Pinned right = unplugged or bad sending unit (possibly bad gauge). If it was plugged in, and you ground the middle terminal in the plug and the gauge goes to zero, bad sending unit.
Could be a bad sender, grounded connection, or bad gauge.
Is it always pegged, or only with the key on ? Or is it reading pressure that high, does it go up as oil pressure builds, or immediately when the key is on ? Disconnect the connector from the sender and see what it does.
But Mike - disconnected from the sender will give a past full oil pressure indication.
Shorted to ground (0 ohms) would be 0 psi
90 ohms to ground would be full scale
Open (infinite ohms to ground) would give an indication of past full scale.
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So Wayne - first thing to see is what the oil pressure gauge does with the key on (engine off) and then you take and stick a paperclip into the middle pin of the oil pressure sender connector of the engine harness and ground it to the engine. The gauge should go to zero. If it does that would indicate your oil pressure sender is bad. If the gauge remains pegged that would indicate either a bad gauge or bad wiring from the gauge to the sender.
[This message has been edited by phonedawgz (edited 04-01-2011).]
I'm bumping this because I have the same issue. The oil pressure gauge reads full while running and with the key on engine off it reads past full as far as the needle can go. Dies this indicate a bad sending unit?
Thanks
-Josh
[This message has been edited by OneSlowFiero (edited 04-27-2011).]
The gauge technology that GM used in the 80's put two electromagnetic fields against each other. They did that instead of an electromagnet against a spring. After time the spring will weaken making a spring/electromagnet gauge inaccurate.
So when the key is off, what happens to the needle has no meaning at all.
A simple test of your oil pressure gauge/sending unit. Unplug the connector from the sending unit. On 85-87 that means squeezing the ring to make it more circular and then pulling up. With the proper squeeze it will pull up fairly easy. Now turn the key on, but leave the engine off. The needle of the oil pressure gauge should read far past high right. Now take a straightened paperclip and insert it into the CENTER pin of the three and hold that against ground. The needle should move to 0 PSI. If you wish to make a third test you can take a 90 ohm 1/4 or 1/2 watt resistor (RadioShack $0.99) and insert it into the middle pin and ground it. With the 90 ohm resistor the needle should read full oil pressure, but not past full.
If your gauge reads what it should for tests 1 & 2, but it doesn't work correctly connected to the sending unit, your most likely problem is a bad sending unit. If your gauge reads low but at some pressure just jumps to full high, your problem most likely is a bad sending unit. That jump-failure is common with the 85-88 sending units.
Rock Auto has some reasonably priced sending units at $12.71 + shipping. Make sure your replacement sending unit looks like the one in your car or it won't fit on the electrical connector. Price check this before buying local. Some stores charge over $50 for this.
Note also - You must use a wrench on the metal part of the base, not that big looking plastic nut thing.
Oil pressure sender/switch for 85-87 Fieros
You can also replace the 85-87 sending unit with an 88 sending unit. You have to also purchase the wire connector and splice it onto your existing engine wiring harness.
[This message has been edited by phonedawgz (edited 04-27-2011).]
For a while the only available 85-87 senders were all in that $50 range. It was cheaper to replace the sender AND purchase the connector rather than just replace the old sender. With these lower priced ones on RockAuto that is no longer the case.
The other thing is the 88+ senders do seem to be more reliable than the earlier ones.
You can get and 88 sender and a three pin oil pressure connector from rock auto for a reasonable price also. Make sure the connector you buy is a 3 or 4 pin connector. There are also 1 and 2 pin connectors that don't work for the Fiero.
How long did the original sender work in your Fiero? idk which way is the better way to go. Clearly when you could buy the newer sender and connector for less than the replacement sender then the choice was easier.
Very informative thank you very much. The oil pressure gauge hasnt worked properly since the day I bought the car. I will order a new sending unit from rock auto. Even if the unlikely happens and it isn't the sending unit, it sounds like it is good to have one on hand. Heck I might even buy 2!
