Dash Oil and Brake Warning light - how are they triggered in the start function?

[4mulas]

Hi there, 1970 console shifted automatic A-body. Wondering how the DASH brake and oil light warning lights are triggered during the ignition start/position/phase in order to show you that they are working correctly. Anyone finitely know?

 

[67Dart273]

Very simple. The key switch only has 3 outputs. ACC is powered in the "ACC" and the "run" positions of the key That line powers stuff like radio, wipers, heater

The "run" position of the key powers things like the gauges, the warning lamps, the ignition system, and the VR and alternator field, and in newer models, smog devices under the hood. THAT "run" power goes DEAD when the key is twisted to start. That means the warning lights go dead during engine cranking

In the start position, there are two separate isolated key switch contacts/ circuits live. 1 is the "start" power which involves the start relay and neutral switch, and energizes the starter solenoid. 2. is the coil "bypass" circuit which feeds full battery power to the coil side of the ballast resistor. If you have electronic ignition instead of breaker points, that power during cranking also BACKFEEDS through the coil ballast to provide power to the ECU for spark.

 

[Lefty71]

The "run" position of the key powers things like the gauges, the warning lamps, the ignition system, and the VR and alternator field, and in newer models, smog devices under the hood. THAT "run" power goes DEAD when the key is twisted to start. That means the warning lights go dead during engine cranking

I thought i was following along here..... but then why dont they come back on when the key is released from "start" position??

 

[67Dart273]

I thought i was following along here..... but then why dont they come back on when the key is released from "start" position??

They won't if the engine starts, has oil pressure, and is charging. Sometimes, if the engine does not start, it will have enough oil pressure to hold the oil lamp off, but the "charge" light should return.

 

[4mulas]

Very simple. The key switch only has 3 outputs. ACC is powered in the "ACC" and the "run" positions of the key That line powers stuff like radio, wipers, heater

The "run" position of the key powers things like the gauges, the warning lamps, the ignition system, and the VR and alternator field, and in newer models, smog devices under the hood. THAT "run" power goes DEAD when the key is twisted to start. That means the warning lights go dead during engine cranking

This isn’t the case. Your general knowledge of the system is good, however, when the car is ‘cranking’ the Brake light warning indicator will illuminate, this is to show you that the light and system is functioning properly. When released (now run position) the only thing that should illuminate the brake warning light is a pressure imbalance (from the pro valve or distribution block), if the e-brake is engaged, or if you have a faulty switch in either.

That said, I’m wondering what, in the start function, grounds the brake warning light, in order to show the temporary illumination and its proper functioning.

Also, there is no ‘charging’ light on these cars, just an ammeter.

Both the brake warning light and oil light have power at the circuit board in the dash in the run position all the time but will only illuminate when the prospective light’s switch is triggered (oil or brake) which does the triggering by means of Ground.

 

[jos51700]

Very simple. The key switch only has 3 outputs. ACC is powered in the "ACC" and the "run" positions of the key That line powers stuff like radio, wipers, heater

The "run" position of the key powers things like the gauges, the warning lamps, the ignition system, and the VR and alternator field, and in newer models, smog devices under the hood. THAT "run" power goes DEAD when the key is twisted to start. That means the warning lights go dead during engine cranking

In the start position, there are two separate isolated key switch contacts/ circuits live. 1 is the "start" power which involves the start relay and neutral switch, and energizes the starter solenoid. 2. is the coil "bypass" circuit which feeds full battery power to the coil side of the ballast resistor. If you have electronic ignition instead of breaker points, that power during cranking also BACKFEEDS through the coil ballast to provide power to the ECU for spark.

I only know my '72, '75, and my '90's pickups, but I have to disagree. The oil light on my cars is on during cranking, only goes out when the key is off or engine has pressure. I know this because if I crank long enough, it'll start winking out.

I'm not sure about accessory, I don't use that function (no cute girls will make out with me in my cars, so no need to sit with radio on and dash lights to grope by...)

My brake light ONLY comes on if there's a pressure problem or the parking brake is set. Low fluid doesn't trigger a light unless it's my later pickups. To be fair, I've never had low fluid in the cars, so I might be wrong on that. My brake warning light does not have a test indication. Only the oil light comes on at startup, if I've released the parking brake.

 

[67Dart273]

This isn’t the case. Your general knowledge of the system is good, however, when the car is ‘cranking’ the Brake light warning indicator will illuminate, this is to show you that the light and system is functioning properly. When released (now run position) the only thing that should illuminate the brake warning light is a pressure imbalance (from the pro valve or distribution block), if the e-brake is engaged, or if you have a faulty switch in either.

That said, I’m wondering what, in the start function, grounds the brake warning light, in order to show the temporary illumination and its proper functioning.

