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MG MGB Technical - LH overdrive has a mind of its own

I have a rebuilt '72 4 synchro/od trans in my '67 B. For 20 years it has performed flawlessly. However, when I took the car out for a drive the other day the od seems to engage sporadically. The od isolation switch was bypassed 15 years ago, being replaced by a power cutout relay that operates when the reverse lights are powered up thus preventing od engaging in reverse. I checked the wiring and I'm getting power to the solonoid all the time when the switch is on. I even drained the 20w50 oil and tried putting ATF in to no avail. I believe that the unit is operating right at the edge of proper pressure, but without the correct gauge and fittings I can only guess what the pressure is. It sometimes engages normally and other times not at all. I'm in a bit of a quandrey as what to pull apart next. The oil and filter were changed only a year ago and the solenoid checks out as being good. RAY
rjm RAY

My only thought went to experience that I had, and it was electrical, which you appear to have eliminated as a possible cause.

My OD cut in and out or would or would not engage at whim. I finally figured out that the wire going to the OD box on transmission was down to just a strand or two of wire left. I replaced wire and it has been fine since. Wire looked fine, but when really getting in there with a light, I finally saw the reduced number of strands being connected.

Good luck.
R.W Anderson

Mine had a similar problem. Cut the wire back to where it was solid and soldered a new wire in place. No change. RAY
rjm RAY

This is one of those rare occasions when an ammeter in series with the solenoid supply is useful - if you see the correct current (about 800mA) but the ratio hasn't changed then the problem is mechanical or hydraulic. You can remove the solenoid - looking out for the little ball-bearing that should be on the top - and check the plunger is free in the solenoid, and lightly tap the ball onto its seat with a hammer and drift, but other than that you won't really know more until you put a pressure gauge on it 400-420psi at 30mph.

Why on earth did you bypass the lockout? You have simply substituted one identical switch with another, but added more components, wiring and connections to fail and wreck the OD. Using OD in 1st and 2nd is not a good idea, it's not strong enough, Triumphs with OD 2nd had a stronger unit and V8s were restricted to 4th only.
Paul Hunt 2010

Paul, I've been running without the isolator switch for almost 20 years with no ill effects. Yes, overloading the od in first and second can cause the sun gear to fail, however I don't abuse the od when it is engaged in second gear. It has performed flawlessly all these years and is much tougher than it is given credit for. My wiring system, which I wrote an article about for AMGBA in '93, is simple and straight forward using an off the shelf cut out relay to open the power circuit to the od whenever the shift lever is placed in reverse. When placed on jack stands and run up to 30 mph the od shifts almost perfectly. It is out on the open road, when driving along the Pacific coast, that it acts up, sometimes operating normally, other times refusing to engage. I guess that I'll have to crawl under the car and start taking things apart. I'm getting too old for this @#$%! RAY
rjm RAY

Hi guys,

I had an intermittent fault on my overdrive the other year - a 1972 roadster. It turned out to be the overdrive switch. I took this apart, cleaned it and put some contact grease in it before re-assembly. Fault gone.

HTH

willyphixitt
W A Nixson

"I've been running without the isolator switch for almost 20 years with no ill effects"

The point is you won't know until it's too late.
Paul Hunt 2010

RJM-
From what you describe, I suspect that Willy is right. Check your Overdrive isolation switch inside the remote control tower. If there is any intermittence in continuity at all, especially by wiggling the shift lever, you can fix the problem from inside the car without having to pull the engine / transmission package out.
Stephen Strange

The overdrive isolation switch was bypassed 20 years ago. I have a cut out relay wired into the reverse light circuit to prevent od engagement in reverse. It is still functioning and carrying current to the solenoid when the od switch is turned on and opening the circuit when the shift lever is moved toward the reverse position. RAY
rjm RAY

Sounds electrical. Your reverse light circuit could be intermittent (seen many a vehicle with reverse lights on/flashing while going forward). Try moving the gear stick around while driving with the OD switched on.
Could be an intermittent on/off switch. My cars OD switch needs pressure to the right when switching on to activate the OD ( one day I’ll fix it job). Try moving the switch toggle around while driving with the OD switched on.
Lastly, use an ammeter as Paul says, or if you don’t have access to one, use a temporary switch from a new power source and bypass every thing to solenoid.
Check the solenoid for lose and dry soldered joints.
You would then have the division between electrical and mechanical.

Richard.

RH Davidson

If you don't have an ammeter use a test-lamp with a low-wattage bulb *in-series*. The relatively low resistance of the winding of about 15 ohms should still allow the bulb to glow to some extent. But an ammeter is best as that will show a high-resistance, as well as an open-circuit.
Paul Hunt 2010

I have all of the test equiptment mentioned above. Moving the shift lever makes no difference since the od isolation switch is no longer wired into the circuit. The reverse light switch is functioning normally as I can see my test light go out when I move the shift lever towards the reverse gate. My od switch is the hooked one that was standard on a '67 with od. It is mounted on my Aamco console in front of the shift lever and is easily accessed when resting my hand on the shift knob. I haven't been able to run a separate power lead to the solenoid yet due to bad weather. I'll keep you updated on my progress. Thanks for all the helpful advice. RAY
rjm RAY

"I haven't been able to run a separate power lead to the solenoid yet"

That still won't give you the full picture as the solenoid itself will still be an unknown quantity. You need an ammeter in series to make sure you have the correct current (or a test-lamp and at least get a go/no-go), and it will be much easier to interrupt the connector where the rear harness joins the main harness by the fusebox and get th full picture than grovel about underneath and only get half.
Paul Hunt 2010

I'm going to run a power lead, through an ammeter, from the starter solenoid mounted on the fender. From there it will go through the firewall and connect to a new switch that will then go back through the firewall and down to the solenoid, bypassing all of the present wiring. This will give me amperage draw and a known good power lead to the solenoid. From there it will be an internal problem, either a faulty, intermittent solenoid or a sticking ball and plunger. Thanks for the advice. RAY
rjm RAY

That seems double the trouble to me, you will prove the solenoid it is true but none of the existing circuitry, which presumably you want to go back to eventually. Just put an ammeter in series with that and you will either prove the whole lot is working as it should, if not then you can start breaking down the circuit to find where the fault is.
Paul Hunt 2010

Paul, thanks. I think that I'll try that first. I can wire the ammeter into the circuit at the reverse cutout relay that's mounted under the dash with a couple of spade connectors on 2 wires going to the gauge. RAY
rjm RAY

Well, it turned out to be the od solenoid coil. Though it tested good at 14 ohms and .8 volts, when removed it turned out to be giving inconsistent readings, indicating an open circuit. I replaced it as well as the 2 O rings on the valve assembly above the solenoid. I decided that it was time to service the trans as well, since according to my records the last time I did this was in 1998. The filter and magnets looked as if they were cleaned a week ago! The one thing that surprised me was how rough the surface of the plunger was. It appeared to have been turned on a lathe, but the final cut had been neglected. I cleaned it up with a flat file, light sand paper and a Scotchhbrite pad. I then replaced the its O ring. I refilled the trans with 7 pts. of 30w non detergent oil that I had a heck of a time locating. The od now shifts instantly and is back to the way it was when I first installed it in 1990. Thanks to all of you who contributed to the fix. RAY
rjm RAY

This thread was discussed between 16/04/2010 and 07/05/2010

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