When I pulled the wiper motor from my car it had an extra wire running from the casing to the body. Why is this? There should be no current passing through it and it works all the same without it.
Is it just an interference thing or noise supressor?
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| Why is my wiper motor earthed? https://www.ausmini.com/forums/viewtopic.php?f=2&t=63818 |
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| Author: | Harley [ Fri Jun 18, 2010 10:37 pm ] |
| Post subject: | Why is my wiper motor earthed? |
When I pulled the wiper motor from my car it had an extra wire running from the casing to the body. Why is this? There should be no current passing through it and it works all the same without it. Is it just an interference thing or noise supressor?
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| Author: | drmini in aust [ Fri Jun 18, 2010 10:48 pm ] |
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It is an earth wire. The wiper motor is mounted on rubber bushes so is insulated from the body. You need the wire to ground the motor circuit back to the battery. Without it, it has a dodgy earth return via the cable and rack wheelboxes to the body. |
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| Author: | Harley [ Fri Jun 18, 2010 10:54 pm ] |
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Sorry I should'e clarified: I have a rover 2 speed wiper motor, and its the aluminium wheelbox housing that is earthed. Works fine without it, all functions, no blown fuses. |
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| Author: | gafmo [ Fri Jun 18, 2010 10:57 pm ] |
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Same, Same..no differant |
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| Author: | drmini in aust [ Fri Jun 18, 2010 10:58 pm ] |
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yes the Oz round motors (1 & 2 speed) have this wire there also. Reason as stated. |
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| Author: | GT mowog [ Sat Jun 19, 2010 7:40 am ] |
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The wiper motor has a live feed, via the ignition switch & fuse box, to it all the time. For running it is earthed via the wiper switch. The Earth on the Body of it is for the dynamic brake when you switch it off and want (expect?) it to self park. So, yes, it will run without the body being earthed, but it may over-run when you try to park them. |
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| Author: | Mick [ Sat Jun 19, 2010 11:36 am ] |
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Edit: Ah yeah...eggzackery what GT said...but more windy 
When the wiper motor is turned off at the switch, the two brushes inside the motor are shorted out by the switch to provide what is called an "Armature Brake". This shorting arrangement turns the motor temporarily into a generator within itself and causes a large shorting current to be formed by the still turning rotor which in turns rapidly decelerates the motor through the magnetic fields created. I'm not sure how it is needed when the wiper mechanism provides so much resistance to movement ...but anyway... When the switch shorts the brushes, the ground is disconnected from the negative brush by the switch. If it did not you would have a dead short between 12volts and earth. Their is a very large short term current created in the wiper motor to provide the braking and the wiper motor case may develop a rising voltage potential with relation to the chassis given the dodgy grounds through the rubber grommets and wiper plinths. Also, while the negative is disconnected (nearly always), then there is no path for the 12 volts to go to ground should the 12 volt supply to the wipermotor devlop a fault and short out the motor case. This ground wire dissapates this voltage. This is why the ground wire has no apparent funtional use, and why the negative to the motor has to be provided through the switch. |
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