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Thread: Sticky Oil Level Sensor, Best Fixes

  1. #1
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    Sticky Oil Level Sensor, Best Fixes

    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.

  2. #2
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    Re: Sticky Oil Level Sensor, Best Fixes

    That was a great post with a ton of information.

    It took me multiple reads, getting a spare sensor from the yard and doing some testing on it myself to get my head around it a bit, so I'm going to share some pics and stuff. Also I believe OLS stands for oil level sense and OMS for oil motor sense.

    This is what the sensor looks like in the orientation it is sitting inside the oil pan, i.e. the float is uppermost at the end of the post when the sump is full, and at the bottom of the post when the sump is empty.

    DSCN4783.jpg

    The two wires are soldered onto separate terminals on the underside where the post attaches, and there are soldered terminals on top of the post too indicating that the switching stuff happens within the post itself. When the float is about 1/3 of the way up and again at 2/3 of the way up you can hear a very faint click/click if you have your ear right next to the post indicating switches opening and closing. The post is plastic but the float has a metal inner ring so I suspect the switches are operated by magnets (total guess).


    Here is a photo showing the 3 prongs of the connector.

    DSCN4784.jpg

    The center terminal is the common one and the side terminals go to the respective switches inside the post.
    i) When the float is at the top, the switch on the right terminal is closed and the resistance between right and center is negligible. The resistance between left and center is 5k ohms. This is all true when the float is in the top 1/3 of its travel.
    ii) when the float is in the middle, both switches are open and the resistance between left/center and right/center is 5k ohms for both. This is true for the center 1/3 of float travel.
    iii) When the float is at the bottom, the switch on the left terminal is closed and the resistance between left/center is negligible. Resistance between right/center is 5k ohms.
    OK, so the following is another of my guesses, but it's the only thing which makes sense to me...
    When the float is at the top, the ECU sees one specific switch closed and the other open. All is good, nothing happens.
    When the float is in the middle, the ECU sees both switches open and turns on the oil transfer pump motor.
    When the float is at the bottom, the ECU sees the reverse of the open/closed switch positions and turns on both the pump motor and the oil level warning light.
    Again, that was guesswork and I'll try to verify next time I change the oil.

    Here is a close up of the scrap yard float and post. Unfortunately this photo is after I had filed the post with a small sharp file. The marks on the float are from my filing. The post had a kink in the upper half, you can still detect a little bit if you squint, and the float was sticking at the top, indicating a full sump regardless of whether it was full or not.
    * Hence the need to always periodically check the dipstick. *

    DSCN4785.jpg


    Finally, the gasket seal is yet another piece of Previa unobtanium. If anyone knows good alternatives please post, otherwise it looks like it's going to have to be a big old smear of silicone and a bit of hoping.
    Also, if you have this thing out and want to eliminate the whole reservoir and pump system from your life without causing other problems, such as a permanently flashing oil level light, and without doing the circuit modifications described by the OP, you can secure the float in the upper position with something non-metallic so that you don't interfere with the magnetic switches. I was thinking of slipping some shrink tube over the float and shrinking it over the post. Don't know if this would work - it is still theory at this point for me.
    Last edited by Jonny; 12-26-2020 at 02:25 PM.

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    Re: Sticky Oil Level Sensor, Best Fixes

    Jonny, what you said to rocketman, I say of your post:

    That was a great post with a ton of information.
    Most of all, by bumping this topic, you brought a great thread to the attention of this forum, adding a lot of good info on the sensor and confirming rocketman's circuitry analysis with pictures showing the sensor implementation. [I'm sure you are right that inside the post are reed switches (or a double-pole reed switch) in parallel with 5K resistors.] What prompted you to find this?

    Rocketman did a nice job of analyzing the issues -- noting that the serious one is the common failure caused by the float sticking low -- and laying out the options comprehensively. Where the Previa is vulnerable to damage caused by a component failure in a protection system, I like his thinking to use the pieces in that protection system more intelligently rather than just defeat them; that's first-rate engineering. I'm tempted to do that. [I already have a couple LEDs added to the dash in front of the combo meter for other functions; for this function it might be nice instead to use the LEDs in a double-throw switch, which would provide the oil feed control.]

    Finally, the gasket seal is yet another piece of Previa unobtanium. If anyone knows good alternatives please post, otherwise it looks like it's going to have to be a big old smear of silicone and a bit of hoping.
    That may not be too difficult: It is Toyota p/n 90430-36005, and it just looks like an o-ring. It was used on all Previas 1991-1997, and also on Yaris 2005-2011, so it still seems to be obtanium, for $2~$3 from dealer.

