Overdrive in 1987 with 4 speed manual tranny

[jerrywagner]

Can anyone tell me on my 1987 Vette with the factory 4 speed transmission if 68 mph and 2800 rpm is normal, it seems that the OD symbol in the tach just above the digital display comes and goes at will and the rpm's stay the same, also what is the switch just to the right of the Headlamp switch control ? Thanks in advance

 

[vetteoz]

O/d light indicates the computer is " trying " to operate the O/d ; not that it is actually engaged
If car is new to you and service history unknown ,
change the fluid and filter in the O/d. Is supposed to be done every 12K but often wasn't resulting in the problems 4+3 have a bad name for.

If O/d not engaging could be
low on fluid
bad O/d relay ( common with age )
bad wiring to O/d ( also common prob )

As most of the O/d mechanical problems have been fixed by this time , incorrect or no O/d operation is usually a control problem ;something simple and cheap to fix

 

[jerrywagner]

so I will change the fluid and filter and then troubleshoot, is there an existing relay on the car that is similar that I can swap out to test ? what is the test to see if the relay is getting the signal and sending it to the OD unit ?

Jerry

 

[vetteoz]

what is the test to see if the relay is getting the signal and sending it to the OD unit ?

Relay is working because O/d light is downstream of the relay.

Power is not getting from relay to O/d ( could be as simple as the connector on the side of O/d case being off or loose )
or O/d does not have enough fluid
or outside possibility the pressure switch in the O/d is faulty ( Switch is a safety device to prevent operation if fluid is low)

 

[jerrywagner]

Overdrive solenoid was unplugged at the overdrive unit so I plugged it in, single prong round plug, test drive was the same RPM at highway speed with OD light up 65mph=2800rpm, BUT now when OD light is lit the car will not use the gear box to slow the car on a hill like a normal manual transmission would do, it acts like an automatic transmission so when you come to a flat part and give gas you can feel the overdrive unit re-attaching with a slight slip but no slipping once you start moving or under a load, so it seem the solenoid is getting a signal, could it be an overdrive in need of an overhaul or is there something else to try ? Jerry

 

[jerrywagner]

and I replaced the filter and added new ATF and the test drive was the same, 65mph and 2800RPM and still while in Overdrive, any other ideas on what to do short of having the unit rebuilt ?

 

[WhalePirot]

Overdrive solenoid was unplugged at the overdrive unit so I plugged it in, single prong round plug, test drive was the same RPM at highway speed with OD light up 65mph=2800rpm, BUT now when OD light is lit the car will not use the gear box to slow the car on a hill like a normal manual transmission would do, it acts like an automatic transmission so when you come to a flat part and give gas you can feel the overdrive unit re-attaching with a slight slip but no slipping once you start moving or under a load, so it seem the solenoid is getting a signal, could it be an overdrive in need of an overhaul or is there something else to try ? Jerry

I wonder why it was unplugged; perhaps the PO had issues with it.

My '84 turned 2000 RPM @ 70mph w/ stock 3.07 rear.

The light illuminating (excellent support, Oz) indicates the electrical portion is working.

The 'acts like an automatic' is confusing to me. Do you mean the car acts like it is in neutral?

 

[jerrywagner]

there is not any "engine compression braking" when letting off on the gas when going down a hill if the OD light is on, if the OD light is not on then it acts like a normal manual transmission and has compression braking, also is this year 1987 suppose to have an overdrive off switch ? there is not one on the stick shift as I took off the top and there is no mechanism there, is this normal ? and yes I believe the prior owner un plugged the plug at the overdrive unit to disable it and have the car act like an older vette, thanks for all your help on this as I really want to get the overdrive unit working properly