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Discussion Starter · #1 · (Edited)
So, my car is a Geo Prizm 90 hatchback 4afe model. So recently I started having my oil light come on when i was idling my car when i got back to it after I got out to do something for a minute and left my car sitting there on. it seemed to blink on and off with the rpms flucuating. Also i had noticed that sometimes when Id take off it would light up for just a second and I'd barely notice it.
Read it might just be the sending unit, so I bought a new one and installed it. I read it was 24mm. When taking mine out, i had to use a 27mm socket to remove it. It looked different and when I compared the one I took out compared to the new Light sending unit i bought, and crossed them with pictures I seen of ones on Rockauto, the one that came out, seemed to be a Oil pressure gauge sending unit.? This is weird, as the light always seemed to work right when it was supposed to on my car. Like, when Id almost kill the motor by throwing it into 5th gear for no other reason going 10mph or something, the light would come on. So, ? what the heck ? If I bought a new pressure GAUGE and installed it, would it run both my light on my dash and an oil pressure gauge both if i ran 2 wires on to it? has anyone ever tried this before? Why or how does teh gauge turn the low pressure light on and off?
My new gauge doesnt even turn on when i try to trip it on, so i dont know why its not working it might just be faulty. i even tried to make sure i had it tigthtened down enough by retightening, or just adding another turn to it, i was afraid to strip it, and i double checked it after it wouldtn trip on the light and it turned another half turn or more, and then I noticed it tightening down; i could feel it getting tighter. so i thought, this is enough here. I tried the ohm meter and tests i read on another topic, ohm to the sensor and body will have resistance car off, ohm from sensor to body will have no resistance motor is on. and the wire while grounded and key on will show oil light on.

Q's are, can the gauge work my light? Can i run 2 things from it both light and gauges? (why odesnt gauges just have a light too?) Does a pressure gauge only need one wire to the sensor output to get the reading to function? maybe pos neg.?
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1991 Corolla DLX 4AFE, 1994 Camry LE 5SFE, 1995 Avalon XLS 1MZFE, 2004 Sienna XLE/LTD, 2011 Camry LE
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have you checked your OIL LEVEL on a flat surface? you'd be surprised...

the switch is a simple pressure transducer device and if there's little or no oil pressure pushing on it, it will activate your dash light and let you know there's a fault. This circuitry is part of the key on, engine not running self test sequence. If its on during self test then goes off after the engine is running you can be fairly certain that the oil pressure switch is working normally. Low oil level, leaking oil pump seals, worn pump bearings or simply a loose wire on the switch are the most common root causes of failure of the oil pressure reporting system. The switch is either open (engine off) or closed (>~7psi oil pressure). The oil pressure transducer's body is the path to ground with the YEL-BLK wire providing the +12.6 vdc and the oil pressure switch is what turns the idiot light on and off in the instrument cluster. The OIL PRESSURE and OIL TEMPERATURE circuits are two totally different circuits, and are not interconnected.

How many miles are on your engine?
 

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Discussion Starter · #3 ·
yea its been checked and topped off. I read through the manual and it said, a front cover gasket (im assuming valve gasket) leak can cause low oil pressure. I checked and noticed, i had just developed a valve cover gasket leak. replaced my valve gasket but, havent been able to tell if it fixed, the oil pressure problem or what it is if its my sensor or not. do oil pressure gauges only have 1 wire that goes to the sender? im thinking the gsi may have had a oil pressure gauge on its cluster. would be nice to see one ever again on ebay
 

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Discussion Starter · #4 ·
I just seen a few pictures and I see, that gsi instrument clusters have both the light and a gauge built in. so the sender "does" have both. well thats great. this guy i bought the car from always was wanting to/ was upgrading it. this was probably one of his ideas. I hope my oil pump isn't going out and it was just my sending unit on it. I mean, I did find the valve "front cover" leak right immediately after this started.
has anyone else ever had thsi happening? It doestn seem right, seems like when i got the car it had valve cover leaks and i never had the oil light coming on. It was appearing on tho at weird times like, just really randomnly it was appear for a split second. Only once when it was parked did i come back and it was flickering on and off. but i had noticed earlier the light just lighting up for a split second on me from really out of nowhere, didnt seem like the right time for it to come on.
I guess ill grab a gauage to go along with the new sending unit to check out what the pressure is at. HOpe i dont need to do a oil pump. Do those slowly go out or are they like a water pump, just kind of goes out fairly quickly? or what?
 

