I want to figure out a way to make a circuit that is only hot when the engine is running. Like, as it is, there is one side of the fuse panel that is hot and one side that is switched. I want one that is only hot after the engine is running. I'm building that 66 bonneville right now with some heavy draw items like a compressor for air suspension, a couple amps for a medium duty stereo, DRLs, and they normally would come on with the switch. How does one go about adding a run-only circuit so those items wait until the start is done and the alternator is making power?
Replies (13)
uochronos on 11/2/2004 03:04:33
well the best way i see to do this is wire a relay to the switched side of the fuse box. if you have a empty spot that would be great. then the hook the devices to the relay they well only get power then the relay gets switched on when you start the car.... i did something similar in my truck with LED's.
Chronos
ttocs on 11/2/2004 08:49:06
you can get that at the fuse box(test with a voltmeter with the key on or off), or you could also get it at the ign switch. I believe GM liked to use yellow for the +12v switched wires but test before you hook it up, and properly fuse it.
curtis73 on 11/2/2004 22:27:52
Good suggestions, but I don't think I described it correctly. I know there are hot and switched circuits. I want a circuit that is on only when the ENGINE is RUNNING. When I turn the switch on, I don't want that circuit hot. AFTER I start the engine and the alternator is making juice, THEN I want that circuit hot. Then I would have hot, switched, and running.
If I just wired those items I listed above to a switched circuit, they would be competing with the starter and drawing amperage while I was cranking. Even if I relay it off the switch (which isn't hot during crank) I would still be running those items before the car starts.
I just want a circuit that ONLY is hot AFTER the engine is running.
ttocs on 11/3/2004 01:38:20
hmmmm.........
that would be a new one to me. Could you tap directly off of the alt? It should not have any pwr output until the car is running but this is a new one to me.
curtis73 on 11/3/2004 01:50:06
The problem with tapping into the alt is that the alt is just connected to the hot side of the battery. Even though its not producing power when its not spinning, its just attached to the hot side. I thought of an idea but I don't know the math behind it. If I had the alt run through a diode to the charge circuit, then connect my "magic" circuit upstream of the diode, it should only have juice when the alt is running. The problem is, how does one design a diode that can handle 140 amps?
If it were an EFI car, they have triggers to tell the computer when the car is running and I could relay from that, but I don't even have a "charge" light in my dash. If I did I could use a DTDP relay to be closed when the light is not on, but I don't know how to engineer that???
uochronos on 11/3/2004 02:20:04
what are you running off this that your starter cant compete with it??? i can haev my 1700watts of amps running and then start the car while they are already running? just wondering,...
Chronos
swez on 11/3/2004 08:31:24
Hummm, this could get very interesting. Am wondering if some sort of sensor would work here.
1. A magnetic pickup type circuit that senses crank shaft rotation via magnetic induction (rotational sensor) that switches on the circuit only when engine is running, connected to a relay and then a power line circuit, back to the electronics toys? Just need to attach a small magnet to harmonic balancer and mount the sensor close enough to pick up the magnetic field as the shaft rotates. This is similar to a tachometer circuit that reads engine RPM off the ignition coil. Basically, a magnetic pickup system.
2. A light emitting diode and detector circuits to sense when there is current flow out of the ALT ONLY. This would require a diode to block unwanted current from the BAT. A "shunt resistor" (high ohmic value)can be used to limit current and voltage to the LED.
Once LED is on, the sensor detects and you have a circuit that can trigger a relay to turn on other devices. LED's are commonly available at most autoparts stores and often operate at very low voltage/current values.
The trick here, is to design a shunt resistor and diode that does not drain the BAT or consume excess power off the ALT. A high resistance value will limit how much current will flow through the diode.
3. A simple switch on the dash with a light to tell you when power is on or off that circuit. The switch would be connected to a relay as well and then power fed to said devices only when switch is in the ON position. This switch can be connected to the fuse panel via ACC/IGN circuit. The lamp in the switch is off when no power is flowing. The light comes on only when you activate the switch. Similar to say add on fog lights or similar accessories.
Comments?
