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Wiper current leak


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Help again please, my wiper circuit seems to have developed a problem, when switching on the ignition only the ampmeter reads approx -10 normally it's been about 3-4 though the amp meter isn't calibrated finer than graduations of 10amps.

By removing red wires from the fuse block I have narrowed the fault down to the wiper circuit, new wiring loom fitted using the original two speed push pull switch and wiper motor. The motor still works ok, should I be looking at replacing the switch or is the switch repairable as I cannot see anything else it could be.

 

Paul

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Hi Paul

 

I think you jeed to methodically check where the current is going.

 

The usual rule of thumb is that 'its whatever you just fixed/changed/fitted' so start with the new wiring.

 

With your location you do have experts in the vicinity if help needed!

 

Steve

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Wires on switch in wrong place.

Photo is recent topic shows correct locations.

 

It's also too stiff to start in FAST mode, but does start in SLOW and continues when you switch to FAST.

 

You will burn out the armature quite shortly if you don't fix it.

 

Need to also check rack-wire run and lube and maybe precise assembly of the motor.

They do this if the armature just touches the pole-piece.

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I have just rebuilt everything the wipers are working as should starts on slow speed then onto a little bit faster. New wheel boxes all well lubricated hopefully I will never need to go there for awhile. This is something that has developed. I will look at the switch tomorrow.

The high amp reading is with the motor switched off so I'm thinking it can't be a problem with the motor it parks ok.

Thanks

Paul

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10A is about right for an armature that's either stalled or has no field current.

Or not enough field current to start from standstill.

 

Of course you can just pull the bullets out to check this.

 

The Jag motors have their field coil completely separated from the armature so they can be made to run backwards when parking.

 

They are always ending up with no field-current and getting overheated.

Edited by AlanT
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10A is about right for an armature that's either stalled or has no field current.

Or not enough field current to start from standstill.

 

Of course you can just pull the bullets out to check this.

 

The Jag motors have their field coil completely separated from the armature so they can be made to run backwards when parking.

 

They are always ending up with no field-current and getting overheated.

Hi Alan

Thanks for your input on this, from reading past threads you are certainly the wiper expert.

If I switch the motor on it will run as normal and wipers work, my electrickery skills are rather limited so I try to work problems out logically.

As in this case as the motor is switched off and parked correctly it should not be consuming any power, but it is approx 6 amps. I will pull the motor bullets tomorrow, it keeps the brain active.

One observation I have made while trying it out with a dry screen the motor did stall on fast speed, then switching to low speed it continued again, I put this down to the dry resistance of the screen in fast speed.

Paul

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Pulled the bullets and the ampmeter has returned to its original position with 4 amps, this is why electrics confuse me so, if the motor is switched off how can it consume power?

I guess the motor is suspect as you pre warned me Alan.

Next step to seek a thread for testing the motor out.

 

Paul

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The problem is that there are five contacts on the switch and only three wires.

These switches make contacts is a very confusing way.

It's easily possible to apply power when the switch is in the OFF position.

 

To run, you want 12V in the GREEN motor wire and BOTH the other motor-wires GROUNDED. This is slow running.

 

You can do this at the bullets with some spare wire.

 

I took a few days to get one to start in FAST mode reliably. It was just the armature grazing against the pole-piece. Once started it would run fine and not make much noise about it either.

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There are two ways for current to flow through the motor:

 

1. armature route - GREEN wire - BRUSH ARM - COMMUTATOR - BRUSH ARM - RED WIRE

 

2. field-coil route - GREEN WIRE - INNER END OF COPPER WINDING - OUTER END OF COPPER WINDING - BROWN WIRE

 

You need both routes to run, hence grounding BOTH RED and BROWN.

 

On FAST there is a route from the BROWN wire to the RED wire using a few turns of resistance wire. This gives a route for field current but the resistance reduces the current from 1A to about 0.5A.

 

If you take off the brushes or interpose a bit of paper insulation, then you only have the field coil route.

 

PM me with an email address for some docs on wiper dismantling.

 

The flying-leads were made from Cotton covered wire, well past its best now and ready for a short.

I'll be surprised if you can actually tell the colours.

 

You may have a thermal cut-out on the inside of he motor body. These like to make short-circuits. The long-bolts pass near to everything. The factory put insulation tubes over some of them. If lost or degraded then Drinking-Straw can rescue you.

Edited by AlanT
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I know Roger. Stuart says so also. But It makes no sense. The motor must start with reduced torque if you do this.

 

Poor thing only just has enough torque to sweep a WET screen on FAST.

 

If you compare with the Rover rotary-switch and the Jaguar switch, for two-speed motors using the same field-coil to get two-speeds, then these are both marked "Park Slow Fast".

 

So I think this is just "folk-lore" based on somebodies past mistake with the switch wiring. It is a bit more "obvious" if you guess the connections to get OFF-FAST-SLOW. But I am sure this is wrong.

 

By the way, for ages I expected that these would park running FAST, because the "parker" grounds only the RED wire.

But NOT SO. If you use what I believe to be the correct switch wiring then it will park SLOW. The switch joins RED and BROWN when OFF.

 

I'd guess we all think parking SLOW is a better option.

