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MG MGB Technical - HELP OD Drop-out

Noticed a couple of thread/responses on OD units, so thought I'd try here before getting out the wrenches.

Have a 68MGB w/1980 blue label OD. Unit works perfect from cold start-up to somewhere in the 30 to 50 mile range. When it gets to full operating temp it begins to drop out on it's own.

What I have done:

1) I pulled everything from the bottom. Sprayed every available passage way with carb cleaner,
2) The pump is clean, unscored and tight and has very good suction when operated against my skin.
3) There were some issues with the relief valve spring. Couldn't buy a new spring by itself, so I sprung for the new valve assembly.
4) When I finally got to my wits end I ASSUMED that the solenoid coil was breaking down under heat. So I replaced it.
5) Obviously, while doing all this, a new filter screen.
6) To eliminate the switches I bypassed the 4th lock-out and installed a new toggle switch in place of the gearshift lever switch.
7) I've tried almost every type of 'recommended' oil. It's currently running 20W-50.

What I haven't done:

I have a new relief valve plug to drill and tap for a pressure gauge. Hopefully I'll have that done this weekend. I don't know the exact pressure it's running at, but it operates so well cold and warm that I have doubts there is problems there.

One last thing:

When it's hot and the OD kicks out I can: Take it to about 50MPH - switch the OD in and coast - RPM drops like it's in OD - but accelerate and have no OD.

BUT: if I accelerate to 50MPH - switch the OD in and coast - RPM drops like it's in OD - then (without accelerating) switch it out - I can feel it engage into non-OD mode and the RPM's come back up.

I have this strange burning sensation in the pit of my stomach that I'm going to have to disassemble the unit and replace the sliding member.

H-E-L-P

Tom
T Lea

You haven't already asked this on the MGs mail list have you? If so you are going to hear the same response I gave there (without all the aggro I hope).

I'm guessing you have an American 68 with a 4-synch gearbox and LH OD and the manual switch on the column

The first thing you should do, as it is the easiest, is to eliminate electrical problems. These could be *anywhere* - any of the many bits of wire, bullet connectors, spade connectors, switches, solenoid and the solenoid connection to earth.

The easiest way of doing that is to insert an ammeter somewhere in the circuit, with the meter positioned in the cabin, and go for a drive.

The simplest place to insert the ammeter is at the manual switch on the dash on those cars that have them, but with the manual switch on the column it will have to be at the bullet connector in the yellow wire where the main harness joins the gearbox harness by the fusebox. Part the connector, and connect the meter to the two halves of the connection, observing polarity i.e. meter +ve to the wire from the main harness and -ve to the gearbox harness. You will need the meter in the cabin so may have to extend the leads. This circuit is unfused so make sure none of the connections can come into contact with anything else including the body.

The LH O/D should draw about 1 amp at 14v (running), or about 800mA at 12v (engine off ignition on). If you see about that with the OD operating, but the ammeter current suddenly drops when the OD drops out, then you have a intermittent electrical connection somewhere in the circuit, which is good news as it should be the easiest and cheapest to cure. If you have significantly less than those currents to begin with, and it doesn't really change when the OD drops out, then there is *permanent* bad connection somewhere giving a marginal current to the solenoid. This could well cause the OD to drop out as it warms up, but ordinarily I'd expect that to cause OD to drift in and out on its own before dropping out altogether.

There are further techniques to determine *where* in the circuit the break is occurring (or if you don't have a suitable ammeter) but I'll leave it there until you come back.
PaulH Solihull

Aggro not from yourself, I should have added, but from another.

As a result of two (one?) questions about the OD in recent days I've expanded electrical diagnosis here:
http://www.mgb-stuff.org.uk/gearstext.htm#diagnosis
plus added a bit of non-electrical diagnosis.
PaulH Solihull

Sounds to me as if the lining has come detached on the cone clutch - the late blue label OD units have the lining bonded on rather than riveted as on the early units.

Another possibility is that the pump ball valve spring is too weak - stretch it so that it reaches the top of the hole in the plug and the ball sits high.
Chris at Octarine Services

Hi Tom,

When you replaced the solenoid, did you replace the rubber o-rings? I have a similar problem with the overdrive on my 73 GTV8. I was told all sorts of things and then I spoke to a guy from the Midlands who used to work for Laycock and now specialises in rebuilding overdrives. He suggested the o-rings. They cost about 50p, took about 15 min and solved the problem. I still have a brand new solenoid in its box!

Cheers, David.

David JM

Yup. Replaced both of them, and tapped the ball into place. And, replaced the non-return valve assy and the ball sits proud on the spring.

0 - 600 PSI gauge is ordered and will be here this week. Sat I'll hook up the gauge and ammeter and see what I get.

Tom
T Lea

Hi Guys,

My 70BGT has the same OD problem as Tom's:

1. first 20-30 miles from cold, the OD is perfect, in & out as I flip the dash switch;
2. Then, the OD would not engaged if I just switch on, I have to back off the gas pedal to let it engaged and the engagement is very slow. And have to keep very light throttle. It would disengage itself if I increase the rev say from 2k to 2.5k;
3. After another 10 miles, the OD then does not work at all. It just does not response.

Today, I just put an ammeter in the circuit as Paul advised in his website, the reading is 850ma when working, it reads 800ma when above condition 3 happens.

