Anybody interested in fixing the nuisance pump running and sticking oil sensor problems once and for all?
I can post my fixes if so.
Summary:
There are at least three levels of intervention. All are based on the assumption that the sensor is sticking causing the ECU to run the oil feeder motor too much and overfill the engine. This is the most common "initial" problem with the auxiliary oil system.
1. Poor man's fix: One resistor, one jumper wire. Fakes oil level OK to the ECU. Forget the oil level sensor. Forget the oil reservoir. Check oil level regularly with the dip stick and add as required just like normal humans do with lessor (normal) cars. ECU will never again command the oil pump to come on.
2. Working man's fix: Two resistors, a SPDT switch, and some wire. Fakes oil level OK or oil level low to the ECU, your choice. Forget the oil level sensor. Allows you to fake oil Low condition to tell the ECU to add oil to the engine from the oil reservoir. Check your oil level regularly with the dip stick just like a normal car, but carry extra oil in the oil reservoir and let the ECU add it for you when you decide.
3. Rocket Scientist fix: Use this fix if you aren't intimidated by soldering. Eight resistors, a SPDT switch, two LEDs, two transistors, and some wire (all very cheap). Fakes oil level OK or oil level low to the ECU, your choice. Allows you to tell the ECU to add oil to the engine from the oil reservoir. Lets you decide whether to add oil based on sensor behavior as read by the LEDs and/or dip stick reading, but carry extra oil in the oil reservoir and let the ECU add it for you at your command.
4. Best Fix: Based on what I learned doing the above fixes, a combination of the Poor man's fix and the Rocket scientist fix. Fakes the sensor input to the ECU (OLS and OMS) so that the ECU never attempts to turns on the Engine Oil Auto Feeder Control Motor. Fakes an Oil Feeder Motor signal to the ECU (OMT) so that the ECU believes the motor circuit is OK. Lets you decide whether to add oil based on sensor behavior as read by two LEDs and/or dip stick reading, but carry extra oil in the oil reservoir and lets you add it by manually pushing a momentary switch to power the engine oil level relay. I like this fix because it takes the ECU out of the control loop for adding oil, and lets you decide when to add oil, intelligently, easily and remotely from the reservoir as intended. No carrying oil bottles in the car, you have two quarts in the reservoir to call upon at the touch of a button.
Now, how to test the oil level sensor.
First a little explanation of how the sensor circuit works.
Inside the ECU, a regulated 5v is delivered to what I will call the OMS ECU resistor and the OLS ECU resistor. Those ECU resistors are each about 5 kohm. The other side of those resistors come out of the ECU as the OMS and OLS sense wires. The ECU reads the current in those wires and can tell three conditions for each sense wire. The sensor in the oil pan also has a 5 kohm resistor in parallel with a switch (see the shop manual diagram of the sensor) for the OMS as well as the same setup for the OLS sense wires. So current can flow from the ECU 5v regulated source thru the OMS ECU resistor to the sensor, and from there either thru the OMS sensor resistor to the E1 ground return wire, or thru the OMS sensor switch to the E1 ground return wire. The OLS sensor circuit is the same.
When the ignition is on:
1. A current of zero in the sense wire means the sensor wire is disconnected or broken (open circuit).
2. A current of approximately 0.5 mA means the sensor is connected and that sensor wire's parallel switch is open.
3. A current of approximately 1.0 mA means the sensor is connected and that sensor wire's parallel switch is closed.
To follow the procedure in the shop manual for testing the sensor circuit: It has you measure voltage between sense wires and ground. Connect the positive probe to the sense wires between the ECU and the sensor with everything connected, the oil level at the full mark on the dip stick, the ignition on, and engine not running. Connect the negative probe to ground. If everything is working correctly, the oil float should be in the high position. The OMS sensor switch should be closed and the OLS sensor switch should be open. This produces a current of ~1.0 mA with a voltage of ~0v to ground on the OMS sense wire, and a current of ~0.5 mA with a voltage of ~2.5v on the OLS sense wire. If you performed this same test with the case empty at an oil change, the reading should be the opposite. That is the oil float should be in the low position. This OMS sensor switch should be open and the OLS sensor switch should be closed. This produces a current of ~0.5 mA with a voltage of ~2.5v on the OMS sense wire, and a current of ~1.0 mA with a voltage of ~0v on the OLS sense wire.