I just upgraded my 87 v6 to the 88 oil sending unit following the instructions on the Ogre's Cave Fiero site. The Car is running fine, but my gauge which had been bouncing around is not pinned to the far right. Car on or off. I pulled the tan (center) line from the sender and grounded it as described in the post below and my gauge went to zero. Sounds like this is a bad sending unit...but it is a brand new AC Delco part. #1808A. Any ideas? I am actually worse off than I had been. LOL.
Thanks, Bill
quote
Originally posted by phonedawgz:
The gauge technology that GM used in the 80's put two electromagnetic fields against each other. They did that instead of an electromagnet against a spring. After time the spring will weaken making a spring/electromagnet gauge inaccurate.
So when the key is off, what happens to the needle has no meaning at all.
A simple test of your oil pressure gauge/sending unit. Unplug the connector from the sending unit. On 85-87 that means squeezing the ring to make it more circular and then pulling up. With the proper squeeze it will pull up fairly easy. Now turn the key on, but leave the engine off. The needle of the oil pressure gauge should read far past high right. Now take a straightened paperclip and insert it into the CENTER pin of the three and hold that against ground. The needle should move to 0 PSI. If you wish to make a third test you can take a 90 ohm 1/4 or 1/2 watt resistor (RadioShack $0.99) and insert it into the middle pin and ground it. With the 90 ohm resistor the needle should read full oil pressure, but not past full.
If your gauge reads what it should for tests 1 & 2, but it doesn't work correctly connected to the sending unit, your most likely problem is a bad sending unit. If your gauge reads low but at some pressure just jumps to full high, your problem most likely is a bad sending unit. That jump-failure is common with the 85-88 sending units.
Rock Auto has some reasonably priced sending units at $12.71 + shipping. Make sure your replacement sending unit looks like the one in your car or it won't fit on the electrical connector. Price check this before buying local. Some stores charge over $50 for this.
Note also - You must use a wrench on the metal part of the base, not that big looking plastic nut thing.
Oil pressure sender/switch for 85-87 Fieros
You can also replace the 85-87 sending unit with an 88 sending unit. You have to also purchase the wire connector and splice it onto your existing engine wiring harness.
Here is a little trick I figured out by accident. Pull the fuse listed gauges on the interior fuse box. if your volt meter stopped working and your oil pressure gauge now works. and If you have changed both your sender and gauge set like I have, you can you can pretty much bet that you are a proud owner of a melted wire.
For a while the only available 85-87 senders were all in that $50 range. It was cheaper to replace the sender AND purchase the connector rather than just replace the old sender. With these lower priced ones on RockAuto that is no longer the case.
The other thing is the 88+ senders do seem to be more reliable than the earlier ones.
Better reason... Type used for 88 and up won't leak pressurize oil and dump all oil on highway because doesn't have leak problem when they fail.
88 just have a longer plastic to fit same high as older units but otherwise is same as short ones shown.
88 need 27mm Wrench for most but many types can work depending just where.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
I have a dumb question as I am looking into what it would entail to get my oil pressure gauge working on my "85 GT 2.8 . Just where is the oil pressure sending unit? From what I gather, from reading theses posts, that if you take off the sender connector and short the middle pin to the block with the key on (motor off) the needle may move to zero. This would mean the sending unit is toast right? My oil pressure gauge needle moves to the center, around 40, when I turn the key on and with the motor off . As soon as the motor starts it goes all the right as far as the needle can go. Any thoughts? Centervilledon
I am having a similar issue with my 85 SE. pressure switch was leaking but worked, so I replaced it with a duralast one and now my gauge reads maxed when running. Don't know if I put the wrong one in but it wasn't the same style as the oem.
Johnny, tightening or loosening on the plastic nut will break the unit. Usually twisting the terminals loose from their internal connections. I can't understand why they made it with a hex case unless it was so someone would use that, resulting in the sale of another sending unit.
