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SIGH... Wonky oil pressure gauge, pegged right by notaguru
Started on: 07-02-2014 11:43 PM
Replies: 11 (273 views)
Last post by: notaguru on 07-04-2014 08:24 PM
notaguru
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Report this Post07-02-2014 11:43 PM Click Here to See the Profile for notaguruClick Here to visit notaguru's HomePageClick Here to Email notaguruSend a Private Message to notaguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
'88GT 2.8L
Just when I thought everything on this car was straightened out, the OP gauge pegs right when the car starts. After things warm up, it works properly. At one point, I might have applied 12V to the gauge assembly by shorting two pins in the cable.

???
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Old Lar
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Report this Post07-03-2014 08:10 AM Click Here to See the Profile for Old LarSend a Private Message to Old LarEdit/Delete MessageReply w/QuoteDirect Link to This Post
Oil sending unit is the first thing to check.
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paulmckibben
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Report this Post07-03-2014 04:15 PM Click Here to See the Profile for paulmckibbenClick Here to visit paulmckibben's HomePageClick Here to Email paulmckibbenSend a Private Message to paulmckibbenEdit/Delete MessageReply w/QuoteDirect Link to This Post
You might want to read this carefully:

http://www.fierosails.com/OilSensor.html

Paul McKibben
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notaguru
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Report this Post07-03-2014 05:05 PM Click Here to See the Profile for notaguruClick Here to visit notaguru's HomePageClick Here to Email notaguruSend a Private Message to notaguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
THANKS

I'd like to put a 100ohm potentiometer across the proper two pins of the cable, then play with it to see if the gauge responds properly. Apparently that potentiometer should go between the pin on the right (looking at the end of the cable) and ground. RIGHT?

ALSO - I've seen comments suggesting that the fuel pump control (in the OP sender) be disabled. Sounds fair to me - how do I do that?

[This message has been edited by notaguru (edited 07-03-2014).]

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Gall757
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Report this Post07-03-2014 06:16 PM Click Here to See the Profile for Gall757Send a Private Message to Gall757Edit/Delete MessageReply w/QuoteDirect Link to This Post
That fuel pump control was put on the car so you could drive it if the FP relay failed. I don't know how many times that has happened, but have heard no complaints about it''s operation. Why would you want to disable it?
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notaguru
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Report this Post07-03-2014 11:24 PM Click Here to See the Profile for notaguruClick Here to visit notaguru's HomePageClick Here to Email notaguruSend a Private Message to notaguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
My ignorance.

One post said that the fuel pump switch in the oil pressure sensor module was originally designed to support an idiot light, and that it does not have the current carrying capacity to handle the fuel pump. I could easily throw a 50amp MANUAL switch in that circuit, or use the oil pressure circuit to run a second relay. But if the original design can handle the amps, then it doesn't matter.
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phonedawgz
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Report this Post07-04-2014 10:02 AM Click Here to See the Profile for phonedawgzClick Here to visit phonedawgz's HomePageClick Here to Email phonedawgzSend a Private Message to phonedawgzEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by notaguru:

THANKS

I'd like to put a 100ohm potentiometer across the proper two pins of the cable, then play with it to see if the gauge responds properly. Apparently that potentiometer should go between the pin on the right (looking at the end of the cable) and ground. RIGHT?

ALSO - I've seen comments suggesting that the fuel pump control (in the OP sender) be disabled. Sounds fair to me - how do I do that?



The two outside pins connect to the fuel pump switch internally in the sender.

The middle pin connects to the gauge. At zero ohms to ground (direct short) the gauge should read 0 psi. At 90 ohms to ground the gauge should read full scale (80 psi). At 100 ohms to ground the gauges should read slightly higher than 80 psi. Unplugged (infinite ohms to ground) the gauge should read pegged high.

I have seen discussions that claim that the contacts inside the oil pressure sender, the contacts that CLOSE with 5 psi of oil, the contacts that were designed to act as a back up to the fuel pump relay were too small since they were designed to run a light on the dash.

Since that dash light would be on all the time the engine was running, since under that conclusion GM engineers would have mis-designed the oil pressure sender, not once, but for two design generations of the oil pressure sender by 1988, and since they ran that type of oil pressure switch for decades afterwards, I conclude that those who make the claim don't know what they are talking about.

Since the Fiero fuel pump supply wire is fused at 10A, a 50A switch would be overkill.

http://www.wellsve.com/cust..._drawings/PS220C.PDF

[This message has been edited by phonedawgz (edited 07-04-2014).]

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notaguru
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Report this Post07-04-2014 10:21 AM Click Here to See the Profile for notaguruClick Here to visit notaguru's HomePageClick Here to Email notaguruSend a Private Message to notaguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
Perfect!

That really helps, and now I can easily diagnose using a potentiometer. I happen to have a 100 ohm unit available, and that should tell me whether my problem is gauge, wiring, or sender.

THANKS!
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notaguru
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Report this Post07-04-2014 12:16 PM Click Here to See the Profile for notaguruClick Here to visit notaguru's HomePageClick Here to Email notaguruSend a Private Message to notaguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
With a paper clip and wire connecting the center pin of the sender to the center pin of the connector, all is well. But when the connector is pushed into the receptacle, the connection is NOT made and the needle pegs right. I've examined both the connector and the sender and there's no visible fault.

I'll solder new wires to the three pins, fill the cavity with RTV, and make 100% connections externally. Should be fine.

Again, thanks for the diagnostic help!
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notaguru
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Report this Post07-04-2014 02:53 PM Click Here to See the Profile for notaguruClick Here to visit notaguru's HomePageClick Here to Email notaguruSend a Private Message to notaguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
Finished....

Something for the gurus to file away:

There was an intermittent OPEN in the connector itself. When bent slightly, connection to the center pin (OP sensor) was lost. At other angles, the connection looked good.

I cut the wires, soldered (quickly) directly to the OP pins, applied heat shrink, filled the cavity with RTV, and it's all good to go.

Later I might install a disconnect, but for now this works fine.
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phonedawgz
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Report this Post07-04-2014 06:16 PM Click Here to See the Profile for phonedawgzClick Here to visit phonedawgz's HomePageClick Here to Email phonedawgzSend a Private Message to phonedawgzEdit/Delete MessageReply w/QuoteDirect Link to This Post
For the 88 oil pressure sender if you are interested I can send you a new 3 pin oil pressure sender connector with new leads that have new terminals and seals on them. $20 to include shipping - paypal me at gofast@reddevilriver.com if you are interested.

On the working pins you don't have to replace them, you can just move them to the new connector. I take a standard paperclip and hammer it slightly and find that it is the perfect size to de-tang the terminals for those connectors.

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notaguru
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Report this Post07-04-2014 08:24 PM Click Here to See the Profile for notaguruClick Here to visit notaguru's HomePageClick Here to Email notaguruSend a Private Message to notaguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
Thanks - but I soldered directly to the pins in the sender, then sealed the cavity with RTV. I'd need your cable+connector, and ALSO a new sender. That would be perfect, which is the enemy of good enough.

Inside the connector, I found the sender wire detached from the clip. What a PITA to sort out... Hopefully this fix is permanent.
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