Swez
ttocs on 11/3/2004 12:22:25
we might be able to use a relay to isolate it from the batt, but it would have to be a big one.... there are two different types of ignition wires. True ignition has pwr on in the acc position, and while the engine is running. 2nd ignition has pwr only in the acc position, and not while the engine is running. The switched wire that goes to the radio is a good example of this. It turns on, then off while the starter is cranking.
Seems we are making this harder then it need to be though, why not add a switch to turn this stuff on after you start the car?
swez on 11/3/2004 19:22:16
That's the simplest way to manage this issue. Sure, a few high tech gizzmos' would be interesting to toy with... but the fail safe approach is a lited dash switch.
Swez
curtis73 on 11/3/2004 20:01:03
You're right; the dash switch is the easiest. Its just that I have a clean slate with my car currently gutted. I have a new harness, fuse panel, switches, lights, the works. Absolute clean slate.
OK, lets go back a little. Someone mentioned the charge light. My old dash had a "charge" light on it. How did it work? If I can recreate that function and use a DT relay, I can have that circuit be active when the light is NOT active. How do car's "charge" lights work?
swez on 11/3/2004 21:05:37
The charge light (dummy light) senses output from the ALT when the engine is running. It has a circuit that detects voltage in a given range of operation. It is usually tied to the Voltage regulator for voltage information. (voltage divider/comparator circuit)
If the ALT is charging above 12.5 volts, the lamp will remain off. However, if the charging rate drops below voltage preset of say 11.5 volts, the light will come on.
This might work... if you can locate the circuit that feeds that "dummy light" and tap off that with a relay coil. Depends on the amount of current needed to run that circuit.
A typical Bosche relay will need ~0.6 A and 12 volts to close the coil and make circuit connections. However, if that lamp has a very low sensing current, the coil current draw may be enough to trigger the light to go on when activated.
Only 2 ways to find out.... get a wiring diagram of that circuit and trace it through or just try probing that circuit until you can determine the right wires to use to trigger the relay.
Swez
curtis73 on 11/3/2004 21:20:06
Great. I have a 66 Pontiac shop manual in the mail right now. It has wiring diagams.
great thread! Thanks, folks!
ttocs on 11/3/2004 22:54:36
the light should be +12v, a relay would work here.
Replies (13)
uochronos on 11/2/2004 03:04:33
well the best way i see to do this is wire a relay to the switched side of the fuse box. if you have a empty spot that would be great. then the hook the devices to the relay they well only get power then the relay gets switched on when you start the car.... i did something similar in my truck with LED's.
Chronos
ttocs on 11/2/2004 08:49:06
you can get that at the fuse box(test with a voltmeter with the key on or off), or you could also get it at the ign switch. I believe GM liked to use yellow for the +12v switched wires but test before you hook it up, and properly fuse it.
curtis73 on 11/2/2004 22:27:52
Good suggestions, but I don't think I described it correctly. I know there are hot and switched circuits. I want a circuit that is on only when the ENGINE is RUNNING. When I turn the switch on, I don't want that circuit hot. AFTER I start the engine and the alternator is making juice, THEN I want that circuit hot. Then I would have hot, switched, and running.
If I just wired those items I listed above to a switched circuit, they would be competing with the starter and drawing amperage while I was cranking. Even if I relay it off the switch (which isn't hot during crank) I would still be running those items before the car starts.
I just want a circuit that ONLY is hot AFTER the engine is running.
ttocs on 11/3/2004 01:38:20
hmmmm.........
that would be a new one to me. Could you tap directly off of the alt? It should not have any pwr output until the car is running but this is a new one to me.
curtis73 on 11/3/2004 01:50:06
The problem with tapping into the alt is that the alt is just connected to the hot side of the battery. Even though its not producing power when its not spinning, its just attached to the hot side. I thought of an idea but I don't know the math behind it. If I had the alt run through a diode to the charge circuit, then connect my "magic" circuit upstream of the diode, it should only have juice when the alt is running. The problem is, how does one design a diode that can handle 140 amps?