Also, there is no ‘charging’ light on these cars, just an ammeter.

Both the brake warning light and oil light have power at the circuit board in the dash in the run position all the time but will only illuminate when the prospective light’s switch is triggered (oil or brake) which does the triggering by means of Ground.

What year is your car? That could be from feed back through the ignition system circuit. My 67 does not do that

 

[4mulas]

It’s a 70, and it’s a pretty standard thing amongst any mopar I’ve owned. Perhaps your car is a little wonkey. Jos51700 reaffirms what I’m Saying.

Hence, my problem is a little confusing, I just want to know what ground the circuit during cranking so I can eliminate that from my trouble shooting and before I change the switch prematurely

 

[jos51700]

It’s a 70, and it’s a pretty standard thing amongst any mopar I’ve owned. Perhaps your car is a little wonkey. Jos51700 reaffirms what I’m Saying.

Hence, my problem is a little confusing, I just want to know what ground the circuit during cranking so I can eliminate that from my trouble shooting and before I change the switch prematurely

I think the oil pressure switch grounds the circuit, and oil pressure moves the contact and breaks the ground so light goes out.

Power to the circuit turns off with the key

 

[67Dart273]

I think the oil pressure switch grounds the circuit, and oil pressure moves the contact and breaks the ground so light goes out.

Power to the circuit turns off with the key

Then you show me where it gets power. IGN1 goes dead during cranking. It may get some power fed back through the ballast resistor backwards during cranking. But you are talking about the battery is down to 11V or less during cranking, the ballast is dropping through the field/ VR circuit and whatever else is on the IGN1 line, so the light can't surely get more than ?? 4V ??

Maybe if you are in a dark/ shaded area opposed to bright sun you will see it. Maybe my engine cranks enough pressure that the starter clicks it right off, or maybe my sender is just more sensitive.

The brake lamp has no ground unless the parking brake is set or the warning/ pressure switch is made, which of course it normally is not.

 

[jos51700]

Then you show me where it gets power. IGN1 goes dead during cranking. It may get some power fed back through the ballast resistor backwards during cranking. But you are talking about the battery is down to 11V or less during cranking, the ballast is dropping through the field/ VR circuit and whatever else is on the IGN1 line, so the light can't surely get more than ?? 4V ??

Maybe if you are in a dark/ shaded area opposed to bright sun you will see it. Maybe my engine cranks enough pressure that the starter clicks it right off, or maybe my sender is just more sensitive.

The brake lamp has no ground unless the parking brake is set or the warning/ pressure switch is made, which of course it normally is not.

I'll look at the diagrams and see, but all I can tell you is what I see when I start the car.

I never said the wiring in my car was right, either.

 

[Mattax]

[URL]https://www.web.imperialclub.info/Repair/Lit/Master/227/Page06.htm[/URL]1966

I think the thing to do is study the circuits for your specific car to figure out why it works that way on a '70 a-body.
Like 67Dart posted, on a 67 the ign 1 from the key switch is dead with key in start. On a 67 Barracuda (ralley dash) the brake warning lamp is powered from the I1 terminal. It can turn on with either the parking brake or the distribution black switch grounding. I do not recall it ever lighting for a self test upon start. It works when I pull the hand brake and the key is in run.

 

[4mulas]

Then you show me where it gets power. IGN1 goes dead during cranking. It may get some power fed back through the ballast resistor backwards during cranking. But you are talking about the battery is down to 11V or less during cranking, the ballast is dropping through the field/ VR circuit and whatever else is on the IGN1 line, so the light can't surely get more than ?? 4V ??

Maybe if you are in a dark/ shaded area opposed to bright sun you will see it. Maybe my engine cranks enough pressure that the starter clicks it right off, or maybe my sender is just more sensitive.

The brake lamp has no ground unless the parking brake is set or the warning/ pressure switch is made, which of course it normally is not.

I don’t have the time to open a schematic to show you that the warning light for brake and oil is powered at the instrument cluster all the time, and when the brake or oil light switch itself gets grounded it completes the circuit which illuminates the light.

It’s in ALL the SERVICE manuals. Look it up yourself.

 

[4mulas]

[URL]https://www.web.imperialclub.info/Repair/Lit/Master/227/Page06.htm[/URL]1966

I think the thing to do is study the circuits for your specific car to figure out why it works that way on a '70 a-body.
Like 67Dart posted, on a 67 the ign 1 from the key switch is dead with key in start. On a 67 Barracuda (ralley dash) the brake warning lamp is powered from the I1 terminal. It can turn on with either the parking brake or the distribution black switch grounding. I do not recall it ever lighting for a self test upon start. It works when I pull the hand brake and the key is in run.

