Author Topic: What happens to oil warning system when W tach not installed (Read 3670 times)

914turboford · « **on:** January 18, 2008, 12:35:52 pm »

I searched extensively before posting.

I have a 1982 Caddy. I had the oil pressure buzzer go off and I checked the output at the oil filter housing and it checked ok. I replaced the filter housing sensor. This kept the light at bay.

Then, not knowing the W terminal had anything to do with the oil warning system I robbed the alternator from this truck to put on another car with a tach.

While driving the truck the other night we got no buzzer/light for a 10 mile trip out but on the return trip the buzzer/light came on briefly a couple of times. I replaced the cylinder head sender and took it for a drive with no buzzer/light.

What should happen when you have a dynamic system but no W terminal? Should the oil light go off all the time or not at all?

RabbitJockey · « **Reply #1 on:** January 18, 2008, 03:20:38 pm »

i didn't realize they were related

Vincent Waldon · « **Reply #2 on:** January 18, 2008, 04:15:34 pm »

Without a W signal the dynamic oil pressure circuit built into the cluster will be guessing as to what RPM the engine is at.... if the W wire is dangling electrical noise could occasionally look like an RPM spike and cause the circuit to look at the wrong signal.

Without a W signal you're not using the dynamic oil pressure system anymore anyway so I'd be inclined to ground the W wire (prevent noise gremlins from looking like RPM) or ground the wire to the high-pressure sensor on the oil filter flange (if the warning circuit does wake up it will alway see a perfect signal from the sensor ie ground).

The buzzer is specific to the high pressure sender btw... the one on the head only trips the light and is always being monitored. This means that you will still receive warning of low pressure... just really really low pressure (0.3bar) as opposed to the early warning you might get from the high pressure sender (1.4 bar).

914turboford · « **Reply #3 on:** January 18, 2008, 05:24:35 pm »

Quote from: "Vincent Waldon"

Without a W signal you're not using the dynamic oil pressure system anymore anyway so I'd be inclined to ground the W wire (prevent noise gremlins from looking like RPM) or ground the wire to the high-pressure sensor on the oil filter flange (if the warning circuit does wake up it will alway see a perfect signal from the sensor ie ground).

The buzzer is specific to the high pressure sender btw... the one on the head only trips the light and is always being monitored. This means that you will still receive warning of low pressure... just really really low pressure (0.3bar) as opposed to the early warning you might get from the high pressure sender (1.4 bar).

My wife drove the truck today and on the return part of a 20 mile round trip the buzzer/light came on 3 times at low speed (20mph or less while coming to a stop). If the dynamic system shouldn't be working with the W disconnected what is causing it to sound?

I doubt this matters but this truck sat for 5 years and we have only driven it 200 miles since we got it 3 weeks ago. It still has the 5 year old oil in it. I changed the filter (in case it was internally collapsed) but not the oil yet. I would be highly doubtful that old oil could cause a low oil pressure situation.

Vincent Waldon · « **Reply #4 on:** January 18, 2008, 07:19:37 pm »

Quote from: "914turboford"

My wife drove the truck today and on the return part of a 20 mile round trip the buzzer/light came on 3 times at low speed (20mph or less while coming to a stop). If the dynamic system shouldn't be working with the W disconnected what is causing it to sound?

If the W terminal is "floating" (not grounded) then random electrical noise (an automotive electrical system is a very spiky place) could easily look like a burst of RPM and fool the dynamic system.

Hence my thought of grounding the unused W wire that used to go to the W terminal on the alternator.

dieselsnowmobile · « **Reply #5 on:** January 19, 2008, 10:38:18 pm »

I have mine unplugged right now. With it unplugged, it will only go off the pressure of the head, it will no longer be dymanic. Since unplugged, I have never ever had the buzzer go off.

saurkraut · « **Reply #6 on:** January 20, 2008, 05:48:03 am »

What did you unplug, sensor, W terminal, or relay?

RabbitJockey · « **Reply #7 on:** January 20, 2008, 09:24:22 am »

my car does this bs too, once its warmed up anytime the engine slows down the buzzer will come on for a second just before the car returns to idle... it never does it under acceleration, i don't understand it

Vincent Waldon · « **Reply #8 on:** January 20, 2008, 10:44:14 am »

Quote from: "Trev0rbr"

my car does this bs too, once its warmed up anytime the engine slows down the buzzer will come on for a second just before the car returns to idle... it never does it under acceleration, i don't understand it

The short answer is that your oil pressure (or perhaps, just perhaps, the high pressure sensor) is on the edge... when the engine is hot and the oil is thin the pressure is low enough around 2000 RPM to just trip the sensor.

So, the buzzer comes on very briefly as the engine drops past 2000 RPM... at 2500 theres enough pressure to keep the sensor happy, but right at 2000 RPM (ish) the pump is turning a bit slower and the pressure is just low enough to blip the sensor.

You don't see it on acceleration because the RPMs climb, the pump spins faster, and the oil pressure goes up before the sensor trips.

Very very common symptom on MK2s... do a search for "dynamic oil pressure" and you'll find a Stephen King novel worth of discussion... more than you ever want to know. ;-)

Black Smokin' Diesel · « **Reply #9 on:** January 20, 2008, 11:22:42 am »

Quote from: "Vincent Waldon"

The short answer is that your oil pressure (or perhaps, just perhaps, the high pressure sensor) is on the edge... when the engine is hot and the oil is thin the pressure is low enough around 2000 RPM to just trip the sensor.

So, the buzzer comes on very briefly as the engine drops past 2000 RPM... at 2500 theres enough pressure to keep the sensor happy, but right at 2000 RPM (ish) the pump is turning a bit slower and the pressure is just low enough to blip the sensor.

You don't see it on acceleration because the RPMs climb, the pump spins faster, and the oil pressure goes up before the sensor trips.

Very very common symptom on MK2s... do a search for "dynamic oil pressure" and you'll find a Stephen King novel worth of discussion... more than you ever want to know. ;-)

Mine used to do that. I upgraded to the 36mm oilpump instead of the 30mm unit found on 87 and older diesels. It fixed the low oil pressure problem. Now I know that the bearings are kinda worn though and will need to be replaced soon.

RabbitJockey · « **Reply #10 on:** January 20, 2008, 04:45:12 pm »

ah thanks guys

Vincent Waldon · « **Reply #11 on:** January 24, 2008, 06:55:54 pm »

The output of the W signal from a diesel alternator is many many times faster than the signal the gasser tach is expecting from a conventional ignition system... hence your gasser tach reading 5K at idle when connected to a diesel engine.

And if the tach thinks the engine is at 5K so does the oil pressure warning system... meaning it's watching the high pressure sensor which is expecting the pressure to be high but is in fact low (engine is idling)... hence the buzzer.

Cut the wire... oil pressure system thinks the car is below 2000 RPM... ignores the high pressure sensor... no buzzer.

And thus the universe unfolds as it should... :wink:

Vince