overdrive disengages when driving
Posted By : David Styles, 08-Feb-2005, 09:49am
Morning Colin,..
welcome to the club...
It probably means that your gearbox oil is low. Overdrive is kept in by oil pressure from a little pump in your gearbox. The first warning you get that the oil is getting low is when overdrive becomes intermittent.
Incidently, this will usually first happen on a motorway, as the oil gets nice and hot, and therefore thin, and therefore if the level is getting low, it won't be able to hold the pressure that nice thick stodgy oil will.. (when its cold).
Get the car up on ramps/axle stands and top it up with some EP90 through that hole in the side of the gearbox. While its up there, its usually a good idea to do the diff too, and if you have an oil gun out.. there's always your trunnions.
btw... definitely don't grease them.
A fun first piece of maintenance...
Have fun driving..
cheers
Dave.
welcome to the club...
It probably means that your gearbox oil is low. Overdrive is kept in by oil pressure from a little pump in your gearbox. The first warning you get that the oil is getting low is when overdrive becomes intermittent.
Incidently, this will usually first happen on a motorway, as the oil gets nice and hot, and therefore thin, and therefore if the level is getting low, it won't be able to hold the pressure that nice thick stodgy oil will.. (when its cold).
Get the car up on ramps/axle stands and top it up with some EP90 through that hole in the side of the gearbox. While its up there, its usually a good idea to do the diff too, and if you have an oil gun out.. there's always your trunnions.
btw... definitely don't grease them.
A fun first piece of maintenance...
Have fun driving..
cheers
Dave.
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overdrive disengages when driving
Posted By : Van Hamlin, 08-Feb-2005, 11:24am
I'd check the electrics first, but you might as well change the oil as well since you will either be underneath or have the tunnel out.
The 2 weakest points are the wire that goes up the gear stick to the switch, and the wires to the solenoid, particularly the earth.
The switch wire gets damaged by being pulled about every time you change gear and the solenoid wires are damaged by heat. There is also the switch on top of the gear box which means the overdrive only works in 3rd and 4th. The wires to this can also work loose.
You can connect a 12V light across the solenoid, if this stays on when your overdrive cuts out then it isn't the wiring.
Van
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overdrive disengages when driving
Posted By : Nigel Gibbins, 08-Feb-2005, 11:55am
I expect the electrics will be okay as the overdrive is engaging initially without any reported problems. Although most o/d problems are traced to the electrics it would be wise to check them if you know what you are looking for (as Vin says).
The recommended oil for O/d is EP80 (not 90), but then again the MG manual recommends a 40 oil for the same overdrive... go figure??
It might be a good time to do an oil change on it though... you'll need 1.35 litres (I think) and atleast you'll have good clean stuff in there then
EP90 is right for the diff though.
If this 'top up' doesn't resolves the problem then the relief valve may need a 'clean' this can be done by removing it from under neath the car (gearbox in situ) but I'll referr you to the manual for the exact proceedure... it's not difficult at all.
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overdrive disengages when driving
Posted By : Chris Wilson, 08-Feb-2005, 01:32pm
One other possibility (if the other suggestions don't work!) is that the top fuse in the fuse holder may have become a bit worse for wear and causing intermittent breaks in the current going through it. (This happened to me which is why I'm changing over to blade type fuses but that's another story!!).
This problem is easily noticeable as the top fuse protects (in my car anyway!?) the overdrive electrics as well as the indicators, fuel/temp gauges, rev counter etc so if the fuse fails, all that lot will too!!
Regards-
overdrive disengages when driving
Posted By : Philip Rosenberg, 08-Feb-2005, 05:58pm
I had exactly the same problem and it was the oil level being low. As stated earlier the overdrive works by using the oil as hydraulic fluid to push the unit into overdrive. If the oil is low it can't do this.
With my box I had an oil leak, so was topping up the oil once a week. If you end up doing the same a little tip is to cut a hole in the drivers side of the fibreboard transmission tunnel, and then either cover it with a screw on plate or something similar. This way you can get to the gearbox filler through the footwell and don't have to jack the car up every time. Of course you do have to be carefull you don't spill oil on your carpets
Phil
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overdrive disengages when driving
Posted By : Michael Blow, 07-Apr-2005, 00:57am
Hi Colin,
This reply's a bit late (!) cuz I don't often browse these boards, but it could be something the others havn't mentioned. I had the same problem with my Mark IV, the O/D just suddenly wouldn't engage sometimes or would switch in-out-in-out-in-out before I lost it altogether. Except that the O/D would function fine later...
