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Overdrive Engage/Disengage question [Problem...]

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noutlaw

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Took the car out yesterday for a little spin and noticed the overdrive not acting correctly. Driving down the road with the switch 'off' (40 mph) then set the OD switch to 'on' - the overdrive engages fine, drops rpm, etc. but only stays engaged for about 60 seconds then disengages. If you apply just a little throttle it disengages... Tried it at different speeds/rpms (3000,3500,4000)...

Thoughts?


/ubbthreads/images/graemlins/patriot.gif
 
Re: Overdrive Engage/Disengage question [Problem..

It sounds as if the pump is starving for oil. Check the oil level in the gearbox first and if it is low then top it up. If the level is OK then clean the pick-up screen in the overdrive drain plug.

Alan T
 
Re: Overdrive Engage/Disengage question [Problem..

There should be a rubber plug under the transmission tunnel carpet to access the transmission oil level.

The problem could be low oil level or it could be the OD operating solenoid is not staying engaged. There is a explanation of the electrical circuits in the "knowledge base" at the top of this page.

It could also be that the hydraulic accumulator is not holding pressure for various reasons, such as the pump non return (check) valve is not fully seating, but check the easier things first. I can give more detailed info if needed.
D
 
Noutlaw ,it sounds a bit like the throttle SW is misadjusted,--Keoke-?
 
I wouldn't think so - it was just at the mechanics getting tuned, etc. - I'll check oil level(s) this afternoon and retest - if that doesn't do it I'll take it to my local $85/hour guy... /ubbthreads/images/graemlins/frown.gif
 
Re: Overdrive Engage/Disengage question [Problem..

Keoke,
The only way the throttle switch could cause the problem is if the OD relay is not operating or it's contact is not making connection. A possibility. All the throttle switch does normally is prevent a shift out of OD when the throttle is closed. As long as the OD switch is on & the relay is working, the throttle switch is bypassed.
See the schematic here:
https://www.britishcarforum.com/lore/attachment.php?id=13
D
 
Re: Overdrive Engage/Disengage question [Problem..

Nope , if it is set less than the 20% value it will do just as his RPM checks show.---Keoke
 
Re: Overdrive Engage/Disengage question [Problem..

Dave R. and Keoke: you guys continue to amaze with me with the depth of your off-hand knowledge of these cars!
 
Re: Overdrive Engage/Disengage question [Problem..

Guess we will have to disagree. If the DASH switch is still on, as he says, AND the OD relay is WORKING - the throttle switch has no bearing on the situation. In this condition, the throttle switch is bypassed by the relay contact. The throttle switch only has control when the dash switch is off. Provided that everything is still wired as originally intended.

The only way that the throttle switch could disengage the OD solenoid with the OD switch still on, is if the OD relay contact is not bypassing the the throttle switch as it should.
D
 
Re: Overdrive Engage/Disengage question [Problem..

switch is on and it's not engaging naturally - e.g. 40+ mph/3500 rpm... the only way to make it engage is to set it off, then back to on(waiting a few secs of course) - then it engages for maybe 30-60 seconds or until you apply some 'gas' then it disengages...
 
Re: Overdrive Engage/Disengage question [Problem..

That is absolutely correct Dave, However, a combination of a weak overdrive relay, the relay is kinda Mickey Mouse any way being a modified standard relay, and a missadjusted throttle switch will cause that problem, On the other hand, his latest post seems to say he has cumulative problems.--Keoke
 
Re: Overdrive Engage/Disengage question [Problem..

[ QUOTE ]
Dave R. and Keoke: you guys continue to amaze with me with the depth of your off-hand knowledge of these cars!

[/ QUOTE ]

/ubbthreads/images/graemlins/shocked.gif Naw we just make all this stuff up---Keoke- /ubbthreads/images/graemlins/lol.gif
 
Re: Overdrive Engage/Disengage question [Problem..

Noutlaw,
You can bypass the throttle switch AND the relay at the same time by jumpering the two connections on the throttle switch. The OD should now engage or disengage anytime you operate the dash switch. If it now works, at least you know that nothing is wrong with the OD itself or the gearbox 3-4 switch. If it doesn't work, either the dash switch is defective or there is something more seriously wrong.

You can "jumper" by putting both wires going to the throttle switch on the same terminal. Doesn't matter which terminal. The throttle switch is the one with the rod link going to the throttle linkage.

It won't hurt anything as long as you don't try to shift out of OD with the throttle closed. Even with throttle closed, it will just jerk the drivetrain a bit. I ran a Healey gear box & OD behind a 302 Chevy for years with the throttle switch & relay completely removed. Not suggesting this as a permanent solution for you, but ok for temporary.
D
To be perfectly clear, jumpering the throttle switch causes the relay to also be bypassed which is the real objective here.
D
 
Re: Overdrive Engage/Disengage question [Problem..

I have to jump in to the middle of this one also. I must agree with Dave.
The only function of the throttle position switch is to INHIBIT the OD relay from disengaging. If the switch fails or is maladjusted the only effect would be that the OD will drop out IMMEDIATELY after the switch is turned off or it would never drop after the dash switch was turned off until the ignition switch was turned off, but it can not CAUSE the OD to disenagage, even if you disconnect the wires from it. You must look elsewhere other than the throttle position switch if the OD is dropping out by itself with the dash switch still in the on position. It could still be an electrical problem in the circut, just not the throttle position switch.
 
Bet you it is lever adjustment or throttle switch adjustment. Walk carefully through your setup adjustment guid in your shop manual first. You might also try Redline MT90 which helped when I used to use ODs. /ubbthreads/images/graemlins/hammer.gif
 
Re: Overdrive Engage/Disengage question [Problem..

After giving this more thought, there is one possible failure mode of the throttle position switch that might cause the overdrive to disengage. If there is an internal short to ground inside the switch or one of the wires on the screw terminals is touching ground, ( like the temp gauge sender ), then the voltage to the hold coil could be sufficiently reduced to cause the OD to disengage. There is an easy test to see if it is causing the problem, unfasten the two wires from the throttle switch and tape the ends so they don't short to ground. Drive the car and test the o/drive. If the OD still drops out as before, then the throttle position switch has nothing to do with it.
 
Re: Overdrive Engage/Disengage question [Problem..

Dear OD Team -

Thanks - I'm going to start with the oil check - and see how it goes from there. My hope is it's something simple...

UNless one of you guys is a colorado local then I'll Bring Her by...
 
Re: Overdrive Engage/Disengage question [Problem..

[ QUOTE ]
Dear OD Team -

Thanks - I'm going to start with the oil check - and see how it goes from there. My hope is it's something simple...

UNless one of you guys is a colorado local then I'll Bring Her by...

[/ QUOTE ]

-- /ubbthreads/images/graemlins/savewave.gif
------ /ubbthreads/images/graemlins/nonono.gif--Keoke--AZ- /ubbthreads/images/graemlins/cowboy.gif- /ubbthreads/images/graemlins/grin.gif
 
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