Electrical help required

[Woznaldo]

As some of you will know, I am still trying to get my GTT complied for road use in Oz and have hit a small electrical problem. The engineer wants the brake warning light to come on when the ignition is switched on (hand brake off) and then go off after the engine has started. A sort off self test so the circuit can be proven before setting off.

To help me work out what needs to be done, I need someone with a known working std instrument panel to tell me what happens with the brake warning light (BWL) with the ignition key at the different positions and with the engine started (all with the handbrake off) i.e.

-ignition off - BWL OFF
-ignition to 1st position - BWL OFF
-ignition to 2nd position - BWL ?, Battery Light ON
-Start Engine - BWL off, Battery Light OFF

I think I might need to make a small circuit that switches the BWL to earth and then back to std circuit when an ignition pulse is felt?

Scoff, this is obviously your area of expertise (well, one of) and i'd appreciate your help to the max to the power of 10!!!

Woz

[Nick k]

What about using a magnetic read switch/with coil around it like on the brake light monitoring circuit for bulb failure. ( Old vauxhalls ) You basically wire it the same and use the handbrake light as the warning light. It would come on with the ignition and go out once you press the brake pedal / You could also power the circuit from the dip stick level circuit so it would go out anyway once the engine is running, but you would have to put a blocking diode in there to stop it feeding back to the dip stick circuit when you put the hand brake on whilst engine running, There are afew ways it can be done to be honest.....

[Scoff]

Sorry woz, I don't know how the lights should work on a normal GTT, so can't predict what you'd need to do.

If you can verify exactly what the lights should do on a standard car then I might be able to tell you how to wire it up.

[Scoff]

Looking at the wiring diagram it looks like there are 2 brake warning lights on the cluster

1) Handbrake & fluid level indicator
2) Brake pad worn lndicator

both are common live (inside the cluster) and are switched to earth via the hand brake lever, fluid level switch and the brake pad wires.

I think you want the handbrake & fluid level lamp (both on the same lamp) to come on with ignition, regardless of weather handbrake is on or off, am I right ? Your not bothered about the pad warning light are you ?

If the above is correct then the scoff bodge would be this:

Stick a diode (1n4001 or similar) between the wire going to the hand brake lever and the live feed to the fuel pump. cathode (end with white band) connected to the fuel pump live and anode (no band) connected to the hand brake wire. Both of these wires should be available in the block connector marked "308" in the haynes, I think that is the connector in the right hand side foot well (UK drivers side), behind trim just in front of the door, so your dioide could be hiden in there.

What should happen then is the brake fluid warning lamp should light up with ignition on and engine NOT running. The second the fuel pump comes to life it should turn the light off. I've only just drempt this up so there may be side effects, like dash lamp staying on very dim when engine running (would idicate low volts to fuel pump) but it isn't likely. it won't harm anything anyway.

[Woznaldo]

Looking at the wiring diagram it looks like there are 2 brake warning lights on the cluster

1) Handbrake & fluid level indicator
2) Brake pad worn lndicator

both are common live (inside the cluster) and are switched to earth via the hand brake lever, fluid level switch and the brake pad wires.

I think you want the handbrake & fluid level lamp (both on the same lamp) to come on with ignition, regardless of weather handbrake is on or off, am I right ? Your not bothered about the pad warning light are you ?

If the above is correct then the scoff bodge would be this:

Stick a diode (1n4001 or similar) between the wire going to the hand brake lever and the live feed to the fuel pump. cathode (end with white band) connected to the fuel pump live and anode (no band) connected to the hand brake wire. Both of these wires should be available in the block connector marked "308" in the haynes, I think that is the connector in the right hand side foot well (UK drivers side), behind trim just in front of the door, so your dioide could be hiden in there.

What should happen then is the brake fluid warning lamp should light up with ignition on and engine NOT running. The second the fuel pump comes to life it should turn the light off. I've only just drempt this up so there may be side effects, like dash lamp staying on very dim when engine running (would idicate low volts to fuel pump) but it isn't likely. it won't harm anything anyway.

Thanks Scoff,

I think I can get that diode circuit sorted.

Yes, I don't care about the brake pad wear light, just the 'Park Brake' ((P)) and BWL ((!)). I had a local Auto Elec take a look and he tried to put a diode across earth circuit tacks of the Battery Light and the BWL which made no sense to me?

This is the last thing I need to square away for the engineer before I get the thumbs up!

I'll have a crack at it tomorrow and report back.

[Scoff]

Well he's sort of right, that would work too but you run the risk of none of the lights working should the alternator warning circtuit fail. the fuel pump method is fool proof (unless the fuel pump go open circuit) and is easier to do since you don't have to fetch clocks out even.

his method will have been a diode between the earth side of the 2 lamps, cathode toward the alternator lamp, meaning that when the alternator switched to earth it would light both lamps, without intefering with the operation of the BWL whilst running.

didn't he get it working ?

[Woznaldo]

The battery light wouldn't come on at all?

[Scoff]

should have done, something might have been wired wrong.

[Woznaldo]

The Auto Elec had the diode the other way. I changed it to the way you said which made some sense. I did pull the bulb holder out to do the soldering so may have to check the battery light bulb?

I'll try your method anyway as it makes more sense to me.

On a plus side, he did manage to get the speedo working. turned out to be something in the instrument panel end?

[Scoff]

ah, see no need to have done it on the bulb holder, you can do it at the wiring that plugs into the clocks, that way you are sure of getting the earth side of the bulb.

[Scoff]

diode the other way means that he probably thought it was all common earth, but it's common live.

[Scoff]

plan B for you tomorrow, should the 1st method have side effects:

find the BWL lamp wire and the battery warning lamp wire in the looms going to the clocks. Fit the diode between the 2 wires, cathode (band) pointing toward the battery warning lamp wire.

[Woznaldo]

Thanks again Scoff. I'll let you know how I get on. Here's a pic of his handy work:

[Scoff]

Thanks again Scoff. I'll let you know how I get on. Here's a pic of his handy work:

its on the right side of the bulbs, so short of it maybe being the wrong way around it should have worked.

[Woznaldo]

The battery light not working was a blown bulb. I didn't have access to the car today (forgot the garage dude took the day off for his son's birthday) so I will hook it all up tomorrow.

I did hook it up to a power supply at work and it all seemed to work as advertised with the diode in place as per the picture above.

[markey b]

or ya coulda wired it to the oil pressure light....

i have done it before on a corsa gsi for its mot, after a few mins the abs light could come on, so we took the bulb out for mot and it failed, so i wired it to oil pressure, so it went off when started...

[Scoff]

or ya coulda wired it to the oil pressure light....

i have done it before on a corsa gsi for its mot, after a few mins the abs light could come on, so we took the bulb out for mot and it failed, so i wired it to oil pressure, so it went off when started...

yes, but I think he would still like the lamp to function properly when driving too, that's what the diode solution is for

[markey b]

oh, i see

[Woznaldo]

Update. Installed the instrument panel at lunchtime today and all is working as advertised. With the handbrake off, the BWL now comes on with the ignition and goes out once the engine has been started.

Thanks for you help Scoff.

In a cruel twist of fate, my brake reservoir cap complete with low fluid warning switch for the above BWL has gone open circuit!!!!! (when lifted out of reservoir)