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1991 Corolla DLX 4AFE, 1994 Camry LE 5SFE, 1995 Avalon XLS 1MZFE, 2004 Sienna XLE/LTD, 2011 Camry LE
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I just seen a few pictures and I see, that gsi instrument clusters have both the light and a gauge built in. so the sender "does" have both. well thats great. this guy i bought the car from always was wanting to/ was upgrading it. this was probably one of his ideas. I hope my oil pump isn't going out and it was just my sending unit on it. I mean, I did find the valve "front cover" leak right immediately after this started.
has anyone else ever had thsi happening? It doestn seem right, seems like when i got the car it had valve cover leaks and i never had the oil light coming on. It was appearing on tho at weird times like, just really randomnly it was appear for a split second. Only once when it was parked did i come back and it was flickering on and off. but i had noticed earlier the light just lighting up for a split second on me from really out of nowhere, didnt seem like the right time for it to come on.
I guess ill grab a gauage to go along with the new sending unit to check out what the pressure is at. HOpe i dont need to do a oil pump. Do those slowly go out or are they like a water pump, just kind of goes out fairly quickly? or what?
The valve cover leaking will cause low oil levels, and not an issue with this oil system's pressure sensing, as it is not a fully contained pressurized system. The oil pump pumps oil to the top of the engine via the oil galleys to the valve cam lobe lubricator "buckets" (highest point) and then gravity fed back into the oil pan. Make sure that the sensor connector is not loose and has no corrosion build up. Likewise the sensor boss should be free of corrosion before it was screwed back in. You may also want to verify the integrity of the sending wire back to the instrument cluster by ensuring you have zero resistance (no opens) throughout the entire length of the wire, and also ensuring there is no paths to ground.
 

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Discussion Starter · #6 ·
I know my valve cover was leaking all over the place when I got it and i didnt have an oil light. but the light also has been coming on randomnly at times lately.. maybe that means the switch or sender is faulty or,were the distributor was off a little when i put it back on and i fixed that might have fixed it. but, it seems to have a lower idle than usual and that seems to have stayed. i never had the light come on before. i guess im going to have to get a new gauge sending unit and get a gauge reader. is all those need to read the oil pressure is nothing more than an 18 gauge wire running from the sending unit to the gauge ? because i dont know were to tap into the line at my cluster to get the reading from my wiring harness going to my instrument cluster which would power a gauge in cluster if it had one. I mean, right? 1 wire any copper wire what it takes to run one? im not sure on these things.
Font Material property Publication Book Paper
 

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1991 Corolla DLX 4AFE, 1994 Camry LE 5SFE, 1995 Avalon XLS 1MZFE, 2004 Sienna XLE/LTD, 2011 Camry LE
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"Poor seal at front cover gasket" can refer to both the "valve cover" seals (gasket) OR the oil pump cover seals (behind timing belt cover). I've addressed both in my above statements. If you have little to no electrical experience you might be in over your head on this one. Your book is for "general' engine information, not specific to, nor necessarily exclusive of TOYOTA 4AFE or 4AGE engines. ie; Your engine does not have "rocker arms" in the valve train, which would necessitate "pushrods." Our engines are overhead cams style.
 

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Discussion Starter · #8 ·
I get that now, oil pumps gasket ok. IM looking to see if my guess is right, would the sending unit behave the same if i got the gauge version instead of the light. Id like to know if its my sending unit working next time or not. And how does an oil pressure gauge get its reading? is it only 1 single copper wire that touches the connection to the gauge taht gives it its oil pressure reading? so if everything is workign i can just reattach my cars cluster units light wire back on to the sending gauge unit for light functionality.
 

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Stick to stock items for now and get those bugs worked out. If you’re having oil pressure issues now that go unchecked you could destroy your engine. Do the “pretty stuff” after you get it all running right. An “idiot light’s” switching mechanism is a pressure transducer that is either closed or open. An oil pressure gauge is a circuit that senses the pressure and indicates its value on a guage. This setup requires additional circuitry to translate a sensed pressure into a meaningful expression on a dial face, in this instance, a psi reading.
 