If it were an EFI car, they have triggers to tell the computer when the car is running and I could relay from that, but I don't even have a "charge" light in my dash. If I did I could use a DTDP relay to be closed when the light is not on, but I don't know how to engineer that???
uochronos on 11/3/2004 02:20:04
what are you running off this that your starter cant compete with it??? i can haev my 1700watts of amps running and then start the car while they are already running? just wondering,...
Chronos
swez on 11/3/2004 08:31:24
Hummm, this could get very interesting. Am wondering if some sort of sensor would work here.
1. A magnetic pickup type circuit that senses crank shaft rotation via magnetic induction (rotational sensor) that switches on the circuit only when engine is running, connected to a relay and then a power line circuit, back to the electronics toys? Just need to attach a small magnet to harmonic balancer and mount the sensor close enough to pick up the magnetic field as the shaft rotates. This is similar to a tachometer circuit that reads engine RPM off the ignition coil. Basically, a magnetic pickup system.
2. A light emitting diode and detector circuits to sense when there is current flow out of the ALT ONLY. This would require a diode to block unwanted current from the BAT. A "shunt resistor" (high ohmic value)can be used to limit current and voltage to the LED.
Once LED is on, the sensor detects and you have a circuit that can trigger a relay to turn on other devices. LED's are commonly available at most autoparts stores and often operate at very low voltage/current values.
The trick here, is to design a shunt resistor and diode that does not drain the BAT or consume excess power off the ALT. A high resistance value will limit how much current will flow through the diode.
3. A simple switch on the dash with a light to tell you when power is on or off that circuit. The switch would be connected to a relay as well and then power fed to said devices only when switch is in the ON position. This switch can be connected to the fuse panel via ACC/IGN circuit. The lamp in the switch is off when no power is flowing. The light comes on only when you activate the switch. Similar to say add on fog lights or similar accessories.
Comments?
Swez
ttocs on 11/3/2004 12:22:25
we might be able to use a relay to isolate it from the batt, but it would have to be a big one.... there are two different types of ignition wires. True ignition has pwr on in the acc position, and while the engine is running. 2nd ignition has pwr only in the acc position, and not while the engine is running. The switched wire that goes to the radio is a good example of this. It turns on, then off while the starter is cranking.
Seems we are making this harder then it need to be though, why not add a switch to turn this stuff on after you start the car?
swez on 11/3/2004 19:22:16
That's the simplest way to manage this issue. Sure, a few high tech gizzmos' would be interesting to toy with... but the fail safe approach is a lited dash switch.
Swez
curtis73 on 11/3/2004 20:01:03
You're right; the dash switch is the easiest. Its just that I have a clean slate with my car currently gutted. I have a new harness, fuse panel, switches, lights, the works. Absolute clean slate.
OK, lets go back a little. Someone mentioned the charge light. My old dash had a "charge" light on it. How did it work? If I can recreate that function and use a DT relay, I can have that circuit be active when the light is NOT active. How do car's "charge" lights work?
swez on 11/3/2004 21:05:37
The charge light (dummy light) senses output from the ALT when the engine is running. It has a circuit that detects voltage in a given range of operation. It is usually tied to the Voltage regulator for voltage information. (voltage divider/comparator circuit)
If the ALT is charging above 12.5 volts, the lamp will remain off. However, if the charging rate drops below voltage preset of say 11.5 volts, the light will come on.
This might work... if you can locate the circuit that feeds that "dummy light" and tap off that with a relay coil. Depends on the amount of current needed to run that circuit.
A typical Bosche relay will need ~0.6 A and 12 volts to close the coil and make circuit connections. However, if that lamp has a very low sensing current, the coil current draw may be enough to trigger the light to go on when activated.
Only 2 ways to find out.... get a wiring diagram of that circuit and trace it through or just try probing that circuit until you can determine the right wires to use to trigger the relay.
Swez
curtis73 on 11/3/2004 21:20:06
Great. I have a 66 Pontiac shop manual in the mail right now. It has wiring diagams.
great thread! Thanks, folks!
ttocs on 11/3/2004 22:54:36
the light should be +12v, a relay would work here.
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