MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - overdrive frustration

Hi All
Im looking for a little guideance regarding the overdrive on my 68 roadster, LH type.
Ive rebuilt the car and covered a couple of hundred miles with no overdrive, the gearbox and o/d was rebuilt in 2008 and lay on my garage floor until 2011 when it was actually mated to the engine and put in the car but its never worked.
this week ive had some time to do some investigation, first of all the oil level is just below the high mark on the dipstick, with the car in third or fourth gear im getting 12+ volts at the inhibitor switch, ive taken the solenoid out which looks new, the 3/16 ballbearing is present and in the correct position as are all 'o' rings, ive taken the pressure relief valve out all looks good.
Ive just taken it for a run and still nothing when i flick the o/d switch.
I know its well out of warrenty, should i be looking at something else on the o/d?
Hoping for some help as i really dont know what to do next.

frustrated Pete
PJ Thompson

Test the amperage draw of the solenoid. It should be 800 ma, just under 1 amp. Mine was acting up and it turned out to be a part time open circuit in the solenoid windings. RAY
rjm RAY

Hi

I don't have an MGB (yet) but I assume the solinoid is the same as the Jaguar MK2. If this is the case the solinoid has 2 windings. The first winding draws a heavy current until the solinoid is closed. The integral switch then changes to the light winding (800ma) just to keep the solinoid closed. Hope this helps.

Ronnie
RA Potter

Thanks Ray, Ronnie

I have a multimeter but not really sure how to check what amperage is being drawn.
Do i select 3rd or 4th gear flick the switch into O/d then put a probe into the solenoid bullet connector and the other to earth.
As you can gather electronics is not a strong point :)

Cheers Pete
PJ Thompson

The D o/d has 2 coils, not the LH
Disconnect the bullet at the winding, check it for resitance to ground 10/20 ohms or so will draw the current mentioned.
Check that 12v is reaching this point. a probe into the bullet will do this, dont forget to change from R to V.
Yuo should also check that when the load is on the current is available. If you have a 1A capable meter, put it in series between the solenoid and feed wire. If not check you still get 12 v when its all connected up and drawing 1A-ish. Yuo can spike the lead to the winding with a pin or use a croc clip arrangenment to do this, just dont get the 12v to gnd.
A final check would be a lead direct to the battery, again be careful! as you connect the solenoid it should click. If it measures 0 ohms dont fo this!
Stan Best

The LH solenoid *may* click, but it will only click once if it has been manually pushed right back while dismantled, and not electrically operated yet. Once it has been electrically operated once it is unlikely to click again, even when it is working correctly. The earlier D-type should click every time.

You don't need to disconnect any bullets, just connect an *voltmeter* between the two connections on the manual switch, which should be off, the ignition switch should be on, and the gearbox in 3rd or 4th. If you see 12v registered then you have electrical continuity through the whole circuit. The reason for using volts rather than amps is one of safety, an ammeter is effectively a short-circuit, if one probe happens to touch earth while the other is on the white wire at the switch you are short-circuiting the battery.

If you do see 12v then very carefully do the same test but this time on the one amp range, and you should see the 800mA or so registered.

If you didn't see 12v, or you did but you see much less than 800mA, then you have an open-circuit in the first case or a bad connection in the other. In either case with the ignition on (remove the white wire from the coil +ve to stop it overheating), the manual switch on, and the gear lever in 3rd or 4th, trace the voltage through both sides of the manual switch, both sides of the bullet connector between the yellow from the main harness and the yellow or yellow/red of the gearbox harness, which is with the mass of other by the fusebox, and both sides of the gearbox switch. Note that you need to check both sides of the gearbox switch, if you only see 12v one side then the switch probably needs resetting or may be faulty. Sometimes pulling the gear-lever to the right and back while in 4th gear can operate the switch if it needs resetting. They were originally fitted with a couple of fibre washers, check it is tight, and if so remove it and remove a washer and try again.