You are correct. And it IS supposed to illuminate during crank to show you the circuit is functional.

67dart273 means well but doesn’t know and is clouding the issue and putting this thread off topic.

 

[Mattax]

The warning lights should not be powered all the time, just when the key is in either start, or run, or accessory.
If a lamp turns on during start, then one method that may have been used is to have a path that is balanced with same voltage on both sides of a lamp during run.

 

[jos51700]

I think the simplest answer for OP, regardless of all this, is that with the key in RUN, the oil light and brake light work normally. There's no self-test for BRAKES like on later vehicles (that I'm aware of), and the default operation for the oil light also coincidentally serves as a self test. If hop in, insert the key, turn your key to RUN, and the oil light isn't on, something is wrong. (or you have an accumulator that hasn't bled down yet.)

You're not going to be driving with the key in START or ACC, so those don't really matter. There was another poster here that recently had the oil light on with the key removed, and that's a whole 'nother issue.

In my personal case, any bright ideas I have don't appear above my head as an Edison lightbulb or even an LED. At best it's an oil light letting me know I'm not as ready to go as I think I am, and in the worst case, it's a BRAKES light letting me know I'm about to hit a Chestnut tree.

 

[67Dart273]

I don’t have the time to open a schematic to show you that the warning light for brake and oil is powered at the instrument cluster all the time, and when the brake or oil light switch itself gets grounded it completes the circuit which illuminates the light.

It’s in ALL the SERVICE manuals. Look it up yourself.

No it is NOT powered "all the time"
It is powered ONLY in RUN by the IGN1 circuit. It is backfed BACKWARDS through the ballast resistor by the IGN2 circuit during cranking BUT the ballast is loaded by whatever else is on that line.

YOU look it up. If you CAN NOT AND WILL NOT prove your point then don't come here and argue.

 

[67Dart273]

You are correct. And it IS supposed to illuminate during crank to show you the circuit is functional.

67dart273 means well but doesn’t know and is clouding the issue and putting this thread off topic.

RE read what I said. YOU are the one who "doesn't have time" to look it up.

There are ONLY two power sources, from the switch----IGN1 and IGN2, PERIOD. If you do not understand how they function, you need to SPEND THE TIME with an ignition switch and figure that out, or else don't come on here and argue that someone is wrong.

 

[Mattax]

Here's a start (no pun intended).
1970 Plymouth FSM
Feed to the lamps is with the key in run (I1) -> J2 -> G5.

G6 ends up here with no splices.

So far, if the oil pressure warning lamp flashes during start - its likely to be a brief back feed through the ballast. Would think it unlikely that current can be sustained through start when there are other paths to ground available.

 

[4mulas]

These are grounds. All the other lights in the dash work off the instrument cluster ground. The warning lights each have their own ground. Ie: when the brake distribution block loses pressure it complete the ground via the switch.

 

[Mattax]

For sake of clarity in this discussion, now we know what you mean by grounds.
I don't call those grounds. To me they are a connecting wire and a ground wire is normally at zero voltage.
While I have in the past described that type of connecting wire as a return or ground when explaining the dome light switches, I refrain from that if I can.
The wires to the sending units are at system voltage when the key is in run and the switch is open. The wires between the dome light and the switches are hot all the time.

Is it important? IDK but it could be. Its kindof like a switch leg in residential wiring. Switch is off but the wire leading to it is hot and so is the fixture.

 

[Mattax]

Only when current flows through the lamp does the voltage drop to zero on the return side.

 

[jos51700]

No it is NOT powered "all the time"
It is powered ONLY in RUN by the IGN1 circuit. It is backfed BACKWARDS through the ballast resistor by the IGN2 circuit during cranking BUT the ballast is loaded by whatever else is on that line.

YOU look it up. If you CAN NOT AND WILL NOT prove your point then don't come here and argue.

RE read what I said. YOU are the one who "doesn't have time" to look it up.

There are ONLY two power sources, from the switch----IGN1 and IGN2, PERIOD. If you do not understand how they function, you need to SPEND THE TIME with an ignition switch and figure that out, or else don't come on here and argue that someone is wrong.

Please calm down a little bit. He's not attacking you, and if he is, that's his problem, not anything wrong with you.

 

[4mulas]

What dart273 does not understand is that the brake light switch, the e-brake or the one on the combination valve, make the light illuminate by means of completing the circuit by Grounding. So yes there is power at the light, at the ready, all the time when the car is on - but the light will only come on when one of the switches goes to ground.

He’s off the chain and even sent me out of line pm’s, but if he calmed down long enough to actually read, and then look at wiring diagram he might see how it actually works.

This thread has gone to crap now because of some people’s persistence to try and get their wrong info across. Thanks to the others who get and kept it on topic