Electrical: Switching Ground. Normal Practice?

[meangreen01]

I've recently had to troubleshoot a non-working blower motor in my Durango. My buddy is now having a similar problem with his Ram. I noticed when reading the schematic that the blower motor circuit is switched on the ground side. Is that standard practice? Is there a good reason for doing it that way?

As someone with some experience in this area, switching the ground side of a circuit is usually a no-no and is more difficult to troubleshoot IMO.

 

[Donald]

The fuel injectors are done that way. I doubt it was the decision of an EE right out of school at Chrysler.

 

[brianl703]

Switching the ground (actually neutral) side of a circuit is only a no-no in wiring that has to comply with building electrical codes.

Otherwise, it is commonly done.

For fuel injectors and other ECM controlled devices, I believe it's because transistors can usually sink more current than they can source.

 

[Dave Sherman]

Nothing wrong with switching the negative side. Solid state drivers for things like solenoids and motors are often open-collector or open-drain output, so they are designed to sink current rather than source current and hence have to switch the negative side of a load.

 

[440Magnum]

Originally Posted By: meangreen01
I've recently had to troubleshoot a non-working blower motor in my Durango. My buddy is now having a similar problem with his Ram. I noticed when reading the schematic that the blower motor circuit is switched on the ground side. Is that standard practice? Is there a good reason for doing it that way?



Ground-side switching is often used where there are multiple conrtol points (switches) for a single device- dome lights are a common example, because only a single wire has to be run TO each door switch and to the dome light dash switch, rather than having to run a power supply wire to each switch and then a feed from each switch to the light.

Its also sometimes used on heavy current draw devices since its something of a failsafe. A heavy switch wire that accidentally shorts to ground just turns the device on, rather than starting a fire or blowing a fusible link and disabling the vehicle.

 

[NHGUY]

That circuit doesnt go thru the body control module does it? Normally all you are switching in that case is a resistance value and the BCM knows what to do from there (when it works).

 

[meangreen01]

I'm not sure what the body control module(BCM) is, but I'm pretty sure it doesn't.

The positive comes out of the ignition switch directly to the blower motor. The blower motor only has two wires, so the other one(neutral) goes to the fan speed selector switch on the dash, then it moves onto the resistor pack where it takes the path determined by the speed selected, and then on to vehicle ground

 

[mechtech2]

what difference does it make with 12V systems?

 

[PandaBear]

Originally Posted By: mechtech2
what difference does it make with 12V systems?


not sure about this particular application, but from device physics (in the silicon) point of view it is easier to make a transistor that pull down, or you can get a bigger current for the same cost or size, than a pull up transistor.

 

[meangreen01]

I think my lack of understanding stems from primarily working on AC circuits. I'll work through it and figure it out. Thanks for the replies.

 

[tom slick]

By AC circuits do you household/industrial electrical? The reason 120/240/277/480 is switched on the "hot" side is for safety. When the switch is off the circuit is "cold".

 

[mechtech2]

Negative ground systems were standardized in automobiles because of one thing:
Spark plugs!

The erosion of the side electrode was the reason. The cooler center [and thus longer lasting] was chosen as the emitter.

 

[Kiwi_ME]

Seems to me the coil secondary polarity is independent of the primary and therefore you could have whichever HT polarity you want with either 12V negative or positive ground.

 

[JetStar]

Old breaker point ignition systems low side switch the coil and have for as long as I can remember. It is common on aircraft systems to low side switch.

 

[George7941]

Originally Posted By: mechtech2
Negative ground systems were standardized in automobiles because of one thing:
Spark plugs!

The erosion of the side electrode was the reason. The cooler center [and thus longer lasting] was chosen as the emitter.


This post is ALL wrong!

As KIWI ME pointed out, there is no difficulty having the centre electrode negative on a positive ground system.

The centre electrode is the HOTTER one and is thus chosen as the emitter(of electrons) since the spark voltage will be lower with a hotter emitter.

BTW, one thing I have noticed is that, while mechtech2 is very knowledgeable on all matters automotive, he will NEVER admit he is wrong.

 

[mechtech2]

George -
No, it is 100% right.
This is why English cars went to negative ground, following everyone else. Spark plug erosion and life. Accessories like radios were a minor consideration.

Actually, I will change in 2 seconds when wrong.
But there is no reason to admit I am wrong when I am right.

The side electrode is not cooled. It is hotter than the center, which is flowing heat to the cooler head. This is how/why heat ranges work - by regulating that heat transfer.
The side electrode is hanging in the breeze of the very hot combustion , and is a thin piece.[relatively].

So you are quite wrong about this - you have it backwards.

 

[andyd]

Weren't positive ground systems done because there is less corrosion on terminals with a + ground. Telephones use -46 VDC.

 

[mechtech2]

Neg ground batteries are not in second place concerning battery terminal corrosion.

 

[brianl703]

I wonder if negative ground systems came about because of solid state circuits, which are almost invariably negative ground. There's probably no reason why they couldn't be positive ground, but very very few are. So perhaps standard solid state circuit design practices sort of made negative ground on cars a requirement.

 

[meep]

cars were around before solid state devices....

but we're getting OT.

Yes, it's common practice to switch on the ground side. From headlamps to points.

And yes, it does work out better for the FETs (transistors) doing the switching to injectors, transmission solenoids, etc., as well. It's cheaper.