1972 Brake Warning to Oil Pressure Warning Conversion

[Rosco_NZ]

So my project is progressing well .. decided seatbelt warning, handbrake warning, brake fail circuits won’t be reinstated as it’s all a bit messy and redundant.

And what’s with the early cars and no oil low pressure warning light .. just the guage. Don’t want to lose a hose off the oil tank and not know about it!!

So let’s install an oil pressure switch into the Carrera tensioner block mount using the mount block from ‘74 on and repurpose that brake warning light to an oil pressure warning light .. it almost looks like it was supposed to be an oil light anyhow .. and often gets mistaken as such.

I’m guessing the oil pressure switch is normally closed and opens with oil pressure, so switch a relay to ground to supply 12V to the warning light, (via the handbrake circuit??). Oh and the buzzer socket in passenger footwell uses standard round Porsche relay mount… so let’s reuse that factory location and wiring to install a standard Porsche relay in place of the buzzer (long gone) and wire in a different oil warning buzzer in parallel with the light, for no oil pressure while we’re there.

So we can probably install with minimal wiring changes and reuse some of the currently redundant wiring.

All sounds like a very cunning but extremely useful plan… other than originality, why not … ??

Surely I’m not the first early 911 geek to dream this up …??

[Rosco_NZ]

Actually looks like the Brake Warning Light is ground switched via the Handbrake Switch or Brake Pedal switch.. so should be able to use oil pressure switch to control lamp without needing a relay. Buzzer may require a relay .. what current is oil pressure switch rated to?

[Walt Fricke]

The idiot light sensor/switch is set at a really low value - maybe 0.2 bar - to open. You can purchase similar switches which have higher open values and adapters to deal with thread differences (metric 10 versus US 1/8" or whatnot pipe).

However, the parts manual would suggest that '72-3 911s had both an oil pressure sender and an idiot light switch. Typically the pressure sender is located at the rear by the fan pulley on a banjo where the right cam oil fitting is. And the idiot is located in the case (with an adapter) back by the oil thermostat and the engine breather cover. However, you could put either one in either location.

And one of the warning lights on the temp/pressure gauge is for the low oil warning. Or it seems it should be, though maybe there is some variance which escaped even Grady Clay's notice?

See this: Oil pressure warning light on 1969

[Rosco_NZ]

For US spec cars at least, there is not any idiot light. There is a temp sender at the rear of the block racing the rear of the car visible when decklid open. And the pressure sender forward in the triangle of death as you state. But no pressure switch. Possible RoW spec cars were different but I can’t be sure. I suspect the park brake light still works but fasten seat belt sign and buzzer possible omitted ..??

[PeteKz]

My US Spec 1973 911T does not have an oil warning light either. I would like to get one and even more importantly, a buzzer. A small light in one of the instruments or elsewhere on the dash won't get my attention fast enough on a sunny day, but an audible warning will. I have an SC engine in it, which has the oil pressure switch by the oil thermostat and the Green/white wire going from the OP switch through the engine harness to the 14-pin connector. On the car side of the 14 pin connector, there is a black/purple wire that goes off into the car somewhere and I don't know what it does or where it terminates. It seems I could repurpose that wire for oil pressure buzzer/warning.

I'm subbed to this thread too.

[thetorch]

Regardless of how you wire it... I would avoid adding a new opportunity for a future oil leak by using the dual pole VDO pressure sensor referenced in Grady's post. I believe it does what you want, and is readily available.

[PeteKz]

That's all good info, thanks. I already have the oil pressure switch on my 3.0 case (next to the thermostat), and it's not leaking. Yet. I'll probably get more motivated to install a combo sensor/switch at the back of the engine when it starts leaking!

[Rosco_NZ]

I think the current the oil pressure switch can operate is minimal .. OK for the 2W indicator light but I think installing a buzzer as well in parallel might exceed its comfortable switching capacity, so looking to switch a std Porsche round relay fitted to the old buzzer location. The original wiring in my car is deeply hidden so may take some work to extract.

[PeteKz]

Use a piezo buzzer. The current draw is very small, fractions of an amp. A relay for that small a current is overkill.

[Rosco_NZ]

Agreed Peter, but I kinda want to retain the handbrake light operation. So I think a relay could avoid the buzzer going whenever the handbrake is on! I want the buzzer to be oil pressure only warning. Still working on the wiring logic to get operation as I want ..
Here’s the new 0-10 bar sender & integrated switch installed.

[panama911]

To my car, a 73.5 CIS, I have also added an oil-pressure switch - but NO idiot light or buzzer or such...

Instead, I have added a fuel pump relay, which the early cars didn't have, and that is controlled by the pressure switch. No need for a light, if pressure drops to where the light would come up, it would cut off the pump instead. Works great.

[panama911]

Originally Posted by Walt Fricke

The idiot light sensor/switch is set at a really low value - maybe 0.2 bar - to open.

...and this is why control of the pump relay with it works perfectly - the pump starts running within the first 1-2 cranks at start-up.

[Rosco_NZ]

I’m going to earth to the brake warning light and to the buzzer via a diode in the earth side. Then then light and buzzer will come on with low oil pressure, but the handbrake and low hydraulic pressure brake circuits will not feed the buzzer. If the buzzer comes on, it’s oil pressure. And of course will squeal at startup. It can be disconnected at the sender when doing work on the car and the ignition needs to be on .. I guess this is the best workaround …