MGB overdrive solenoid - advice please
Chris Dennis
I've been having some trouble with the overdrive on my 1974 chrome
bumper MG BGT.
The overdrive suddenly disengaged at 70mph on the M4, and I traced the
immediate cause to the fact that all the oil had drained out of the
gearbox. (oops)
Having sorted out the leak and replaced the oil, the gearbox is fine but
the overdrive still won't engage, despite having checked the overdrive
sump and valves and replaced various filters and o-rings. I've also
checked the electrics - 12 volts are applied to the solenoid at the
appropriate times, but there's no sound of it operating.
I've now taken out the solenoid. The coil looks fine, but it shows no
sign of generating any magnetism when 12 volts are applied. A meter
records the resistance of the coil as 13 ohms, which doesn't seem
unreasonable - the operating current would then be about an amp - is
that a bit high?
Can anyone tell me what the resistance of the coil should be, or suggest
any other tests I could do on it? (I haven't got an ammeter, but maybe
I could get hold of one.)
Any advice would be appreciated before I shell out 50 quid on a new
coil.
cheers
Chris
--
Chris Dennis
Bristol, England
Ian Edwards
Chris Dennis <chris....@secret.co.uk.invalid> wrote in message
news:1e5dz8z.1ayutjhiomwxeN%chris....@secret.co.uk.invalid...
I don't think from your description this is your problem, but . . . .
Have you checked the overdrive inhibitor switch?. It went high resistance
on my B, which meant that if you disconnected the supply to the solenoid at
the o/d, you could still measure 12v with a meter, but when the solenoid was
in circuit, the volts disappeared. Temporarily connecting a 12v 21w bulb in
place of the solenoid revealed the problem.
Good luck,
Ian Edwards
Good luck,
Oops - sorry, forgot to add my sig. last post.
--
Ian Edwards
Substitute IanE for NOSPAM to reply
Bob Walker
: Hello folks
: I've been having some trouble with the overdrive on my 1974 chrome
: bumper MG BGT.
: The overdrive suddenly disengaged at 70mph on the M4, and I traced the
: immediate cause to the fact that all the oil had drained out of the
: gearbox. (oops)
: Having sorted out the leak and replaced the oil, the gearbox is fine but
: the overdrive still won't engage, despite having checked the overdrive
: sump and valves and replaced various filters and o-rings. I've also
: checked the electrics - 12 volts are applied to the solenoid at the
: appropriate times, but there's no sound of it operating.
: I've now taken out the solenoid. The coil looks fine, but it shows no
: sign of generating any magnetism when 12 volts are applied. A meter
: records the resistance of the coil as 13 ohms, which doesn't seem
: unreasonable - the operating current would then be about an amp - is
: that a bit high?
: Can anyone tell me what the resistance of the coil should be, or suggest
: any other tests I could do on it? (I haven't got an ammeter, but maybe
: I could get hold of one.)
: Any advice would be appreciated before I shell out 50 quid on a new
: coil.
AFAIR - and its a long time ago now - the Laycock-type
overdrive solenoid unit had two coils. The first was to do
the initial pull in and took around 20A and the second was
the holding coil. It sounds like the second of these is still
connected. There should be a changeover switch operated by
the plunger.
I would take off the solenoid, check it out as above on the
bench and with no load. Also try pulling on the operating arm
of the overdrive to see if that's jammed - it shouldn't take
much force to move it.
It sounds like an electrical failure. The loss of oil may
have been a coincidence. I would have thought that low oil
would make it slip erratically before finally giving up,
unless the oil loss was very rapid. If it was electrical -
weren't you lucky !!
-------------------------------------------------------------
The opinions expressed are personal. They do not necessarily
represent those of my employer.
Bob Walker, BBC Research and Development Department.
Kingswood Warren, Tadworth, Surrey, UK.
Martin Watson
<rwa...@sung3.rd.bbc.co.uk> writes
>In uk.rec.cars.maintenance Chris Dennis <chris....@secret.co.uk.invalid>
>wrote:
>: Hello folks
>
>: I've been having some trouble with the overdrive on my 1974 chrome
>: bumper MG BGT.
>
>: The overdrive suddenly disengaged at 70mph on the M4, and I traced the
>: immediate cause to the fact that all the oil had drained out of the
>: gearbox. (oops)
>
>: Having sorted out the leak and replaced the oil, the gearbox is fine but
>: the overdrive still won't engage, despite having checked the overdrive
>: sump and valves and replaced various filters and o-rings. I've also
>: checked the electrics - 12 volts are applied to the solenoid at the
>: appropriate times, but there's no sound of it operating.
>
>: I've now taken out the solenoid. The coil looks fine, but it shows no
>: sign of generating any magnetism when 12 volts are applied. A meter
>: records the resistance of the coil as 13 ohms, which doesn't seem
>: unreasonable - the operating current would then be about an amp - is
>: that a bit high?
been able to find a definitive figure.
>
>: Can anyone tell me what the resistance of the coil should be, or suggest
>: any other tests I could do on it? (I haven't got an ammeter, but maybe
>: I could get hold of one.)
>
>: Any advice would be appreciated before I shell out 50 quid on a new
>: coil.
>
>AFAIR - and its a long time ago now - the Laycock-type
>overdrive solenoid unit had two coils. The first was to do
>the initial pull in and took around 20A and the second was
>the holding coil. It sounds like the second of these is still
>connected. There should be a changeover switch operated by
>the plunger.
