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MG Midget and Sprite Technical - Electrics

Took the sprite to MGCC midget Register rolling road event at Aldon Automotive in Birmingham today. As I have a low compression Morris 1000 engine installed I was having a mini competition with Neil Thomas and his Frogeye to see who was making the lowest power. The frog won with a devastating 39.5bhp to my 45.5!

It was a wet day and on the way back I had just got on the motorway when the wipers ceased. As did the fuel gauge, rev counter flashers and brake lights. Groped my home through the road spray...

I see in the archives there is plenty about these symptoms, but little as the the proven successful fix.

So tomorrow I shall investigate the fuse box and connections.

I fail to see what the connection (pun) is between the wiper motor and the other items as on the wiring diagram the wiper motor appears to have its own discreet circuit.

Assume that the wiring is MK 4 Sprite. Where are the main earth points to the body? So that I can check them.

Also there a couple of twin bullet connectors shown on the wiring diagram, one for wiper motor and the other for the other failing items. Where are these connectors?

Oh and my cheapo sat nav fell to bits and would work anymore!

Thanks

Roly
Roly Alcock

Good grief Roly,

I hope your dog didnt die also yesterday ...

I cant offer any help, but I like your thinking "grounds and bullets"

Id probably start with the main ground strap between the transmission and the chassis rail any stater issues

Is your car a pos or neg ground (earth) ?

Wishing you some better karma

Prop
Prop and the Blackhole Midget

Roly

There were a number of different configurations of Sprite Mk4 wiring, so you need the chassis number to pin down the correct wiring diagram.

Do you definitely have a separate fuse feeding the wiper circuit?
Dave O'Neill 2

Chassis number won't help. It's a MK 3 Sprite that has been rebuilt with a new MK4 back end and MK 4 front wings. The loom looks recent nut as to which, I cannot say. The previous (now deceased) owner was an aeronautical electrical engineer and he appears to have made some semi documented '"improvements" which may have involved dismantling the loom. Screen washer pump and hazard light control are two I can think of.

I have not found inline fuse for wiper yet. That's not to say there is not one. The wipers have never self parked......

Roly
Roly Alcock

The main feed for the wiper is off the same main fuse as the other items which failed - the green wires off the ignition-switched fuse. (There may be an in-line fuse on the parking circuit, I don't receall)

Common causes of failure are the fuse itself (not just blown - a faulty connection between fuse wire & cap seems distrubingly common), the connector it clips into being tired or dirty or both, and the fuse box itself having bad connections where the terminals are riveted together in behind.
Paul Walbran

Hiya,

Probably a poor connection in the green wires at the fusebox, or a dirty fuse.



The wiper self park circuit is not fused individually from the rest of the system.

Assuming it's a round bodied 14W motor.........check you have an ignition feed on the green wire at the motor. If that proves good, either the self park switch has failed or, more likely, the wires are not connected correctly at the dash switch.



Good luck with tis.



Regards Steve
SR Smith 1

Pick a wiring diagram, any diagram, but preferably one later than a Mkiii. Essentially they are all pretty much the same, but since you think it's been altered, you can use it as a guide only, until you establish and mark the changes.

How many fuses in the fuse box 2 or 4?

" --- the wipers ceased. As did the fuel gauge, rev counter flashers and brake lights. --- "

This tells me it's likely a feed problem and not an earth problem, since these are the only circuits affected. And, all these work when the ignition is live.

So consult the diagram for say the rev counter live feed, and see where it comes from. It should be white. It goes back to the fuse box, and dependant on which fuse box you have, you'll find others with it including green.

To me too it sounds like a poor connection in the fuse box, or a poorly seated fuse, or even a blown fuse that looks visually ok.

Start there if you haven't already and report back.
Lawrence Slater

Edit.

Beggin yer pardon. :) The rev counter feed is green. It's fed by a white on the other side of the fuse. So read that into what I've written below, and correct accordingly. The rest will still make sense. At least to me. lol.
Lawrence Slater

Lawrence,


You correct in saying the green wires are fed via the white wire from the ig switch.


That crt should have an impulse type tacho which is triggered by being in series with the ig switch and the coil +.

If the tacho loses the feed on the green wire it will cease to indicate but the car will still run as the coil is live via the "loop" on the rear, or inside the tacho.
SR Smith 1

Thank you guys for all the input.

The main prize goes to Paul who stated:-
Common causes of failure are the fuse itself (not just blown - a faulty connection between fuse wire & cap seems distrubingly common)

Yes spot on. The fuse measured 12v through it. But when I removed it from the fuse holder and gave it a tug, it just came apart.

