Earth/Positive switched Relays ??
Discussion
Just looking at re-wiring my race car and was wondering which option is best. I'm sure when I worked at Jaguar they used some earth switched circuits.
Opinions and options. Pluses and minuses Please !!
I don't have much to power.
Electric Fans
Electric water Pump
Gauges
2x Fuel pumps
ECU and MSD ignition
ECU relates relays ( x3 )
2 lambda sensors
Thanks for reading.
Opinions and options. Pluses and minuses Please !!
I don't have much to power.
Electric Fans
Electric water Pump
Gauges
2x Fuel pumps
ECU and MSD ignition
ECU relates relays ( x3 )
2 lambda sensors
Thanks for reading.
It is of the greatest importance that the relays switch the positives to all the circuits that you have listed, if you switch the negatives you will have problems
Example
If a fuel pump was permanently supplied with a positive and the relay switched the negative, should the negative wire to the fuel pump short to earth, the fuel pump would run all the time
If a fuel pump was permanently supplied with a negative and the relay switched the positive, should the positive wire to the fuel pump short to earth, the fuel pump fuse would blow
The above is only used as an example and going on the vehicle being of negative earth
As far as ECU switching goes.....The ECU would normally be activated from a relay switched positive and then do its positive and negative output switching as written to do
Switch the positives and you will get a satisfactory result
Switch the negatives and you have created possible problems
Example
If a fuel pump was permanently supplied with a positive and the relay switched the negative, should the negative wire to the fuel pump short to earth, the fuel pump would run all the time
If a fuel pump was permanently supplied with a negative and the relay switched the positive, should the positive wire to the fuel pump short to earth, the fuel pump fuse would blow
The above is only used as an example and going on the vehicle being of negative earth
As far as ECU switching goes.....The ECU would normally be activated from a relay switched positive and then do its positive and negative output switching as written to do
Switch the positives and you will get a satisfactory result
Switch the negatives and you have created possible problems
TheEnd said:
Most of the stuff I see on ECUs is earth switched, not sure if there was a good reason for it.
If the ECU is involved then I suspect this might be because it uses power transistor based switches which often switch the ground (sinking the current). Then connected devices can take their + supply from a common source.It makes sense though to use relays to switch + due to what was mentioned above about it being much easier for a stray ground side cable to touch the chassis earth.
Penelope Stopit said:
It is of the greatest importance that the relays switch the positives to all the circuits that you have listed, if you switch the negatives you will have problems
Example
If a fuel pump was permanently supplied with a positive and the relay switched the negative, should the negative wire to the fuel pump short to earth, the fuel pump would run all the time
If a fuel pump was permanently supplied with a negative and the relay switched the positive, should the positive wire to the fuel pump short to earth, the fuel pump fuse would blow
The above is only used as an example and going on the vehicle being of negative earth
As far as ECU switching goes.....The ECU would normally be activated from a relay switched positive and then do its positive and negative output switching as written to do
Switch the positives and you will get a satisfactory result
Switch the negatives and you have created possible problems
The car has a battery cut off key so circuits are only energised when in use. All other times the key is off and no power going to ancillaries.Example
If a fuel pump was permanently supplied with a positive and the relay switched the negative, should the negative wire to the fuel pump short to earth, the fuel pump would run all the time
If a fuel pump was permanently supplied with a negative and the relay switched the positive, should the positive wire to the fuel pump short to earth, the fuel pump fuse would blow
The above is only used as an example and going on the vehicle being of negative earth
As far as ECU switching goes.....The ECU would normally be activated from a relay switched positive and then do its positive and negative output switching as written to do
Switch the positives and you will get a satisfactory result
Switch the negatives and you have created possible problems
melhookv12 said:
The car has a battery cut off key so circuits are only energised when in use. All other times the key is off and no power going to ancillaries.
WrongWhen you turn the battery cut off key to the on position, any circuit with a relay switched negative that was shorted to the body would kick in before you had operated the relay for that circuit
There is nothing to gain from relay switching the negatives but I am intrigued to know why you should wish to do so
melhookv12 said:
Just looking at re-wiring my race car and was wondering which option is best. I'm sure when I worked at Jaguar they used some earth switched circuits.
Opinions and options. Pluses and minuses Please !!
I don't have much to power.
Electric Fans
Electric water Pump
Gauges
2x Fuel pumps
ECU and MSD ignition
ECU relates relays ( x3 )
2 lambda sensors
Thanks for reading.
In answer to your original questionOpinions and options. Pluses and minuses Please !!
I don't have much to power.
Electric Fans
Electric water Pump
Gauges
2x Fuel pumps
ECU and MSD ignition
ECU relates relays ( x3 )
2 lambda sensors
Thanks for reading.
Switching the positives has more advantages than switching the negatives to the above circuits
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