Chg and brake light

[AZDatsun]

78 280z I was Driving along tonight and the car almost died at the stop light and all the headlights got dim, and the charge and brake warning light came on. I reved the engine up and the charge light went back off and the headlights went back to normal but the brake warning light stayed illuminated. Not sure what is going on any ideas?

 

 

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[Zed Head]

@Captain Obvious knows the 78's well.  I remember something different about how the brake warning light works on the 78's because they have the internally regulated alternator.  Not sure.  It's after midnight east coast.  You might explore the wiring diagram in the Engine Electrical chapter.

[Captain Obvious]

Thanks Zed for the hook. I'm not "hands on familiar" with the 78 (I have a 77), but I'll do what I can.

For 78, there are only a couple things that can turn on the brake light. 1) Pulling up on the emergency brake handle. 2) Low brake fluid level. 3) The brake check warning relay.

As Zed Head mentioned, the third one (brake check relay) is tied into the alternator charging system and will light up the brake light if the alternator output drops too low. But it's SUPPOSED to go back out when/if the alternator output rises to normal again.

So, I don't have any conclusive explanation for the latched warning light you experienced, but I would start with the easiest... If your brake fluid level is normal and your parking brake handle switch isn't "touchy", then I'd guess your brake warning relay may be having a problem. They fixed a lot of the electrical gremlins by the time they got to 78, but the earlier years have had issues with that relay.

[AZDatsun]

Thanks so much, yeah my brake fluid wiring isn’t even connected so I doubt it would be that with both the charge and the brake light coming on at the same time I will check the relay first and go from there I haven’t checked this morning if the light is still on but it went off when I turned the car off. So there is that much. Maybe the connection on my alternator is loose that’s what caused all the dimming also I will check and report back


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[Zed Head]

1 hour ago, AZDatsun said:

Thanks so much, yeah my brake fluid wiring isn’t even connected 

Might as well check the wire ends to be sure something didn't short.  Not connected doesn't necessarily mean not powered.

Here's some FSM stuff.  They show the relay locations in there also.

 

[gwri8]

You've probably already done this and I didn't see it in this thread,  but did you check you alternator output with a multimeter?  The alt or belt might be at the beginning stages of leaving the arena, so to speak. Just because the lights don't dim as much doesn't mean that the alternator is performing to spec and could cause your brake light to stay on.

[AZDatsun]

Hmm ok so this is odd with the relay under the seat unplugged the brake light goes off with the parking brake handle down but lights up properly when I lift the parking brake handle... I didn’t touch anything else... I am so bad at electronics I don’t know how it works at all with the relay unplugged


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[Captain Obvious]

That operation with the relay removed is correct.

There are three possible ways to light the warning lamp and they are "in parallel". If any of them connects to ground, the light comes on. By disconnecting that relay, you have simply removed one of the possible ways to connect the lamp to ground.

The parking brake handle still works because it's still able to pull the lamp to ground even if one of the other paths (like the relay) have been completely removed. In fact, you said you have disconnected the brake fluid level switch as well, and the parking brake lever still lights the light. Same thing for the warning relay.

Does that make sense? I'm trying to describe things for someone who doesn't do a lot of electrical work. Hope I'm not oversimplifying and or insulting...

Edited April 22, 2020 by Captain Obvious
grammer

[Zed Head]

It's shown in the diagram.  There are three possible paths to ground the light circuit so the light comes on.  What you did suggests that the relay is stuck or the alternator "L" circuit is messed up, like gwri8 suggested.  Check the solenoid's circuit to ground on the relay and see if it's stuck.  It should be open circuit when it's unplugged.   If the relay is right then your alternator might be messed up.

CO just replied but I'm going to hit Submit Reply anyway...

[AZDatsun]

How do I check the alternators output on the L circuit put a probe on the L line and one to ground and see what the volts are? What should I expect to see?

 

And to check the relay what do I do exactly move the meter to ohm and probe between the ground prong and all the other prongs?

 

If it is the relay I need to find a 4 prong normally open 20 amp relay?

 

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[Zed Head]

Here is a more simple way to check the relay.  Plug it back in, turn the key on and see if the Charge light on your voltmeter comes on.  It should.  If it does not then the ground circuit through the relay and the alternator  is bad.  That would explain two problems.  Could be just a bad connection at the T plug on the alt, or a bad alternator or a bad relay.

I think. If I  understand the circuits correctly.

Edited April 22, 2020 by Zed Head

[AZDatsun]

Ok I went out plugged my relay back in and started the car my parking brake is light is now working again like normal but the charge light does not come on when starting the car

I think we are onto something not sure what but something


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