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100M overdrive disengages on hot engine/transmission

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PatBig

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A few weeks ago my overdrive has started to disengage in 4th, after I have driven roughly 20 to 30 miles. What happens is typically the overdrive works fine when I start, the throttle switch and dash switch do their job, I can turn on the OD in 3rd or 4th and it will stay on until I switch it off and rev the motor up a bit. When the car has been driven 20 to 30 miles the OD in 4th will disengage, sometimes nearly immediately, sometimes after a couple of miles or more. It does not make any difference if I am accelerating or not, it just disengages, and often will come back on then disengage again. The oil level, SAE30 mineral, is OK. I have cleaned the filter under the drain plug and adjusted the solenoid lever per the factory book instructions. I have installed a new Lucas solenoid, makes no difference. This started to happen recently, with no change whatsoever to the electricals, before the OD worked well. Strangely it does it in 4th but so far not in 3rd, the OD in 3rd works as it should, though not be honest I do not use it as often in 3rd.
Here it's nearly impossible to find a mechanic who is knowledgeable about OD's so I need to get a few pointers on what is most likely to be wrong. Thanks
Patrick
 
There are dozens of threads on here about overdrive issues . The search feature is your friend .
I think SAE 30 is to thin an oil for the OD ??
Whats the pump pressure , you need to install a gauge to check it . You need 450psi .
The shop manual set up for the OD linkage may have worked when all the items were brand new ,now with more than 50yrs of wear pretty much guarantee it wont work .
You need to measure with a depth micrometer how much lift there is of the OD operating valve ball. If memory serves me well its 5/64" to make it work .
The only reason I can think for the OD to work in 3rd and not 4th would be pump pressure as the rpm will be higher and maybe the pump pressure is marginal and the higher rpm is just enough to make it work .
My BJ8 ODs all have 20/50 in them same as the engine and no issues ,
Your first test is pump pressure and then operating valve lift height check .
 
EV2239 said:
Fit a new solenoid and if that doesn't work Overdrive needs a rebuild. Usually the solenoid is the trouble.
Click to expand...
He already did the solenoid .
 
See if pushing the gear lever over to the right a bit with the switch on makes it stay on in 4th gear. There might be some wear against the ball switch - not enough pressure. Since third gear stays engaged, there is a different contact surface that touches. The ball switch can be adjusted with the fiber washer(s), or the switch ,may be faulty. The tabs holding the switch together may have loosened and can be bent down carefully to tighten things up.
Good luck.
 
British_Recovery said:
See if pushing the gear lever over to the right a bit with the switch on makes it stay on in 4th gear. There might be some wear against the ball switch - not enough pressure. Since third gear stays engaged, there is a different contact surface that touches. The ball switch can be adjusted with the fiber washer(s), or the switch ,may be faulty. The tabs holding the switch together may have loosened and can be bent down carefully to tighten things up.
Good luck.
Click to expand...

I would also bet on the problem being with the switch at the gear lever being the issue, especially since it occurs in 4th and not 3rd. I could not believe how sensitive that switch can be. On my car just the tiniest bit of movement of the gear lever when in 4th would result in the overdrive switching on and off. I solved my problem at least temporarily by removing the fiber washer and using a smear of Permatex. I have bought a new switch and plan on installing it soon.
 
I'd change out the dash switch first, but I'm lazy.

Edit: Then, I'd change out the relay (or open it up and file the points clean).
 
Last edited:
Before I'd start replacing switches or solenoids, just turn on the ignition, put the gearbox in neutral, flip on the OD switch and move the stick from left to right. You should hear the solenoid clicking in and out as you do this. If it doesnt give you a solid click each time it may be the switch. Obviously if you actually put it in 3rd or forth you should hear the same click. Rules out the switch but not the rigging of the solenoid to the operating valve. If you have a spare cap for the operating valve, drill a small hole it and weld a pipe adapter to allow a 0 to 600psi glycerine filled gage to it. Install this in place of the regular cap. The operating valve will work the same but you'll be able to drive and watch the pressure reading of the oil system in the transmission. Once you make the adapter it is a snap to see if you have a pressure issue. Otherwise you're just guessing. Several things can cause inadequate pressure. Will the OD re-engage if you really rev it in 4th after it has stopped working?
 
Thanks for your answers. I don't recall it re engaged when reving up in 4th but I need to check again. I'll also look at the gear lever switch, easy to test and a simple fix before I move on to the more complex stuff. It works when tested without starting the engine but I did not pay any attention to how sensitive it can be.
 
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