I soon worked out that if the O/D wasn't playing ball then I could often 'coax' it into engaging by wiggling the gear lever slightly in 3rd or top. I also noticed that the O/D only failed to work after a heavy traffic jam or on a hot day.
I traced it to the inhibitor switch which is mounted on top of the gearbox casing. The purpose of the switch is to only allow the O/D to be used in 3rd and top. My theory is that as all the gear linkage etc. got very hot then it expanded/loosened/whatever and thus stopped making proper contact with the switch, thus causing the O/D to disengage.
I solved the problem by bending the inhibitor switch bracket by 1mm or so, until it made a better contact.
Mike
This reply's a bit late (!) cuz I don't often browse these boards, but it could be something the others havn't mentioned. I had the same problem with my Mark IV, the O/D just suddenly wouldn't engage sometimes or would switch in-out-in-out-in-out before I lost it altogether. Except that the O/D would function fine later...
I soon worked out that if the O/D wasn't playing ball then I could often 'coax' it into engaging by wiggling the gear lever slightly in 3rd or top. I also noticed that the O/D only failed to work after a heavy traffic jam or on a hot day.
I traced it to the inhibitor switch which is mounted on top of the gearbox casing. The purpose of the switch is to only allow the O/D to be used in 3rd and top. My theory is that as all the gear linkage etc. got very hot then it expanded/loosened/whatever and thus stopped making proper contact with the switch, thus causing the O/D to disengage.
I solved the problem by bending the inhibitor switch bracket by 1mm or so, until it made a better contact.
Mike
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Overdrive Disengages When Driving
Posted By : Andrew Stanton, 21-Feb-2006, 06:59pm
There are three principle causes for o/d failure
1. Electrical problems, on which folk have already waxed lyrical.
2. Oil pressure failure, caused by insufficient oil in the gearbox. I'm a heretic on this. EP90 does the business. It's thick and it's gooey.
3. Oil pressure failure cause by clogged up oil filters in the overdrive unit.
If your o/d failure is intermittent I would suspect (2) above. Oil's viscosity changes with temp. If levels are marginal in your box then a cold oil will do the business but then fail as it warms up when it gets thinner. It's easily remedied. Top up the gearbox, and experience the improvement.
(1) above might give an intermittent problem. In my experience it's the switch at the top of the gear lever that is likely to be the culprit. You can test this by jury rigging a bypass switch. If this works with no problem the n replace the switch in the gear lever.
(2) is easier to diagnose. If the solenoid works (see earlier postings) and the oil level is ok, then it is likely that the filters need clearing. The following illustration is for a J type o/d. You can get at it under the car. The manuals tell you that you'll need a specialist tool to unscrew the large plug which retains one of the seals. I fond a pair of circlip pliers did the trick nicely.
There's two filters. 1. A flat magnetic filter just under the sump plate, and a corrugated cylindrical filter behind the largest of the three screw in plugs that are exposed once you get the sump plate off. Take both of these out and give them a damn good clean in petrol. Reassemble, top up the gearbox with nice treacly oil, and Bob's your uncle.
Overdrive Disengages When Driving -
Overdrive Disengages When Driving
Posted By : Andrew Stanton, 21-Feb-2006, 07:00pm
There are three principle causes for o/d failure
1. Electrical problems, on which folk have already waxed lyrical.
2. Oil pressure failure, caused by insufficient oil in the gearbox. I'm a heretic on this. EP90 does the business. It's thick and it's gooey.
3. Oil pressure failure cause by clogged up oil filters in the overdrive unit.
If your o/d failure is intermittent I would suspect (2) above. Oil's viscosity changes with temp. If levels are marginal in your box then a cold oil will do the business but then fail as it warms up when it gets thinner. It's easily remedied. Top up the gearbox, and experience the improvement.
(1) above might give an intermittent problem. In my experience it's the switch at the top of the gear lever that is likely to be the culprit. You can test this by jury rigging a bypass switch. If this works with no problem the n replace the switch in the gear lever.
(2) is easier to diagnose. If the solenoid works (see earlier postings) and the oil level is ok, then it is likely that the filters need clearing. The following illustration is for a J type o/d. You can get at it under the car. The manuals tell you that you'll need a specialist tool to unscrew the large plug which retains one of the seals. I fond a pair of circlip pliers did the trick nicely.
There's two filters. 1. A flat magnetic filter just under the sump plate, and a corrugated cylindrical filter behind the largest of the three screw in plugs that are exposed once you get the sump plate off. Take both of these out and give them a damn good clean in petrol. Reassemble, top up the gearbox with nice treacly oil, and Bob's your uncle.
Overdrive Disengages When Driving