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Discussion Starter · #10 ·
but can you answer, is that all a pressure gauge needs to work is one copper wire running from it to the sender unit? i already did the stock items thing and i got a non-working i cant even find a way to check it out if it works right or not procedure. it just seems common sense that the light part will work on a pressure sender unit the same functional way as it would on a light sending unit without any modifications and i can figure that out when i get it.
id just like to get a pressure gauge to give me a good reading of what i do have in case i have other electric gremlins (which I do have some of). it doesn't seem complicated. you'd think the light would work the same off of either sending unit, because the wiring would have to be different, different wiring harness or the other instrument cluster would have to read the two connections differently. light is light. i dont think the wires would be crossed. if so, not a hard fix. little bit of investigation.
 

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First off, if you have a 1990 Geo Prism "GSI" hatchback, then you should not have the 4A-FE engine in it. You should have the 4A-GE engine in it. I'm pretty sure the Geo Prism Base and LSI models had the 4A-FE, but not the GSI.

If you like purchasing from RockAuto, the oil pressure sending unit (for the GSI, with a gauge) is within the red rectangle in the view below:
Rectangle Font Parallel Slope Screenshot
 

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First off, if you have a 1990 Geo Prism "GSI" hatchback, then you should not have the 4A-FE engine in it. You should have the 4A-GE engine in it. I'm pretty sure the Geo Prism Base and LSI models had the 4A-FE, but not the GSI.

If you like purchasing from RockAuto, the oil pressure sending unit (for the GSI, with a gauge) is within the red rectangle in the view below:
View attachment 383147
Yup, we replaced the oil pressure light on my 4A-FE engine in my 92 Geo Prizm about 13 years ago. It would come on whenever it felt like it. I did have to remove the oil filter to access the switch (no room otherwise). Since then I've had no issues with it in the last 11 years and 40K miles that I've owned it. I think my son got the switch from the Zone at the time, since he owned the car when the switch was acting up.
 

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Discussion Starter · #13 ·
its a base model hatchback. it did come on once when it was idling and flickered with the engine rpms. I replaced it with an autozone switch and that one never comes on. not like the old one did. im at 190k and i hope i dont have to do the oil pump. the engine doesnt sound bad or anything. no pinging or knocking sounds. and also the light would sometimes just come on occasionally, not while flat out giong down the road, but taking off i remember, but i wasnt at a low rpm and the lights never lit up before like that ever. and i tried to see if it was consistent and let the rpms get down to low in a high gear or something or just letting it run and the rpms get low by slowing down, it is manual transmission i should hve mentioned this. and the light wouldn't behave diferent than normal.
but after putting in a facet brand they got at rockauto too, light switch, it doesn't work at all. yea it lights up when the motors turned off with key in. the basic tests i said up there passed. but it cant be triggered with low rpms to light up, i know this is wrong and isn't helping me with detecting a failing oil pump.

I fixed btw, the no start geo prizm problem. i ran a wire from the starter spade connection, inside the car and another from the positive on the battery to inside, and added a push button start. Now i dont get stuck. about the only way ive figured out how to fix it.
 

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The base model hatchback came with the 4A-FE, did not have an oil pressure gauge, and only had an oil pressure switch for the oil galley on the engine, so the base never had an analog oil pressure sender unit. So are you saying the original owner put a GSI instrument cluster into your car that has the oil pressure gauge? And now you are trying to get both the gauge and oil pressure warning light to work on the cluster you have?
 

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The base model hatchback came with the 4A-FE, did not have an oil pressure gauge, and only had an oil pressure switch for the oil galley on the engine, so the base never had an analog oil pressure sender unit. So are you saying the original owner put a GSI instrument cluster into your car that has the oil pressure gauge? And now you are trying to get both the gauge and oil pressure warning light to work on the cluster you have?
If so, he'll need to "T" into the oil port above the oil filter. There's not a lot of room in that spot either. If he's trying to figure oil if his pump is getting ready to crap out, I'd remove the existing switch from that port, and put in a fitting and plumb in a mechanical gauge to see what kind of oil pressure I have, keeping in mind that I want 10 psi for every 1000 rpm minimum. If I didn't see that, then I'd plan on replacing the oil pump. If I had more than that, I wouldn't really worry about it. This is also keeping in mind the amount of miles on the engine, knowing it already has 190K miles on the bearings and cylinder walls. It might be a good idea to start pricing out a rebuild kit, and start saving up some money to purchase one, with a plan to rebuild it. Just something to keep in mind.
 

