4+3 Trans

[DrBecker]

Hi all,
Im new to vettes and i just got a 88 with a 4+3 transmission. Prob is, it will not shift into any of the overdrive gears. Sometymes when letting off the gas going 55, the trans seems to be locked to the motor and other tymes, i can let off the gas and it seems as though the trans is not hooked up to the motor....kinda like it just is freewheeling...darn it is hard to explain... sometymes the light on the speedometer (the "D") for overdrive is on, and other tymes its off, but anytyme i try the switch on the shifter, it does nothing... one tyme the overdrive kicked in...but most tymes im driving the thing in 4th and at 2900 or 3000 rpm. I live out in the sticks (closest McDonalds and walmart are 90 miles away) No one here can help...could it be just a simple switch gone bad? What could I check out before it goes to the dealership...The only chevy dealership close to here has a REALLY bad reputation Help! By the way... im 90 miles from Witchita, Kansas...Does anyone know of anybody out this way where Im not gonna get the shaft when taking the car to get fixed?

Thanks Much,
Doc

 

[MBDiagMan]

Yes, it most definitely could be the switch. The circuit is quite simple. There is a momentary switch at the bottom of the shifter stick that when pressed makes a circuit to the computer. The computer decides when to enable the overdrive. An output from the computer goes to a connector on the drivers side of the transmission. From this connector it goes through a pressure switch inside the pan and then to a coil inside the pan that when actuated, magnetically draws a check ball toward it, enabling the overdrive.

To begin with, pull the pipe plug near that electrical connector and check to be sure the overdrive unit is full of ATF. While you're there make sure the connector is connected (It will occasionally come off or get loose.)

If those two things check out, you need to connect a voltmeter or test light to that wire that goes to the transmission and safely get the meter or test light into the passenger compartment so you can see if it is getting voltage. If it is, you either have a bad pressure switch or coil. The coil is easy to replace by pulling pan from the overdrive unit.

If you're not getting voltage to the transmission, pull up the shifter boot and check the momentary switch at the bottom of the stick. It is a typical GM jerry rig, and the switch may very well be bad. You can get a much higher quality micro switch from any electronics supply house.

This transmission got a bad reputation, and I personally believe that it was due to ignorance. If not driven crazy, this is a durable, fun to drive transmission. Keep the overdrive fluid and filter changed and learn the circuits and components that I described. If you do this, you will enjoy driving it forever. Don't let anyone tell you that they are no good.

A factory manual would be a great investment.

Hope this helps,
PS, I just reread your message. The similarity between the two of us appears to be uncanny. I too live in the sticks, 20 miles from the nearest four lane highway or railroad track. I also go by Doc and drive a very similar if not identical Vette.

Also from rereading your message, it sounds like the wire at the transmission connector is loose. That's my hunch. LB

 

[JonM]

Check out this thread

http://www.corvetteactioncenter.com/forums/showthread.php?t=44175

 

[MBDiagMan]

That is a good thread regarding the switch. But don't focus on the switch until you've determined whether the wire to the transmission is getting a 12 Volt signal. That's a good place to start your troubleshooting. If you're NOT getting the 12 volts to the transmission, the switch is definitely the first thing to check. Simply lift up the boot and use an ohmmeter to see if the switch makes and breaks when pressed.

Good luck,

 

[MBDiagMan]

I knew I was forgetting something. My curiosity was killing me so I looked in my book.

There is also a relay near the master cylinder. So, here are the circuits in their entirety.

You start with the pushbutton switch that simply transfers a ground to the computer when you make the switch.

On the other side of the computer there is an output that energizes the overdrive relay near the master cylinder (BTW, this is the same part number as the fuel pump relay and was updated with a better relay some 8 or so years ago.) The contacts of the relay supply 12 volts to the connector on the transmission. From there it goes through the pressure switch inside the transmission. If there is no pressure this switch will not make, thus will prevent overdrive. The most common reason for the pressure switch to not be made is low ATF level in the overdrive. From the pressure switch it goes to the coil that activates the overdrive by drawing the check ball.

Since the car is new to you, it would be well worth the $20 for a filter and pan gasket and a few quarts of ATF. That overdrive unit, pound for pound is probably more expensive to rebuild than anything on the car including the engine, take care of it.

Good luck,

 

[DrBecker]

Thanks!

WOW! Thanks so much for all the info... I cant wait to get home and get greasy to find out if it is any of these things! I am driving her to Ft Worth next tuesday to the Nascar race and hopefully I will find this prob before I leave. At any rate, I will be sure to post here my findings. I am almost liking this forum as much as my corvette!

Warmest Regards,
Doc

 

[MBDiagMan]

Dr. Becker,

I just looked a little harder at my book. There is one more circuit that you need to be aware of. There is a switch in the gearshift linkage that is closed when the car is in first gear. When this switch is closed it feeds a ground to the ECM. When the ECM sees this ground it will not engage the overdrive. This means you can disconnect the wire from the switch and make sure that it does not inhibit overdrive. By the same token, if the switch were jammed closed or the wire between the switch and computer were shorted to ground you would have no overdrive.

Good luck,