New fuse inserted and all is well.

Cheers

Roly
Roly Alcock

Found that one the hard way, electric fan fuse on the B did that and it overheated in rush hour traffic, cracking the head :-(
Paul Walbran

Glad you made it back Roly, was good to meet you :) we had a very wet trip back!
Rob Armstrong

Congrats roly,

Wow... your the man today, just in time for that frigid winter blast comming down from the north this weekend...I wish I had been as productive as you this weekend...I should have winterized the car, but I drove it about 100 miles instead...listening to a bunch of new illegally downloaded music on the phone

Does the wiper park function now work ? Anyway... good job

Prop

Prop and the Blackhole Midget

Thanks Prop, I hesitated the first reply - neither dog died! Phew....

No wiper park does not work, I will investigate to see what is wired up to the switch - once my arthritic mobility improves. Its not not conducive to lying upside down staring under a dash.


Rob, I had water pour onto my left knee on my run back, presumably coming in through the wiper wheelbox.

Roly
Roly Alcock

Roly,
you could pull the switch forward from the dash to check the connections - I'd also suggest at the time you clean the connections and check they're all tight
Nigel Atkins

Sprites never had rocker switches, so the switch has to come out backwards.
Dave O'Neill 2

Roly,

When you get around to messing with the wiper park switch... please post.what you find and your solution...mine dosnt work ither and keeps.poping the the fuse when I turn it off and it parks...currently im just manually parking as ive cut the park wire...not a big issue, just one of those splinters in the back.of the mind id like to remove...just knowing why and.how to fix would give me piece of mind


Thanks...and glad the dogs survived the day
®¿~

Prop
Prop and the Blackhole Midget

Even the Austin Sprite had toggles. I wish they'd kept them for the following Midgets too.
Lawrence Slater

Hi guys,


Would anyone be interested in a simple (well hopefully) explanation of how the parking circuit functions and how to test it?



Regards Steve
SR Smith 1

I would please Steve!
Glynn (1275RWA) Williams

I spent some time a few weeks back figuring out how the park works. It is one of these things that is utterly baffling until you figure it out at which point it is staggeringly simple!

I won't steal your thunder though Steve, fire away :-)

Malcolm
M Le Chevalier

My switches are not toggles. I look forward to the detail description of the parking method.

Roly
Roly Alcock

Ditto X 3

I think it involves virgin goat blood, a black cat, and some naked fat chicks dancing around a midnight camp fire

Prop
Prop and the Blackhole Midget

Ok here goes,

This is for the round bodied Lucas 14W type motor.


The motor has an ignition feed on the green wire via the fuse.

It has an earth on the black wire.


The same fuse also provides a live supply to the switch.



When the switch is moved to "on", the power flows via the light green/red to the first speed brush and the motor starts.


When the switch is moved to "off" the switch removes the feed via the green wire, but the self park switch then supplies a feed via the light green/brown back to the switch.


This is the clever bit. The switch in the "off" position has an internal connection to the light green/red so the motor continues to run. As soon as the drive wheel opens the park switch, the feed is broken on the light grren/brown and the motor stops.


If you have a 2 speed motor, the second speed is via a light green/blue wire to the high speed brush.



Hope that males sense.


You can normally work out how the wires attach to the switch by trial and error.



The biggest problem I get on old cars is where someone has replaced the toggle wiper switch with a lighting switch, they don't have the internal connection and you need to fit a relay to make the park work.
SR Smith 1

Not all round body (14W) motors were 2 speed. Although I think I read somewhere that all rebuilt units are now 2 speed.

More useful I reckon is single, or delayed sweep. I've been meaning to fit a single sweep push button to my Sprite for over 35 years. It can't have bothered me that much though, as I never got around to it, and anyway, it's just as easy to toggle the switch once.

But I do still fancy a time delayed sweep. Maybe whilst my sprite is off the road, I'll get around to buying or building a timed circuit and shove it on a relay.

Here's a nice simple one. But I'll bet these days you can probably buy a complete kit as cheap (if not cheaper)as you can build one.



Lawrence Slater

Takes me back. I used a similar circuit to build a wiper delay device for my isetta bubble car back in 1972!
Bob Beaumont

Steve, thanks for the detailed write up. I shall explore the vagaries this weekend.
Roly
Roly Alcock

Lawrence,
yup, Maplin do a self build timer kit for a tenner. I bought one when it was a fiver and it's still in the box :-D
I'm sure I'll get around to it one day....

Best of ...
MGmike
M McAndrew

This thread was discussed between 08/11/2014 and 13/11/2014

MG Midget and Sprite Technical index

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