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The switch is either open (engine off) or closed (>~7psi oil pressure).
It's the opposite. The pressure switch is in series with the low oil pressure warning bulb. At no/low pressure the bulb is grounded through the switch (electrically closed). At high pressure the switch opens and the bulb goes off. If the wire ever comes off the switch the bulb will never light and the driver may not realize the switch isn't attached because they may not realize the oil warning light is supposed to be on before the engine is started.

A gauge can work with a sender with only one wire. That's how the coolant temperature gauge works. The coolant temperature sender is just a thermistor that changes resistance with temperature, and the switch is self-grounded to the engine.

As to the question of whether a low pressure warning light and guage can work off of a single wire, it would take some electronics in the gauge to turn the light on. I would thing Toyota would go the cheaper route of having a separate warning switch and pressure gauge, maybe combined into one with 2 wires. The fuel lever gauge and warning bulb have separate senders.

It sounds like the old switch may have gotten gummed up, so it's not switching off at the correct pressure. There is probably a spring in the sender switch and that may have broken or gotten weaker. Hooking up a pressure gauge would help diagnose is the switch is bad, but if the new switch makes the warning bulb light with engine off and it goes off whenever the engine is above 600 RPMs then your pressure is probably good.
 
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Oil pressure switches are not fail safe, for some reason. But it was a cheaper setup to put the light in series with the switch. But like stated, not good if the wire disconnects from the switch in some way - most I bet would never know.

To check a light though, in a factory setup w/o a gauge, one could disconnect the wire from the oil pressure switch, then ground the wiring harness terminal, then turn the ignition switch ON - the oil pressure warning light should come on and stay on. If it doesn't the instrument cluster bulb for the low oil pressure warning and burned out, or there is a wiring problem between the oil pressure switch and the low oil pressure warning bulb.

With rancer4's setup, it isn't really known what the previous owner did when he/she installed a GSI instrument cluster with an oil pressure gauge. If the oil pressure gauge isn't that important to rancer4, maybe an original instrument cluster could be installed. However, the previous owner may have done some wiring modifications as well. Who knows.
 

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Discussion Starter · #18 ·
i was wrong. i looked at it wrong; the original light sender switch used a 27mm socket and i read online that the sender was 24mm, and i had it in my mind that that switch was the gauge version. and my cluster is not a gsi version. I wished. coughcough * I remember seeing them on ebay but they are never there any more.
I ordered a gauge sender and I got a oil pressure gauge to hook up to check what my pressure looks like. This was probably the original light sender unit so it might got weaker and started to wig out on me. The motors not making a whole lot of noise or anything. A steady miss is all i ever hear. Dont know what causes that.
 

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Discussion Starter · #19 · (Edited)
i got the gauge sending unit and a oil gauge. I'm trying to figure out if its not installed right.
I get the oil light on all the time as the motor is running after putting in the gauge and hooking up the wire. I tried to hook up my oil gauge and its only got 4 connections, 12vdc, ignition, ground, and sensor. i didnt hook up ignition as i figured thats when its dark and you turn on the lights and it switches the light on it on when you give it voltage. WHen the motor got turned on, teh gauge just went to 100 PSI. This isnt right, right? What is this a sign of anyone ever seen this before? Do i not have the oil sending unit screwed down tight enough? i didnt want to strip it but, anyone else ever had this problem? It was kind of hard to get the wrench behind it and not hit the motor as i was tightening, i thought I had it tight. So yea, the gauge reads 100psi when on and the instrument panels oil light comes on. doesnt flicker just stays on. no revving does anything. motor sounds fine
So, anybody ever noticed this behaviour before? is it sign i dont have it tight enough?

wow theres no way this is workign right. not tight enough? its all over the place. low rpms makes it go out and high rpms makes it come on? doesnt it only work by not having over a certain amount of pressure? i mean, my oil pressure gauge just shows 100 psi at idle. i revved it a little but i noticed nothing on the gauge. Ill have to check it to see if its tight
 
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