If you do have 12v both sides, then check both sides of the bullet between the switch and the solenoid, and if you see 12v there then either the solenoid winding is open-circuit, or it's making a poor connection with earth. I think this is simply the end of the winding trapped between the solenoid and the body of the OD, so could have broken off.
PaulH Solihull

Thanks Stan, Paul

Ill carry out these tests tomorrow and report my findings, ive been too busy driving the car today (happy days) even better if O/d was working

Cheers Pete
PJ Thompson

its been a long time since starting this thread but ive managed to do some checks today with a new meter.
In 3rd and 4th ignition on im underneath with a meter on the inhibitor swith checking voltage, got the wife to flick the o/d swith and getting 12v.
Next check i pull apart the yellow wire connecting the solonoid to check amps, connect my meter to the yellow wire and brown solonoid lead in 3rd and 4th get the wife to flick o/d switch, checking amps with the o/d selected its drawing 830mA but still no o/d

whats next to check ( oil level spot on)

Cheers Pete
PJ Thompson

I ran into a similar problem where the solenoid was getting the correct amount of power, but the OD failed to engage. It turned out to be a sticking solenoid plunger. Upon removal of the solenoid base plate, the plunger and ball came out. I was amazed at the rough condition of the plunger. It was very poorly machined from the factory. I smoothed down the perimeter of the plunger using an oil stone. Upon re-installation, the OD worked perfectly again. RAY
rjm RAY

Pete-
If the Overdrive unit is not operating, the best electrical test to perform is one of the few occasions when you should check for current rather than voltage. Even if 12 Volts is measured at the solenoid, you do not know if the solenoid windings are good. You could measure the resistance (about 15 Ohms), but that is at a very low current, and the standard current may be making it go open-circuit. The same problem is likewise encountered with the lockout switch. These both can appear to be of high resistance when measured with a test meter, or appear to have continuity when measuring open-circuit voltage. The only accurate test is performed with both its design voltage and design current, i.e., with 12 Volts and the solenoid as a load. Having voltage at the solenoid isn't enough if the solenoid itself is faulty. This is one of the very few occasions when an ammeter is useful for diagnostics. It is important to perform all testing at the appropriate connection. Just which connection is appropriate depends upon with which version of the wiring system your car is equipped. On a system that employs a dashboard switch, the connection on the back of the switch is best. On a system that employs a switch in the knob of the gear change lever (gear shift lever), find where the connector in the engine bay where the yellow wire from the main wiring loom (harness) joins the yellow or yellow / red wire in the transmission wiring loom (harness). Disconnect this and insert an Ammeter, set to 1 Amp. Turn on both the ignition and the manual switch, and then select an Overdrive gear. You should see 800 mA being drawn, indicating that there is full circuit continuity, and so the problem is either mechanical or hydraulic. If you do not have an ammeter, then insert a test-lamp instead, and hopefully that will glow, again confirming continuity although not the correct current.

If no current is flowing only then should you start looking at voltages. The first place that you should examine is at the connectors that you have just parted. There should be 12 Volts coming from the main harness. If not, then you need to check at the manual switch if on the dashboard or on the steering column. If you have voltage at those connectors, then you need to check at the transmission lockout switch, and the manual switch if on the gearlever.

However, if you find that no current being drawn, then check to be sure that you have 12 Volts on the yellow wire that comes from the main wiring loom (harness). If it is not being drawn, then check both sides of the manual switch on the dashboard. If you find 12 Volts there, then check the resistance to ground (earth) of the yellow / red wire or the yellow wire in the transmission wiring loom (harness). You should read 15 Ohms of resistance, if the circuit is open, then you will need to check both sides of the lock-out switch on the gear change lever (gear shift lever) extension housing of the transmission. This is tricky to get access to since you will need to remove both the center console and the tunnel carpeting, and then remove the access panel on the top of the tunnel. You may even have to undo the machine bolts that fasten the transmission support crossmember to the chassis rails, and then lower the tail of the transmission with a bottle jack. If you see the correct current, push the connectors back together again. Test for 12 Volts on both sides, and then try driving it, as the problem could have been a just bad connection in that particular connector which may have been cleared by disconnecting and then reconnecting it. If you do not see the correct current, even with the correct current applied, the plunger could be sticking, so the first step is to remove the solenoid, taking care not to lose the little ball bearing on top of the plunger. Push the plunger to the bottom of the solenoid, apply power to it, and then check to see if the plunger is pulled to the middle of the solenoid. If that is the result, then the problem is most likely hydraulic in nature.
Stephen Strange

Ray
Ive had the cover plate off and solenoid out 3 times now, the solenoid is new but have not taken much notice of the surface finish so ill remove it again and check this
Thanks for your advice

Steve
Thanks for your very comprehensive instructions, it seems i need to carry out a few more electrical checks.
Ill let you know the results once ive done this.
Thanks to everyone so far for taking the time to explain things

pete
PJ Thompson

I have seen this happen on several LH types that either the pump plunger roller is reversed or the pump plunger is stuck down. I had a few conversations with John Esposito regarding these types of issues.

Rich 67B
Rich Boris

This thread was discussed between 08/06/2012 and 09/08/2012

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.