Is it soleniod problem or the relief valve spring or the non-return valve spring?

Thanks in advance
Ennio
Hong Kong
Ennio Wong

Sounds like a hydraulic or clutch (OD clutch, not the driver operated clutch) problem in your case Ennio, next step is to do a pressure test.

However I'm surprised it is only 850mA when driving, with the system voltage increased from 12v with the engine stopped to up to 14.5v when driving it should be closer to 1 amp. In tests both my ODs operated reliably at half normal current, I had to reduce it to about 300mA before operation started getting erratic, but that was with wheel in the air i.e. minimal load on the transmission. Also if it consistently drops from 850mA to 800mA when the problem occurs that could be indicative of a problem with the solenoid plunger, the ball or its seat. These would all be revealed by a pressure test, if that is normal when the fault is evident then the problem lies further into into the hydraulics and clutch.
PaulH Solihull

Hi Paul,

thanks for your comment, tommorrow I'll go to buy a pressure gauge and make an adaptor to test the oil pressure.

Thanks!
Ennio
Hong Kong
Ennio Wong

Along with the O-rings, replace the ball bearing.
werner haussmann

"Along with the O-rings, replace the ball bearing."

Did that the first go-around. Tapped it into place also.
T Lea

OK: The light is beginning to shine in.

Attached a pressure gauge. At cold start (about 80F) the OD produced 440 PSI. The farther I drove it and the warmer it got, the lower the pressure dropped. When I got about 15 miles the pressure at 55MPH and 2,500 RPM was a little less that 325PSI. Coming home I started out with the engine temp higher, and at the end of the 13 miles the pressure at 55MPH and 2,500 RPM was closer to 300PSI.

ALL the O-Rings in the lower end are new. The solenoid and ball are new and the ball was seated. The pressure relief valve is new, as well as a new non-return valve kit in the pump.

Where do I go from here????????????

As far as I can surmise, the only things left that I can get to from underneath are the pump body ($43), the pump plunger ($80) and the pump spring ($11).

The only other place I can see where fluid would by-pass would be the 45 cent piston O-Rings, but I have to disassemble the unit to replace them.

H-E-L-P

Tom
T Lea

It does seem to me then that fluid and pressure is leaking out from the high pressure circuit as the oil gets hotter and thinner, which could well be piston seals hence removal and dismantling unfortunately. It could be the solenoid failing to hold the ball against its seat hard enough but you have replaced both, which leaves possible damage to the seat although you have tapped the ball onto it. But leaving the possibly good news until last the pump has its own non-return valve involving a ball and seat, which is essential to develop the correct pressure. Like the solenoid this can also be accessed from the bottom of the OD, by removing the circular plug with the two holes inside the sump. Above this plug (with O-ring) is a spring, ball, seat, pump body and another O-ring, all of which should be forced out by the pump spring above those. I'm not sure if you can get the pump plunger out from this side, but at least you can examine the pump body. Again tap the ball onto the seat using a suitable drift. Also make sure the passage from the sump (looks like it goes via the lower part of the pump body) is clear.

I'm still slightly concerned about the solenoid current, even though it should operate reliably as less current than that. Clutching at straws, but just in case it saves a major job, I'd run a new wire up from the solenoid wire, using a new connector and cleaning its bullet, via the ammeter, to say the brown at the fusebox or direct onto the battery. If that shows much closer to an amp with the engine running at a fast idle, then you do have some bad connections somewhere in the normal circuit. But as I say, that is clutching at straws.
PaulH Solihull

Hate to say this, but the non-return valve in the pump assembly is new also. Looks like a tear down is in order (someday).

Last check I guess is the tension on the pump spring. I don't know what difference it would make, but as we're grasping at straws...............
T Lea

Well.... Went back through everything in the bottom end AGAIN, and could find no faults. Everything seems to work fine, but the OD pressure still drops as the transmission gets hotter. Drove on the Interstate (motorway) yesterday at about 80 degrees F and at about the 20 mile mark the OD surged out and in once or twice and then dropped completely out. I stopped for about 2 hours and then started for home. OD worked fine for 15 or so miles and dropped out.

Looks like a winter tear-down for piston seals and maybe a sliding member replacement.

DARN!

Would like to know if Ennio Wong from Hong Kong, got his fixed though.

Tom
T Lea

Mine was doing pretty much the same thing. After confirming that everything was working up to par, I re-checked the solenoid and this time it read as an open circuit. I replaced the solenoid and everything worked as normal. I know that you replaced the solenoid, but was it a new one or a spare that you had at hand? Mine was failing when the internal temperature went up. RAY
rjm RAY

I was thinking Tom had measured the current but I see that was Ennio. Tom should definitely measure the current from cold and through to when it starts failing, weak pressure from the solenoid could well be allowing oil to leak back past the ball valve. You should be getting at least 800mA, and in practice closer to 1 amp with the engine running. Low current, which could be due to a bad connection anywhere in the circuit, will prevent the solenoid plunger from applying the correct pressure to the ball. May well come to nothing, but it is another straw to clutch at and should definitely be eliminated before removing and dismantling the unit.
PaulH Solihull

Hi Guys,

Just back from a China Drive, so not too much work with the BGT. I ordered the non-return valve and pressure relief valve but not yet arrived. Will let you know the result.

Ennio
Ennio Wong

This thread was discussed between 11/09/2010 and 11/10/2010

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