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MG MGB Technical - Best means of bench testing LH overdrive solenoid

Hi,

I'm asking quite a lot of questions at the mo. What's the best way; I know there is some info by Paul Walbran but I don't quite understand it.

Ta

Iwan
Iwan Jones

Hi Iwan - if you energize the lead wire that comes out of the solenoid, you will be able to check for movement of the small cylinder that holds the small ball bearing pressure valve.

I removed the plate (Blue or Black depending on LH model) and you can actually see the piece move inside the solenoid.

There is one gasket and three o-rings as part of that assembly.

You will also want to check the overdrive switch and the possible other switches on your gearbox shift extension.

John Twist has excellent videos on YouTube about these tests. John Twist University Motors.

Good luck
DLD

Hi Iwan - being clear as mud was I? Which part of the explanation did you want help with?
Paul Walbran

Should take 800mA to 1 amp when energised from 12-14v.

On-car inserting an ammeter where the gearbox harness wire joins the main harness will tell you the state of the supply, the manual switch, the gearbox switch as well as the solenoid and all the connections in between. Solenoid earthing like any connection or component can be iffy sometimes as some say it is only the end of the winding trapped between the solenoid body and the OD case, which is why it is best to test it in-situ rather than on the bench.

When checking for solenoid plunger movement you may have to pull it back (down) manually in order to see it rise when powered. Unlike the D-type there is no return spring.
paulh4

Hi Paul, like your tech stuff on your website. I'm in the process of altering some brake levers to your specs!!

Here is the quote from your website re: solenoids

I find the best way to check it is to connect the coil wire and a suitable casing earth to a battery so it is activated, then sit the unit on some scales (bathroom scales will do) and push down into the little hole in the end of the snout until the solenoid plunger moves, noting the load on the scales when it does so. Anything less than 7 lb means the solenoid is marginal.

a) Casing earth .... I take it that means earth the solenoid housing to the battery?

b) When pushing down the plunger, are you trying to push down on the little ball through the hole and not touch the sides i.e. the plunger itself; or, push down through touching the sides the hole thus moving both the plunger and the ball?

Not quite sure.

Ta

Iwan
Iwan Jones

Hi Iwan

Yes, earth the solenoid casing to the battery.

The idea of the push test is to replicate what the oil pressure is doing - trying to blow the ball back off its seat. So push through the little hole in the very end onto the ball itself, and not any part of the housing. Of course, the ball in turn is pushing against the plunger so when one moves the other will also.
Paul Walbran

Mmmm, looking back, a little clarification:
I use a pin punch as the prod through the little hole onto the ball.
The handle end of the punch I sit on the scales, with the pin end upwards and then push the solenoid downwards onto the pin until the plunger gets pushed back.
It's a little bit of a juggle, especially by the time the wires carrying the current to and from are floating around in the mix, but well worth it as it gives a positive check on solenoid strength and whether it is strong enough to hold the necessary ~400psi.
The test has to be done with the complete solenoid assembly, leving bits off weakens the magnetic field through the core and gives a false reading.
Paul Walbran

Thanks Paul,

Great these little tips, I'll give it another go.

Iwan
Iwan Jones

This thread was discussed between 18/07/2016 and 20/07/2016

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