Overdrive electric shock
Discussion
I've just got an MGB, when I disengage the overdrive if I'm touching any metal part of the steering wheel I get an electric shock. It doesn't happen when I engage the overdrive. Does anyone have any ideas why, the only thing I can think of is a bad earth somewhere but if that were the case then surely the O/D wouldn't work in the first place.
Must stop laughing at your expense, but I am finding this v amusing.
What age MG? Where is the o/d switch? Top of gear lever, dashboard or steering column? The gear lever one is a bit of a b
h to get at the wiring, the rest marginally easier. But yes, it does sound like a bad earth.
An other alternative is static shock. I got zapped a few times last week, but realised I was wearing plastic poly nylon something walking trousers and a fleece and it was just static shock. I don't get it when I wear jeans and a leather jacket.
What age MG? Where is the o/d switch? Top of gear lever, dashboard or steering column? The gear lever one is a bit of a b
h to get at the wiring, the rest marginally easier. But yes, it does sound like a bad earth.An other alternative is static shock. I got zapped a few times last week, but realised I was wearing plastic poly nylon something walking trousers and a fleece and it was just static shock. I don't get it when I wear jeans and a leather jacket.
alfa pint said:
Must stop laughing at your expense, but I am finding this v amusing.
What age MG? Where is the o/d switch? Top of gear lever, dashboard or steering column? The gear lever one is a bit of a bh to get at the wiring, the rest marginally easier. But yes, it does sound like a bad earth.
An other alternative is static shock. I got zapped a few times last week, but realised I was wearing plastic poly nylon something walking trousers and a fleece and it was just static shock. I don't get it when I wear jeans and a leather jacket.
The silly thing is that I warned my wife when she drove it today, I should have kept quiet and seen the reaction.What age MG? Where is the o/d switch? Top of gear lever, dashboard or steering column? The gear lever one is a bit of a b
h to get at the wiring, the rest marginally easier. But yes, it does sound like a bad earth.An other alternative is static shock. I got zapped a few times last week, but realised I was wearing plastic poly nylon something walking trousers and a fleece and it was just static shock. I don't get it when I wear jeans and a leather jacket.
It's a 1969 with the switch in the gearlever, the steering wheel is a wood covered motolita type.
1969 with the switch on the gear lever? Doesn't sound original - I thought only the post 75 ones had that arrangement. Suspect you're going to have some fun working out where the bad earth is - when it comes to electrical gremlins, I have a vague bash around with my voltmeter for a bit and then call in a pro as it's not my forte.
AtticusFinch said:
If you had a stray 12v would that really be enough to give you a shock??
I'm assuming It's not 12 volts, how many volts does an ignition coil generate? I admit I know very little about electrickery but I thought that a solenoid works by a coil creating a magnetic field when it is energised. I can only think that in this case when it is disengaged the electric charge is earthing via me, I do stand to be corrected though. It won't be the 12v causing a shock (unless OP has particularly delicate and moist skin!). Much more likely is the back EMF from the overdrive solenoid when it's powered off which could be pretty high (maybe 100v or more) which would explain why it only happens when the OD is disengaged. There is obviously a leakage path through the switch somewhere so this voltage spike is finding it's way to ground through the driver.
A rectifier diode connected in parallel with the solenoid coil would snub this voltage (make sure polarity of diode is correct otherwise it will just smoke and blow the fuse!)
A rectifier diode connected in parallel with the solenoid coil would snub this voltage (make sure polarity of diode is correct otherwise it will just smoke and blow the fuse!)
Mr2Mike said:
It won't be the 12v causing a shock (unless OP has particularly delicate and moist skin!). Much more likely is the back EMF from the overdrive solenoid when it's powered off which could be pretty high (maybe 100v or more) which would explain why it only happens when the OD is disengaged. There is obviously a leakage path through the switch somewhere so this voltage spike is finding it's way to ground through the driver.
A rectifier diode connected in parallel with the solenoid coil would snub this voltage (make sure polarity of diode is correct otherwise it will just smoke and blow the fuse!)
Tanks for that, someone else mentioned EMF but being totally stupid where the magic of electricity is concerned how do I wire the diode in parallel, which wires does it run between?A rectifier diode connected in parallel with the solenoid coil would snub this voltage (make sure polarity of diode is correct otherwise it will just smoke and blow the fuse!)
It should ideally be wired across the solenoid coil. Something like a 1N4002 (or 1N4003,4004,4005) will cost a few pennies. I think the MGB overdrive solenoid is always grounded on one side and the switch supplies +12v to the other side of the coil? One end of the diode will be marked with a white band (this is the Cathode) and needs to be connected to switched +12v side of the solenoid coil, the other end should be grounded (0v).
You could simply connect it between the output of the switch on the gearlever and ground. This means it won't provide any clamping action if you disengage the OD by shifting from e.g. 3rd to 2nd, since the gearbox OD switch is downstream of the gearlever switch, but the gearbox switch should prevent any shocks as long as that isn't leaky as well! Have you tried shifting from 3rd Overdrive to second without manually disengaging the overdrive? Do you still get a shock?
I think this is how the MGB O/D is wired, and I've added suggested locations for the diode. I don't know the wire colours, perhaps someone else can provide this?
If you don't have any it will probably be cheaper to buy a pack from eBay (e.g. here) as buying a single device from an electronics distributor like Farnell or RS will cost a fortune with minimum order quantities and high postage.
You could simply connect it between the output of the switch on the gearlever and ground. This means it won't provide any clamping action if you disengage the OD by shifting from e.g. 3rd to 2nd, since the gearbox OD switch is downstream of the gearlever switch, but the gearbox switch should prevent any shocks as long as that isn't leaky as well! Have you tried shifting from 3rd Overdrive to second without manually disengaging the overdrive? Do you still get a shock?
I think this is how the MGB O/D is wired, and I've added suggested locations for the diode. I don't know the wire colours, perhaps someone else can provide this?
If you don't have any it will probably be cheaper to buy a pack from eBay (e.g. here) as buying a single device from an electronics distributor like Farnell or RS will cost a fortune with minimum order quantities and high postage.
Many thanks for that, the solenoid earths through it's body. I think I would be able to pick them up from Maplin, I will try looking online later. Thanks for the diagram, I thought that may be the way to do it but like I say I'm not to good with electrics beyond simple wiring systems.
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