Isn't the center lead the constant 12 volt lead? If it is, grounding it should result in blowing something or burning something.
If a conversion to the 88 sending unit is done, steps need to be taken to prevent water from getting into the top. My conversion shorted out with the first rainfall, killing my battery. My initial solution was to put a piece of large wire harness sheathing over it. The best method is to get a brass street elbow and install the sender horizontally instead of vertically.
The center lead has 12vdc applied through one of the coils in the Gage, the 'sending' unit provides a variable resistance (to ground), which varies the voltage across the coil, causing the needle on the gage to deflect.
Grounding the wire will not blow a fuse because the coil acts as the load on the circuit.
The 88 oil pressure sending unit faces straight up, has no problems with water intrusion when the WeatherPak is installed properly. A little silicone lube on the ribbed seal may be applied for added water resistance.
Perhaps your connector didn't seal properly, causing your battery drain problem.
The weather pak connector I got would actually let water enter along the wiring, even though it had the rubber seal. These are the part numbers from Rock Auto that I used. Oil pressure sensor. Rock Auto Part Number AC Delco D1808A. Oil pressure sensor harness connector. Rock Auto Part Number AC Delco PT121.
When I replaced it, I put the sheathing over it and also packed the top end with dielectric grease.
My oil pressure gauge is starting to dance a bit again; I'm suspecting the 88 sender catching rain from the decklid vent. My sender is pointing straight up, with the connector up top.
Last time I packed the sender's connector cavity with dielectric grease, until it oozed out from everywhere when I plugged in the connector.
I'm on my second Standard PS245, and not the AC Delco D1808A. Would AC Delco be better?
Also I'm running 15W-40 oil. When the senders are new, the needle points to 80 psi with any revs, so I don't know if high oil pressure could be damaging the senders.
I'm not sure if it's time to make a protective cap...
The top of the early sending units had a pressure relief button on the top of them. This could allow water to get inside the inner workings and short the sender. When I changed over to the new style sending unit, I had a similar problem in that the terminals in the harness aren't waterproof. Water again shorted the sender. My solution was to obtain a brass street elbow 90-degree fitting to screw into the sending tube. Now, my 88 style sending unit is horizontal and no water can get into the terminals.
As far as AC Delco over Standard, I personally go with Standard Ignition after buying an AC Delco and finding a BWD box inside with the sender in it. Echlin fro NAPA is also a good choice.
Something that should be done on maybe a yearly basis is to attach a mechanical oil gauge to the sending tube and get a reading at operating temperature while the engine is idling. Then you have something to compare your in-car gauge to.
[This message has been edited by fierofool (edited 05-03-2021).]
My oil pressure gauge is starting to dance a bit again; I'm suspecting the 88 sender catching rain from the decklid vent. My sender is pointing straight up, with the connector up top.
Last time I packed the sender's connector cavity with dielectric grease, until it oozed out from everywhere when I plugged in the connector.
I'm on my second Standard PS245, and not the AC Delco D1808A. Would AC Delco be better?
Also I'm running 15W-40 oil. When the senders are new, the needle points to 80 psi with any revs, so I don't know if high oil pressure could be damaging the senders.
I'm not sure if it's time to make a protective cap...
Heavy Oil can wreck an engine. Even on old engine w/o variably valve and other things that Hate this. More so driving in Cold Weather. read OM, my cave Oil and Filter or use api.org for charts of oil vs weather temp
Never trust a dash gauge. Use a real gauge. More so on V6 w/ known oil problems and 88+ "Dukes" w/ balancers. High reading can be anything wrong w/ dash wiring, real problem w/ engine, or both.
Si grease is likely still keeping water out. Did you grease the plug w/ back seal out? No? Then back of plug likely hold water and rot on V6 w/ AC because still soak the back when water get in thru the vents.
A car that Georgia Fieros acquired to restore for a disabled vet had the 88 harness end inserted through the top section of a water bottle then filled with silicone RTV.