The sensor has three credible states:
1. Float high, OMS sense switch closed, OLS sense switch open.
2. Float NOT high and NOT low, OMS sense switch open, OLS sense switch open.
3. Float low, OMS sense switch open, OLS sense switch closed.
As you can imagine, the float position with the engine running is influenced by more than just the oil quantity in the engine. It is affected by engine RPM ("wind" caused by cshaft rotation), vehicle acceleration, vehicle slope, cornering and braking as well as oil temperature and air bubbles in the oil (foaming). As these change, so does the float position. The most common failure of the sensor is that it becomes dirty and worn. It does not slide as easy as it should, so becomes “sticky” and slow to respond. Because of the way the ECU interprets the sensor output, this is often read as a low oil level. For example, when braking, the oil sloshes forward and the float drops down. The OMS switch opens. When braking stops, a slow to respond sensor may stick for a while with the OMS switch open longer than expected. The longer the float remains down, the more likely the ECU is to interpret it as a low oil condition. Any of the above conditions can cause the float to drop. The longer it stays down when it should be up, the more likely the ECU will turn on the pump to add oil.
Faking the float high position is as easy as cutting the OMS and OLS sense wires and adding one resistor and one jumper to ground to the OMS and OLS ECU input side of that cut.
Reading the float position in the driver seat is as easy as combining the OMS and OLS sensor side wires cut above, +B and E1 ground with $0.50 worth of simple components (2 LEDs, 4 resistors, and two transistors).
Faking a Engine Oil Motor good signal to the ECU is as simple as cutting the OMT ECU wire and routing the ECU side to ground.
Controlling the Engine Oil Motor is as simple as cutting the ECU OMR wire, insulating the ECU side and running the Engine Oil Relay side to a push button switch to ground.
I can post a schematic with instructions and pictures if there is enough interest.
BTW, there are two reasons I decided to do these fixes on my vans.
1. I was tired of listening to the oil motor run for no reason with an empty reservoir. I had earlier found the sensor bad, cleaned the sensor, found it bad again, ran all the oil out of the reservoir so it would not dump anything into the engine and overfill it.
2. I subsequently had my first ECU failure suspected as being caused by the nuisance oil motor running on that same Previa. The failure showed a burnt place on the main circuit board of the ECU. The failure manifested as an otherwise great running Previa suddenly "winking out." By winking out, I mean, the Oil Level Warning Light would flash on abnormally bright and all ECU functions would cease. When this happened while driving, the engine would die, the transmission would pop out of gear into neutral, and I could hear relays dropping out. It started as intermittent dropout and if I kept the engine reved, I could "play thru" and keep going, but it was very unnerving and I was worried about the catastrophic damage I imagined a haywire ECU could do to my beloved van. The final straw was when it died completely in Austin. That 95 van uses a 89661-28352 ECU, and at home in the Houston area, I had a spare -28351 (the ECU just before mine), and a spare -28600 (the ECU just after mine). I decided to perform a "hail-Mary", left the Previa overnight in the Walmart parking lot where it died, borrowed my daughter's car, drove to Houston and back retrieving my spares, and installed the -28351 in the parking lot. It worked. No more winking out and I now control the oil adds.
I will start a new thread on ECU substitutions and required mods if anyone is interested. My interest starts with my experience with my -28352 (-28351) ECU van (95 LE SC rwd) performance and my -28600 ECU van (96 DX SC rwd) performance. The -28600 has always been much peppier during the 330k of combined comparable miles I have owned and driver them, and I want that for my 95. I suspect the ECU is the reason. I am going to make the simple mods to insure the ECU substitution will work (no expensive $$ peripheral component switchout if I can). I also would like to consider the 97 year ECU but need the 97 shop manual and electrical schematics manual for that model year to do it. So for now, only doing the 96 ECU upgrade to my 95.