OIL light off, Brake light ON

Hi guys, Latelly my bug has a problem. When I turn the ignition key, only the generator light goes on, the oil pressure light stays off. But the light is OK, if I connect the sensor wire to ground the light goes on, so I suspect its a problem in the wireing from the ignition key to the speedo lights. When I bought the car the wireing was a mess, several terminals disconnected, so after restouring it, I had doughts in some connections, but after I was done, al was right. Bouth lights (generator, oil) came on on startup, and went off right after. This problem is very recent. Coincidence, ou maybe not, the brake light is allways on now, and my brakes feel fine. Any thougts? Thanks. MM

Reply to
Manuel Macedo
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if the light lights up when you ground the sensor wire(at the engine i assume) then the sender is not doing it's job...

Reply to
Joey Tribiani

But when you turn the key and both lights go on, there is no sender test involved, the lights just get current so we can know that the lights are ok. If I turn the key and only one light goes on, but the light itself is ok, I see no indication of sensor trouble.

Reply to
Manuel Macedo

You see it is not working so change it. Dennis

Reply to
Dennis

It sounds like a bad oil pressure switch. A lack of a ground path will keep the warning light off. If you have a voltmeter around with a continuity checker you can verify this very easily. Look for continuity between the oil pressure sensor terminal and the switch housing/engine block with the engine off. If it reads as an open circuit your oil pressure switch is defective or you have lost the ground path between the switch and the block. Just make sure you have a good ground connection on the block before you condemn the switch.

I'd need to know what year your vehicle is to help you with this. On the later models there is a transistorized 'switch' behind the brake warning light that is supposed to turn on the brake lamp when the oil light comes on as a bulb-test. The brake lamp will stay on if you have the emergency brake lever pulled on these models, but if the oil light comes on for any reason the brake light will also come on. The brake light will also come on if one of the brake circuits fails(based on readings from the three-pin brake switches).

It's possible that you have a problem with that module, a problem with one of the brake light switches, or maybe just a problem with the emergency brake lever switch.

Chris

Reply to
halatos

Thanks,

But if this was the problem, the light would still be lit with the turn of the key. As I understand it, when you turn the key, current goes directly to the light to check it, the switch is bypassed. So if me bulb is ok (it lights up if I ground switch connector) but it doesn't lit with the turn of the key, we can't tell if the switch is faulty, right?

I have a 1970 VW 1300L. My brake light is one of those that lits if I push it.

Reply to
Manuel Macedo

You are correct that voltage is available to light up the lamp when you turn the key to the on position, but if the voltage supply does not have a ground path you will get nothing, i.e. NO warning lamp. With a bad pressure switch, assuming everything else is ok, you will have voltage, but you do not have a complete circuit.

Unfortunately, what you are telling me now is starting to muddy the waters. Are you saying that with the key off if you ground the sensor wire the light comes on? If so you have a problem somewhere else in the wiring of your car, the oil pressure lamp should not have 12 volts available when the key is in the off position.

I think, based on what you are saying, that you are reading more into this problem than is really there. Turn the key to run, do not start the engine. Does the light come on? If not, hop out, pull the wire from the oil pressure sender, and touch it to a ground in the engine bay. Light on now? If so, the most likely problem is that you have a bad pressure switch. Change it out, it is a cheap and easy thing to try and I would almost bet money that it will solve your problem.

Good luck with it.

Chris

Reply to
halatos

you are thinking too "modern"...what i mean by this is with today's vehicles that are all computer controlled, they light up lights in the instrument cluster while they "test" their operation.... our old VW's do not do this. the oil light is a very simple circuit....it gets positive power from the fusebox when the key is turned to the on position. the negative portion of the circuit(required to light the light) comes from the oil pressure sensor....in operation, the sender completes the circuit to ground when oil pressure is below the predetermined pressure. this is why the oil pressure switch always sends the negative signal when you first turn the key on, there is no oil pressure...

its easy to test(and i think this is what you said you did)....turn your key on, but don't start the engine. if the light comes on, the sender is working(usually, sometimes they can fail in the "no pressure" mode)...if no light, then ground the sender wire to the engine block...if the light lights up with the key on and the sender wire grounded your sender is bad....if it doesn't light up, but you have 12 volts to the positive side, you have a wiring problem in the sender wire.

Reply to
Joey Tribiani

As you mentioned, the three lights at the bottom of your speedo have constant 12v+ as long as the ignition switch is ON.

Before you start your engine, the Gen light is grounding thru the Gen/ Alt/Reg and will light up. The Oil light is grounding thru the oil press sending unit since it has zero pressure. Both of these light up before your engine starts and make for a good "bulb test". The middle turn signal light is also getting 12v+ but since the Flasher and Turn Signal lever are both off, this light does NOT light up (not all bulbs are tested).

Once the engine starts, the blue wire from the Gen/Alt/Reg gets some voltage close to 12v+ from the Gen/Alt side. This reduces the current flow (potential goes to zero) and the bulb goes out (two positive leads of equal voltage cannot light a bulb). Once running, the engine oil pressure also goes up and the oil press sending unit increases resistance between ground and it's terminal, so the Oil light goes out as well. Not sure if this is an on/off switch or a variable resistance switch.

Reply to
AshMan

Oil press sender is a switch. The rest of your analysis is spot on too.

Jim

Reply to
Speedy Jim

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