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The problem is that aiui the wiper switch is the same as the three position lighting switch used on some models. First position has one wire grounded (cf sidelights only) and therefore fast speed. Second position grounds two wires (cf side and headlights) and slow speed. What's needed is a switch that operates in the opposite sense. This type of switch makes no connections when off so the motor parks fast.

 

I'd be delighted to be corrected and pointed towards the correct switch and wiring if I've got it wrong. Btw my motor is happy to run at both speeds on a dry screen so I guess I'm lucky.

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Mine rightly or wrongly starts on slow first click, I just assumed this to be the correct way, there you go so wrong to make assumptions.

I have the motor off seems I will also need to remove the switch.

Thanks all

Paul

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You have a pm Alan

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Lucas part 34477 aka 58SA

 

http://www.ebay.co.uk/itm/PUSH-PULL-light-switch-3-postition-off-on-on-replaces-Lucas-34477-58SA-for-MGB-/252019402966?hash=item3aad86e8d6:g:g7UAAOxyrM5TGalp

 

You can wire OFF-FAST-SLOW or OFF-SLOW-FAST as you wish.

 

Variants of this with toggle to suit Mini and Jag for the fan. Contact block is the same on all of them.

 

These were rare NOS only for a while but now remade in Lucas green-boxes.

As far as I can tell the repros are identical. I mean really identical.

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A well lubricated motor, with good bearings, good wheel-boxes, lubed tubes that fit perfectly and these motors do OK.

 

But they will struggle and burn out if fighting a stiff rack.

 

Another common trouble is rust around the shaft of the main gear.

 

I get seized gears that have gone rock-solid. These can be rectified but it's a nuisance.

Lucas did not lubricate later motors properly and these shafts run dry.

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10A is rather a lot.

Running light on the bench I look for 2.5A and pass them out at 3A.

 

Will take more on a rack of course, but 10A suggests stiffness somewhere.

 

These actually use wire that is a bit thin for the current taken. According to the wire tables they are twice the recommended current/sq-mm for a NON fan-cooled motor. This is why they get a bit hot.

 

I suppose they get a bit of breeze when driving.

 

But don't push your luck. Photos from a forumites TR3 recently fixed:

 

31372572493_00543c014e_b.jpg

 

32144827136_74490673a1_b.jpg

 

32144826406_9436471db2_b.jpg

Edited by AlanT
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I always admit it when I'm wrong.

 

A Rover wiper-switch is actually marked P N H for Park, Normal, High but I really think that's the same as OFF SLOW FAST.

 

Incidentally these are reverse-parkers and you must wire them exactly as Lucas says.

 

22915150613_507cc4537b_b.jpg

 

Sorry it's upside down but I wasn't photographing the knob.

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I have now replaced the three wires with new, still the same fault but with ignition on wiper switch off I have a very slow motor speed, it's three speed (in case it rains).

Must be a switch fault, only mounted the h frame yesterday.

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For a DR3A two speed wiper motor:

For slow speed operation - the switch must ground both wires RED/GREEN and BROWN/GREEN

For fast speed operation - the switch only grounds the fast speed BROWN/GREEN wire.

The switch is crucially important. get it wrong and you will burn out the resistance winding in the motor.

 

That you have current flow with the switch off would indicate to me a problem with the parking circuit. If the park position is not correct, the wipers may be located in the park position, but the park switch may still be conducting. In which case there will be a current through one of the armature windings. The park switch is adjustable - loosen the retaining screws (but don't remove them) covering the gear and rotate the metal dome with the red wire attached that sits over the gear, until the current stops flowing. (You may need to fiddle about to get the parking position of this unit exactly right).

 

Good luck

 

TT

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I think you may be only a few minutes away from ending up with an armature like the one in my photos.

 

Maybe you already have done this. Failure of insulation of the armature would give a current path from the GREEN wire to the motor body.

 

Usually cooked armatures look black and have a tar smell.

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I would quite like to meet the guy who designed the two speed pull out switch, I thought it would be interesting to have a look inside to see if anything was amiss, very well made but so complex. After much ado it's back together with its four springs and working just as it did before.

 

I have been trying it connected to the switch but without the park connected or the wheel rack, next stage is to connect up the parking perhaps as Tony has mentioned there could be a problem there. The motor seems to be working better since I replaced the wires.

With the motor running the ampmeter registers approx 9 amps but this includes the ignition coil drawing 4-5 amps.

 

Don't think it's cooked as yet Alan, it seems quite happy when working no heat generated at all. Internally the components appear unworn.

Thanks again all

Paul

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The slow speed setting gives more starting torque because of the extra current it draws. There is an inherent feature of dc motors called back emf where the motor works as a dynamo opposing the applied voltage, and the faster it goes the more back emf opposing the applied voltage, and limiting its speed. Switching the resistor into the circuit reduces the field current and field strength and hence the back emf, enabling it to go faster before the back emf is enough to offset the applied voltage.

 

I might have been able to explain it better and in more detail 50 years ago as an undergrduate.

 

This explains how it's supposed to work, but not why there is a problem here when the motor is supposed to be off. Sounds like a problem with the parking contacts or their wiring.

Edited by acaie
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Field weakening is/was a recognised DC motor speed control strategy as indeed was field forcing to get more torque such as is required in traction applications although those motors usually had series wound field coils for starting.

Edited by peejay4A
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