Oil Warning Lamp at 2k rpm's

[Helge Wunderlich]

On Fri, 27 Sep 2002 11:56:35 -0300, Sean Shoemaker wrote:

>Now that I finally had a chance to test drive my '84 CGT I found a problem.  As soon as I hit 2k rpms on the tach the oil warning lamp comes on and then goes back off when the rpm's drop.

There's no buzzer ? Normally, when that part of the oil pressure
warning kicks in, there should also be an annoying buzzer going. At
least there is in mine ('84 100 with WC engine).

>Several theories abound, from a clogged pickup screen to a faulty sensor.  Any known problems in this area.

I have had the problem twice. The first time, it was one of the oil
pressure sensors AND the oil warning control unit. I was able to
"reverse engineer" the control unit and repair it. The new oil
pressure sensor only lasted a few months. It turned out I had a tiny
coolant leak, which dribbled onto the sensor and somehow damaged it.

>I would just like a way to narrow the problem before I go dropping the oil pan and my brand new 20W50.  Also I have a factory analog oil pressure gauge in my console and it is pegged at 5 bar anywhere off idle.

Leave your oil in.

Yours may be different than mine, but I think it is the same. Mine
works like this:

There are two oil pressure sensors. Both are located on the left side
of the engine, one above the other, between the thermostat housing and
the oil dipstick. They (especially the upper one) may be hidden behind
the coolant tube for the heater.

One of the sensors is a "normally closed" switch which opens at a
pressure of about .3 bar. This sensor is connected directly to the oil
pressure warning light. If the warning light comes on when you switch
on the ignition, and goes off after you start, this sensor is OK. The
wire going to this sensor is blue and black.

The other sensor is a "normally open" switch which closes at a
pressure of about 1.8 bar. This one is connected to the "Oil Pressure
Warning Control Unit" (This is a translation, as my Haynes book is
Swedish. The unit is labeled J114 in the diagram). The control unit is
also connected to the ignition coil so it can sense engine revs. If
the revs exceed a certain level (2000 RPM seems about right), and this
other sensor still has not closed, the alarm goes off. The wire going
to this sensor is white and grey.

The pressure levels could be different for the turbo engine.

Here's how to test:

- Disconnect the white/grey wire from the sensor and connect it to
ground. You probably need to fabricate some sort of extension to reach
good ground.

- If the problem goes away, either the sensor is the problem or you
actually have low oil pressure. The sensor is not expensive and is
easy to replace. It *could* theoretically also be the control unit,
but it is very unlikely.

- If the problem does NOT go away, disconnect the blue/black wire and
leave it disconnected.

- If the problem still does not go away, the control unit is probably
the problem. It is quite expensive, but a reasonably experienced
electronics engineer should be able to repair it. There is only
conventional stuff inside. If you want the schematics for it, let me
know.

- If the problem did go away, either the sensor or the control unit
may be the problem.

--
Helge Wunderlich