter and new fluid in the OD.

 

[CGM's1988vette]

I'm currently rebuilding my 1988 project vette and have a couple of 4+3 tran questions:

For the overdrive:
1) How much trans oil should be added with a filter change - service manual did not specify?
2) Do you only have to fill up to the fill plug? If yes, this must include all the fluid in the lines and cooler when drained? I'm concerned that the cooler and lines are now dry and will take fluid when first staring the engine then unit will be low as the cooler and lines are higher than the fill plug? Do you need to run the car after filling then recheck?
3) What is the best brand of trans fluid for these overdrive? I presume that dextron II recommended in the service manual has been replaced by dextron III is this safe?
4) My filter kit came with a gasket, but he service manual states just using RTV? What works best?
5) How offen should you change this filter?

Manual tran:
1) My drain plug and fill plug are increadable tight and was only able to loosen the drain plug. How safe is it to apply limited heat to the upper drain plug to help loosen? I I have been spraying "Blaster" on the plug but unable to loosed and have started to bend by 9/16 open end wrench and large cresent wrench started to turn over the plug's sq corner. ? Pipe wrench next or leave it along as it only has 75,000 miles?

Thanks,
Craig

Need to talk to these people
DARRELL

SK Speed - Peformance Auto Parts and Racing Accessories

Read
Corvette 4 + 3 Overdrives

May want to contact this guy.He rebuilt his own with parts from above

86 vette switching an auto trans to a manual transmission [Forums - C4 Technical] - Corvette-Guru.com

 

[vetteoz]

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