Oil Pressure Gauge Not Working On 3800 Swap (Page 1/2)
Crazy_ones24 FEB 19, 09:49 PM
I have an 85 Fiero with a Series 2 SC 3800 4T65E-HD swapped into it. It was someone else's project. It came with a low quality digital dash. I replaced it with a stock instrument cluster from an 87 Fiero. It just has the 85 mph speedo. I've gotten every single gauge to work EXCEPT for the oil pressure. I believe I have narrowed the issue down to a bad ground. To preface, I am using a stock Grand Prix oil pressure sender. Also, the gauge is pinned to the right, meaning it is getting full resistance. As if there were an open in the wire. I have verified that the sender is good and that the gauge itself is good. There are two wires coming from the oil pressure sender, a tan (positive/signal), and a black wire. The black, I presume, is a negative/ground wire. I can't find a wiring diagram that shows BOTH of these wires coming from the sender. Diagrams for this swap on Fieros only ever show the tan wire needing to be run to the gauge directly (which it is). I also can't find any diagrams of the Grand Prix's harness relating to the oil pressure sender. I've verified that the positive/signal wire to the gauge is good (gave it power from the end of the plug and the gauge went to 0). When measuring the resistance of the black wire, I got 44 ohms. Since it's a ground wire, it should be 0. I know the battery's ground is good (measures 0 ohms). I can't find anything on where this black wire is supposed to go into so I don't know where its grounding point is. The original owner spliced the harness together himself. It's possible I'm the only one who can find it by cutting open the looms (which I really really don't want to do).

Any help would be greatly appreciated.
Thanks in advance.
jelly2m8 FEB 20, 02:10 AM
You use the Fiero OPS for the Fiero Oil Pressure gauge Wired as a stock Fiero.
olejoedad FEB 20, 08:35 AM
The sender you are using is actually a switch to turn on the low oil pressure light.

As stated above, use the three wire 88 Fiero sender, and run terminal A of the sender to C203 E.
Crazy_ones24 FEB 20, 08:09 PM
I see. So do I need an adapter to make the 88 Fiero sender fit in the 3800? Also, Since I have 2 wires, one going to E C203, and the other presumably going to a ground, I should only need to add one more. I assume this is for +12v to the oil pressure sender? I found on the Ogre's Cave that terminal C of the Fiero OPS goes to the fuel pump's hot side. Do I need to take power specifically from there? Or can I just get +12v from anywhere? Also, terminal D goes to "F pump to tank." Is that supposed to be a ground on the fuel tank? Or is that something else?
olejoedad FEB 21, 06:46 AM
No adapter needed, it will thread in.

OPS 'C' and 'D' are a backup power circuit for fuel pump power.
They are not needed, as the fuel pump is already receiving power from the fuel pump relay.

The two wires on your existing OPS are not needed.
The gage only needs OPS 'A' connected to C203 'E'.

If your wiring harness is not wrapped, you could easily add OPS 'C' and 'D'.
OPS 'C' is fed power from the orange wire on the FP relay.
OPS 'D' is attached to C-203 terminal that feeds power to fuel pump. (Sorry, I am traveling and can't access my notes) It is the tan/white wire from the FPR that goes to C-203.
OPS 'D' is not a ground for the pump.
The Fiero OPS is also a pressure activated switch that transmits fuel pump power as a backup circuit in the event the fuel pump relay fails.
theogre FEB 21, 05:47 PM
"Also, the gauge is pinned to the right, meaning it is getting full resistance. As if there were an open in the wire."
No. It means gauge Is Not connected to the Sender.

"I have verified that the sender is good and that the gauge itself is good. "
Just how you tested this?
See https://www.fiero.nl/forum/Forum2/HTML/146784.html Fuel, Engine Temp or Oil Pressure gauges have problems?

88 sender needs 88 Plug but screws in the block same way as other GM OP senders
see https://web.archive.org/web...rocave/oilsender.htm

------------------
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)

The Ogre's Fiero Cave

Crazy_ones24 FEB 21, 07:20 PM

quote
Originally posted by olejoedad:

No adapter needed, it will thread in.

OPS 'C' and 'D' are a backup power circuit for fuel pump power.
They are not needed, as the fuel pump is already receiving power from the fuel pump relay.

The two wires on your existing OPS are not needed.
The gage only needs OPS 'A' connected to C203 'E'.

If your wiring harness is not wrapped, you could easily add OPS 'C' and 'D'.
OPS 'C' is fed power from the orange wire on the FP relay.
OPS 'D' is attached to C-203 terminal that feeds power to fuel pump. (Sorry, I am traveling and can't access my notes) It is the tan/white wire from the FPR that goes to C-203.
OPS 'D' is not a ground for the pump.
The Fiero OPS is also a pressure activated switch that transmits fuel pump power as a backup circuit in the event the fuel pump relay fails.



Awesome, thanks. The loom is wrapped and taped. Partially redone by me while fixing all the other gauges. Evidently foresight is not a gift I've been given.

Crazy_ones24 FEB 21, 07:23 PM

quote
Originally posted by theogre:

"Also, the gauge is pinned to the right, meaning it is getting full resistance. As if there were an open in the wire."
No. It means gauge Is Not connected to the Sender.

"I have verified that the sender is good and that the gauge itself is good. "
Just how you tested this?
See https://www.fiero.nl/forum/Forum2/HTML/146784.html Fuel, Engine Temp or Oil Pressure gauges have problems?

88 sender needs 88 Plug but screws in the block same way as other GM OP senders
see https://web.archive.org/web...rocave/oilsender.htm




I verified that the gauge is good by applying power to the wire that runs into E C203, the gauge moved to 0. The sender being good evidently doesn't matter because it's the wrong OPS for the job.
olejoedad FEB 21, 08:42 PM
Where are the wires from the two wire sender terminated?

The gage supplies 12VDC to the sending unit, the sending unit puts a variable resistance on the gage lead on its way to ground. The resistance is what makes the gage needle deflect.

[This message has been edited by olejoedad (edited 02-21-2024).]

theogre FEB 22, 09:14 AM

quote
Originally posted by Crazy_ones24:
I verified that the gauge is good by applying power to the wire that runs into E C203, the gauge moved to 0. The sender being good evidently doesn't matter because it's the wrong OPS for the job.

Means gauge sees 0 ok & is very easy... Check gauge w/ resisters tell you wiring etc reads right. Resistor & Sender max Ω moves gauge 80psi. higher then other wiring or sender has problems.

OE 85 dash had a Light & Switch for oil pressure w/ Solid Tan wire at switch/sender & ground thru threads to the block. 87 Dash expect same wiring w/ 0-90Ω sender.
OP switch/sender Also have 2 others running power to F-pump.
All thru C203.

Old and 88 OPS socket only have 3 wires. Some aftermarket 88 plugs has 4 wires, 4th goes nowhere. See cave sender page.

OE OP F-pump switch is Parallel w/ F-pump Relay.
You have to find out what mess the swap did to run F-pump before finding out what is wrong w/ wiring the OP sender for that.