Checking Your Overdrive Unit

Submitted by several people on the list

Brad Kahler wrote: This write up was faxed to me by Carl Sereda who received it from someone else on the list. I would like to thank both of them for providing the info.

I could be wrong but I would think that this same information would apply to the D-type and the J-type OD's as well. Below is the write up reproduced exactly as it was sent to me.

It is possible to "bench test" a transmission with an A-type overdrive and insure the overdrive mechanicals are functioning, before the unit is installed in the car. I ALWAYS do this before installing an OD tranny. It is easy to do, and is much preferred to discovering a mechanical problem after the tranny is installed.

You'll need, of course, the assembled tranny with the overdrive; a 1/2" chuck drill (the higher RPM, the better); and a functional tachometer or speedometer with drive cable.

First fill the transmission with the recommended grade of oil. By the way, the OD manufacturer Laycock recommends SAE 30 whilst the car manuals usually call for SAE 90! I usually run SAE 50. Chuck the 1/2" drill to the transmission input shaft. Screw the tachometer/speedometer to a drive cable - any length will do - and screw the other end of the cable to the speedo drive on the tranny.

With the tranny full of oil you're ready to test the overdrive. First make sure the tranny IS NOT IN REVERSE! Engage any of the forward gears 1 thru 4 and turn the transmission with the drill. There should be some kind of reading on the tach/speedo when this is done. Note this reading. Now with the drill turning the tranny over, engage the overdrive manually by rotating the operating lever clockwise. This is the little lever mounted to the operating shaft, on the side opposite where the solenoid goes. It will have a small hole in the end which when aligned with a similar hole in the side of the OD should engage the overdrive. A 12% lower reading should result on the tach/speedo when the overdrive is engaged.

Actually I usually dispense with the gauge when performing this test, having found that in first gear it is easy to discern the change in the pitch of the whine the drill makes when the OD is engaged. Since the reduction is a simple ratio, first gear gives the most change in RPM.

If you have a battery laying about you can use this setup to adjust the solenoid operating lever and make sure it actually works. I have found a few instances when simply aligning the holes in the operating lever and OD block was not sufficient to engage the unit. You can also wire up the inhibitor switches and insure they work too.

Michael Porter wrote:

There's not much to see without disassembly, as best as I can gather from the GT6 overdrive. I honestly don't know what the effect of a kerosene slosh might be, but I would guess that, unless there's a ton of sludge in the thing, it won't do a lot of good. From what I've seen inside mine, and some of the comments on the list recently, there seem to be three or four things to be concerned about:

1) Condition of the facings on the sliding member and the condition of the annulus (derived from inspection after disassembly on mine)

2) Operation of the solenoid (both windings)

3) Operation of the gear selector cut-out switches

4) Pump operation

Apart from checking the solenoid and switches, there's not much more that can be done on the bench without either disassembly or making a helluva mess. I'd say you're looking at changing it out, insuring that the fluid is changed and then wiring up the OD, and doing a test drive... if it works, great, if not....

Brian Kemp wrote:

Put clean oil in, run it, change the oil after a good drive or two. Change the oil again after several hundred miles. Don't forget the magnet in the bottom of the O/D. All I've done is check the electrics. With it hooked up (ignition on, engine off), you can hear the OD relay click on when the trans is put in the appropriate gears when the switch is on. You should also make sure the relay doesn't click in 1st and reverse. You can also watch the solenoid operation and verify it is set. My OD hasn't had any mechanical problems, so I can't tell you anything there.

Ken Nuelle wrote:

Fill the unit w/ oil, get a 1/2" chuck drill, and chuck it to the input shaft. Whilst spinning the tranny gears in 1st, in the forward direction, operate the OD lever and engage it. The rpm and hence the whine should go down a bit. If you have a speedo or tach about you can connect it to the trans and check the reading this way as well. Don't know any good way to check the gearbox out besides the obvious.

Bill Kelly wrote:

I'm forwarding a bit I wrote in early '97. Unfortunaely, the only thing I know about OD is that you can't see squat through the top cover, but at least this might help with the basic gearbox. (Bill sent me a fax and I will try and post that info to the list sometime this week) BTW, don't bother "flushing" with kerosene. Drain the box, and mop out any bits of crud you can see afterward, then refill with GL4.

And finally the ever helpful Fred Thomas wrote :

It would be best to have me put it in my car and test it for about a year just to make sure it's O/K for you to use, that way you will not have to ever worry about a let down on the road side. Just one of many options you have. Thanks Fred!!!!

And Now, A Little About Overdrive Wiring

The D-type transmission overdrive unit is on the right side of the transmission and can be accessed from below the car. The J-type transmission overdrive unit is on the left side of the transmission and can be accessed from inside the drivers compartment after removing the transmission cover.

Vic Whitmore writes

Before you start chasing the solenoid or pulling the tranny cover, you should check the switch in the gear shift. It is prone to corrosion. Inside the switch is a rolling contact that rolls onto two contact surfaces. Over time, the roller and contacts tend to corrode and you will get intermittent engagement of the OD.

The easiest way to diagnose this is to remove the shift knob to reveal two wires that connect to the switch. Connect the two wires together thus by-passing the switch. Take the car for a run to see if the OD engages and stays engaged. You can open and close the circuit to check for consistent operation. If this works OK, your problem is in the switch. You can very carefully open the switch by prying the bent over tabs. Take care as there is a spring inside that will fly off into "Never to be Found Land". You can clean the surfaces with fine emery paper and reassemble. Or buy a new switch (too much money).

If the shifting is still erratic with the wires joined together, then you can start looking at the solenoid on the firewall, 3rd/4th gear switch on the tranny, then the solenoid. The solenoid can also have pitted and corroded contacts inside and is also easy to open and clean the contact inside with the same emery paper. The tranny switch and solenoid require significantly more effort to get at and replace so leave these to the last. You may even find an intermittent wiring connection during the early process and save a lot of work.

Dean Dashwood writes

First remove the gearbox tunnel as described in Haynes, but replace the gearstick. Towards the front of the car, mounted on the gearbox, is a unit with two wires coming out of it (the inhibitor switch). The resistance between to two wires should be 0 when in 3rd or 4th, open circuit at all other times (test with a multi-meter or a test lamp). One of the two wires should have 12V when the ignition is on. The other goes to the switch in the gearstick. There should be 0 resistance between this and one of the wires on the solenoid when the switch is in the "In" position. Finally, there's an earth wire that goes from the solenoid to the gearbox case.

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