>
>I would take off the solenoid, check it out as above on the
>bench and with no load. Also try pulling on the operating arm
>of the overdrive to see if that's jammed - it shouldn't take
>much force to move it.
not of this design. The coil moves a plunger which pushes a ball bearing
upwards. This ball bearing effectively operates the overdrive by
altering the flow of oil.
--
Martin Watson, Cheshire Classic Car Spares
E-mail --> mar...@cheshireclassic.demon.co.uk
Web page --> http://www.cheshireclassic.demon.co.uk
Voice --> +44 (0)1244 300270
Fax --> +44 (0)1244 300270
Mail --> Plemstall Lane, Mickle Trafford, Chester, CH2 4EN, U.K.
Chris Dennis
> In article <87c0c4$84g$1...@nntp0.reith.bbc.co.uk>, Bob Walker
> <rwa...@sung3.rd.bbc.co.uk> writes
> >overdrive solenoid unit had two coils. The first was to do
> >the initial pull in and took around 20A and the second was
> >the holding coil. It sounds like the second of these is still
> >connected. There should be a changeover switch operated by
> >the plunger.
> not of this design. The coil moves a plunger which pushes a ball bearing
> upwards. This ball bearing effectively operates the overdrive by
> altering the flow of oil.
Indeed. As I've now discovered (having mended my ammeter), the solenoid
coil on this Laycock LH overdrive (1974 BGT) takes about 0.8 amps when
connected straight across the 12V battery.
And the plunger only moves about 2mm: it's held in the centre of the
coil when the current is flowing, and drops down to rest on the face
plate when it's not.
Chris Dennis
> Chris Dennis <chris....@secret.co.uk.invalid> wrote in message
> news:1e5dz8z.1ayutjhiomwxeN%chris....@secret.co.uk.invalid...
> >...
> >
> > The overdrive suddenly disengaged at 70mph on the M4, and I traced the
> > immediate cause to the fact that all the oil had drained out of the
> > gearbox. (oops)
> >
> > Having sorted out the leak and replaced the oil, the gearbox is fine but
> > the overdrive still won't engage, despite having checked the overdrive
> > sump and valves and replaced various filters and o-rings. I've also
> > checked the electrics - 12 volts are applied to the solenoid at the
> > appropriate times, but there's no sound of it operating.
> >...
>
> I don't think from your description this is your problem, but . . . .
>
> Have you checked the overdrive inhibitor switch?. It went high resistance
> on my B, which meant that if you disconnected the supply to the solenoid at
> the o/d, you could still measure 12v with a meter, but when the solenoid was
> in circuit, the volts disappeared. Temporarily connecting a 12v 21w bulb in
> place of the solenoid revealed the problem.
Thanks Ian - you pointed me in the right direction. In fact I think it
was just a wobbly bullet connector that was causing too much resistance.
I replaced the bullet connector and cleaned up the spade terminals on
the inhibitor switch (not easy, given its convenient position), and it
works.
(Except that it was a bit slow to disengage which is a worry...)
NANS
> (Except that it was a bit slow to disengage which is a worry...)
>
> cheers
>
> Chris
> --
> Chris Dennis
> Bristol, England
>
Dear Chris,
I own a 1970 Volvo P1800E and although I didn't have electrical
problems the OD refused to engage for the first year I drove the car,
And probably hadn't worked for the previous owner as well. I solved
this problem when I used Bardahl's gear oil additive which seems to
have cleared up some small drillings and passageways. I have since
changed the oil and added more of this additive.
Regarding the slow disengagement, I found that this occured idf there
was incorrect level of oil , too high and too low. It also occured if I
used too heavy an oil. Volvo recommends using engine oil. Using engine
oil has solved the problem again.
What kind of oil should you be using?
Good luck
Nanda
Sent via Deja.com http://www.deja.com/
Before you buy.
Ian Edwards
> Thanks Ian - you pointed me in the right direction.
Glad to hear you're sorted.
In fact I think it
> was just a wobbly bullet connector that was causing too much
resistance.
> I replaced the bullet connector and cleaned up the spade terminals
on
> the inhibitor switch (not easy, given its convenient position), and
it
> works.
Yes, it's in a lousy position isn't it, I thought I might have to tilt
engine and gearbox to get at it - managed in the end though - removing
the reversing light switch helped.
>
> (Except that it was a bit slow to disengage which is a worry...)
>
> cheers
>
> Chris
Mine drops out slowly sometimes as well and always has done from new,
it seems to depend on the throttle position when I disengage the o/d.
I use the clutch when I disengage o/d now - OK engaging though.
Regards,
Chris Dennis
> > (Except that it was a bit slow to disengage which is a worry...)
> >
>
> Dear Chris,
> I own a 1970 Volvo P1800E and although I didn't have electrical
> problems the OD refused to engage for the first year I drove the car,
> And probably hadn't worked for the previous owner as well. I solved
> this problem when I used Bardahl's gear oil additive which seems to
> have cleared up some small drillings and passageways. I have since
> changed the oil and added more of this additive.
>
> Regarding the slow disengagement, I found that this occured idf there
> was incorrect level of oil , too high and too low. It also occured if I
> used too heavy an oil. Volvo recommends using engine oil. Using engine
> oil has solved the problem again.
>
> What kind of oil should you be using?
I'm using Duckhams 20W/50 engine oil as specified, and I've checked the
level recently. I think I'll just have to keep an eye (and an ear) on
the overdrive for a while and see how it behaves.