No-Splice or Splice, Oil Pressure Switch Upgrade

[Fierofool]

We all know that the 85-87 V6 oil pressure sending units are notoriously inaccurate and prone to failure.  The 88 oil pressure sending unit is superior in its design and is a common upgrade for the earlier Fieros, but the new style can also be inaccurate, so I suggest while you have the old sending unit removed, install a mechanical gauge and get a hot and cold oil pressure reading for reference.

Since the harness for the oil pressure sending unit is different on the 88, it requires cutting out the old harness connector and splicing and soldiering in a new 88 style harness connector.

This is how I accomplished the upgrade without cutting my original 86 engine harness.

What you will need:

An 88 oil pressure sensor.  Rock Auto Part Number AC Delco D1808A
An 88 oil pressure sensor harness connector.  Rock Auto Part Number AC Delco PT121.
Crimping tool.
Three (3) male spade terminals.  I suggest you use a different color terminal on the center lead.  This is a ground.  The other two leads are interchangeable.
Soldering iron and solder and something to hold the spade terminal while you solder.  My kit has a stand with clips.
A length of 1 inch diameter split wire loom.


First, crimp the spade terminals onto the new harness.


Once you have the terminals securely crimped to the harness, solder the connection.


Now, with the old style sending unit removed, screw in the new unit securely.  Do not use teflon tape on the threads.  The sending unit uses the threaded base as a ground for the gauge and fuel pump. 

Once you have the new sending unit installed, look at the harness for the old style unit.  With the terminals oriented the same as this:

Photo from RockAuto.Com

The center terminal will be the ground.  That's the terminal that I used the odd color spade terminal on.  Plug it into the center terminal on the old harness.
The lead for the terminal shown closest to you will plug into the corresponding female terminal on the old harness.  This will be the orange wire on the old harness.  It is constant unswitched 12 Volt.  It's always hot.  Power is transmitted to the other terminals when oil pressure exceeds 4 PSI.

The terminal lead furthest away will plug into the remaining terminal on the engine harness.

Now, I have found that the new harness terminal isn't waterproof, even though it has the Weatherpak seal.  You can see there are provisions for 3 other terminals.  Those are open.  Plus the 3 terminals we're using aren't waterproof.  The sending unit isn't protected from rain and water will get down into the top of the sending unit, possibly causing a short.  This can cause the fuel pump to continue running or it can drain your battery.

This is where the 1 inch split wire loom comes into play.  Slide the wire loom down over the new sending unit and insert the new harness wire into the loom.  Cut the wire loom to the necessary length that you can insert the original engine harness.  Orient the split on the wire loom so that it's downward and toward the engine.  It also dresses up the conversion a bit.  As added precaution, you can install a 90 degree fitting to put the sending unit on its side instead of upright.

If you want, as added protection from water, you can pack the top of the new sending unit with dielectric grease.  That's DIELECTRIC GREASE.  Not heat sink grease or silicone.  It's a clear compound that doesn't dry and is waterproof.  It will insulate the terminals in the event water does get into the top of the new sending unit.     

[Fierofool]

So, now that we have everything ready, let's remove the old sending unit.  Remove the battery and it's heat shield.  Using an 8 inch adjustable wrench, hold the sending tube and with a 9/16 open end wrench, loosen and remove the sending unit.
For reference, note the 3 male terminals on top of the sending unit.  The one on the right is the orange/black tracer wire that is unswitched 12V.
Your new connector terminals are positioned in the same manner. 


Using Dielectric Grease, liberally fill the top of the new sending unit around the terminals.  This is to insulate from any water that may seep into the top.


Install the new sending unit.  You will need a 10 inch adjustable to fit the base of the new sender.  Again, at the same time, with an 8 inch adjustable, hold the sending tube while tightening the new sending unit.

Next, it's time to make your connections to the engine harness.  Turn the original connector up and hold it so the female terminals are
positioned as such----    I _ I   


Hold the new terminal in the same position and plug the center male terminal into the center female terminal. Next, plug the left male terminal into the left female terminal and the remaining male terminal into the remaining female terminal.  This is a trial fit so that you can determine the length to cut the wire loom cover.  I found that about 8 inches will cover the sending unit and allow enough room to push the new wiring down inside.


Once you've cut your harness cover to the desired length, plug the new harness into the top of the new sending unit and slide the loom down over it, leaving the new wires exposed.  Turn the split of the harness cover toward the engine to reduce the possibility of water entering.  Plug your new wires into the original engine harness as previously described.  Turn the split of the harness cover toward the engine to reduce the possibility of water entering.


Tuck the wires down inside the wire loom and push the original harness into the top to make a seal.


Reinstall the battery and it's heat shield.

My reading with my old sending unit at hot idle.


My reading with my new sending unit at hot idle.

[scottb]

Can you tell me which connector was used? I am about to order the sending unit from RockAuto and they list a bunch of different connectors.

Thanks, Scott

[Fierofool]

I just edited my post to include the information, but it is worth repeating. 

Oil pressure sensor.  Rock Auto Part Number AC Delco D1808A.
Oil pressure sensor harness connector.  Rock Auto Part Number AC Delco PT121.

[scottb]

Thank you Charlie. I'll get that ordered this week. Hopefully that will cure the inaccurate reading and stop the idiot light from yelling at me.

Thanks, Scott

[Fierofool]

Be sure to check the hot idle oil pressure with a mechanical gauge before swapping over. 

[Fierofool]

At the time I did my writeup, I couldn't find a part that I really wanted to use.  That was a brass 90 degree elbow so that the sending unit could be positioned horizontally.  I've since found them, and I would recommend that the split plastic wire loom be discarded in favor of the brass fitting.  It allows easier install and removal of the 88 sending unit, while at the same time eliminating the chance of water getting into the contact area.

[TopNotch]

I think I'm going to have to get one of those. My Formula shows good oil pressure when the weather is dry, and sometimes no pressure when it's raining. And I know the rain isn't affecting the actual pressure.
Is that some kind of NPT thread?

[Fierofool]

Yes.  It's 1/4 NPT male and female.  Same thread as the sending unit and tube.  I have 1 spare one if you'd like to have it.

[Fierofool]

All the above info is of course for keeping your engine harness original while upgrading to the new style oil pressure sensor.  You can use the no-splice or splicing method with this installation method.

Remember the 90 degree brass elbow?  Well, in the conversion, you have the option of screwing the new sending unit into the sending tube and letting it stand upright or setting it horizontally.  I feel there is a possibility of water getting into the harness connection in an upright position, as happened with me.  That was why I covered the sender with the plastic wire loom cover.  Another way to avoid water seepage is to lay the sending unit on it's side using the elbow.  The following shows this installation.

Because the threads on the elbows don't all begin at the same place on the nipple, you may have to buy 3 or 4 to find one that will be oriented correctly once it's tight.  They're cheap, so go ahead and get the extras. 

I turned mine to a position that the sending unit was clear of making contact with anything.  I set the plastic battery heat shield in place on the battery tray to be sure of positioning.  With the elbow and sending unit installed, I then cut the early style harness end from the harness and soldered the new style harness end onto it. 

This is the elbow that screws into the top of the sending tube.


The sending unit installed into the elbow and angled for clearance of engine bay components.


And finally, a piece of ribbed split plastic wire loom covering for the wires.  The wires were soldered and the connections insulated with heat shrink tubing slid onto the wires before soldering. 

[GTRS Fiero]

No electrical tape over the space between the sections of loom?

[Fierofool]

Not yet.  The gray was the longest piece I had.  I have to buy some more black to cover the full length of wire.  Then I'll tape it.