  4. #4
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    Re: Sticky Oil Level Sensor, Best Fixes

    Here is a way to do the simplest "fake it full" mod, without cutting anything. It's all reversible in case you want to do something else. You'll be making yourself your very own plug for the harness connector instead of using the oil pan sensor.
    Though it is not essential, I would recommend a trip to the scrap yard to cut off a few inches of the wiring harness connector with it's mounting clip so that you can use it both as a jig for making the plug and a weather resistant cap for the oil pan sensor once it is abandoned. Maybe get the sensor too - they're cheap and you never know. While you're there you can also get the tail light pigtail harness which I used for the prongs of the plug, because they happen to fit great. Of course another identical Toyota 3 pin plug on a cable would be perfect for this job but I don't know if there is one.


    This is the tail light pigtail harness I used for the prongs. You need a fine but sturdy straight pick to get inside the connector from the open side to pry the tiny plastic thing which holds the individual terminal. A magnifying glass helps when you do this for the first time and don't know exactly what you're looking for.

    DSCN4789.jpg


    Here's your plug. The center prong is a simple cable, as is one of the side prongs. The other side prong has the 5k ohm resistor. Solder them all together. Thats it! There are 2 resistors in my plug because I didn't have a 5k ohm resistor, but I did have two 10k ohm resistors, and 1/R = 1/R1 + 1/R2 + 1/R3 etc for resistors in parallel. It's always worth keeping a small assortment of electronic circuit boards and switches - you never know when they'll be useful but eventually they always are.

    DSCN4790.jpg


    Now you have to make sure you get the plug in the right way round. Here is a photo lying under the van showing what you'll see and how it plugs in.

    DSCN4791.jpg


    I taped the crap out of it to protect the soldering, and waterproofed it with 6" of bicycle inner tube. Fold 2 inches of tube over and tape it like a dry bag and stretch the open end carefully bit by bit over the connector, and tie the whole shebang up out of the way. I like to use coated copper electrical wire rather than zip ties - zip ties deteriorate and fall off. Make sure it is oriented so that the inner tube sheds water, cap the now-unused oil level sensor with your scrap yard connector and voila.

    DSCN4792.jpg


    Oh, and just so that nobody gets any nasty surprises in the future...

    DSCN4793.jpg


    John - to answer your question - I did this because last time I changed my oil the system overfilled. It is after an oil change that this will happen if it is going to because that's when the float will be sitting hard at the bottom of the post and potentially get stuck there. Mine started working again, must have jiggled loose, but I don't trust it anymore. As my scrap yard sensor showed, though, it can also stick at the top. The scrap yard one was quite visibly warped. I wonder if years of hot oil is what distorted it. When I read posts by Previa owners saying they've never had to do anything with the oil reserve tank because the level never moves, perhaps the engine doesn't burn enough oil to engage the system between oil changes, or perhaps the float is stuck. With old vehicles like ours it is debatable which is more likely.

    Merry Christmas, happy holidays, groggy New Year, everybody, and don't forget we have dipsticks.
    Last edited by Jonny; 12-28-2020 at 06:55 PM.

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    Re: Sticky Oil Level Sensor, Best Fixes

    Quote Originally Posted by Jonny View Post
    Here is a way to do the simplest "fake it full" mod, without cutting anything. It's all reversible in case you want to do something else. ...
    To each his own, Jonny. I kinda like Rocketman's basic approach of using as much of the protection system as practicable -- but insert yourself into the control loop, in place of the ECU, to avoid the hazard caused by sensor (or ECU) failure. I've already added a couple of functions using lighted double-throw switches (top and bottom 14V lights), that look like a good candidate for this function as well. The additional circuitry seems not too demanding, and can probably live down in the ECU/igniter area.

    The other consideration is that I live in the rust belt of western PA; there just are not any Previas (or much else over 15 years old) in local yards. [I've been told by the parts guy at my local dealer that this is the last Previa in western PA.] So picking up parts as you did is not a casual thing. [Earlier this year, when the air intake/blend door was just too rusted to be made workable again, do you know where I found a replacement? Dixie Salvage in UT. Of course shipping cost as much as the part.] So I will be putting the take-the-ECU-out-of-the-loop mod on my to-do list.

  6. #6
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    Question Re: Sticky Oil Level Sensor, Best Fixes

    Quote Originally Posted by rocketman View Post
    [...] I will start a new thread on ECU substitutions and required mods if anyone is interested. [...]

    Did you start a new thread on how to do this fix? Just got a 95 and on maiden voyage got a complete dump of Reservoir but when checking at the stick as I was raised the sump was over full. I had gone over a couple mountain passes so I made the mistake of thinking I was using or throwing oil so filled the reservoir but after another 300 miles the reservoir was the same but the engine dip stick showed double above full. Tried the fuse and disconnect and got the flashing light. Just doing some research and found this and only this thread that seems to have a fix. thanks

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