Wooohooo. I got the new oil sensor in, runs perfect, so I did no damage.... except all the oil was spilt over the exhaust and cat. I degreased what I could and am now 'burning' off the rest....stinks a lot! BUT there is no need to remove the dizzy to do it! If you need to know ..ASK

 

My '91 has the "check gages" light on and all the gages work fine and theres no codes set. I took the wire off my pressure switch next to the distributer and the car wouldn't start. Doesn't the computer need to get a minimum 4psi oil pressure signal from this sending unit to start the car? I couldnt start it with the wire off the sending unit. Or am I supposed to disconnect it after the car's started? Where is the other sending unit, by the oil filter? Thanks

 

I don't think I have read so much misinformation in a single thread in my 10+ years on this forum. There's so much I don't know where to start -- so I won't.

You guys need to get a FSM (Factory Shop/Service Manual) and read it.

 

Pretty interesting posts.
Anyway, I have the FSM and got to the 2nd step "disconnect the oil pressure switch connector". Now do I do that while the engine is running, or not running? I assumed I disconnect the connector then start the engine. I disconnected the "single wire" switch connector by the distributor and my car wouldn't start. So I thought I would go through these posts looking for clues, and I found this post: "the switch/sender with three wires supplies the guage, tan wire, the red wire, ckt 458, is backup in parallel with FP relay, orange is from battery. The FP relay works whether oil pressure is observed or not, as long as key is on 2 secs and/or cranking, or engine running. If relay fails, the switch will power the FP once 4 lbs pressure is sensed *** page 8a-20-6 in 91 fsm". So now I'm wondering if maybe my switch is powering my FP and the relay is dead.
I noticed in my aux fuse pnl (under glove box) that my FP2 fuse is missing, but the FSM says it's only used on VIN J (I have a lowly 8).
So for now I can't get past the 2nd step in the FSM (disconnect oil pressure switch connector). Next I will have to check my fuel pump relay, unless I am misunderstanding why my car won't run when I disconnect the oil pressure switch. Could this be why my check gages light is on? BTW, I see in the manual that the FP relay is below RH side of IP next to many others, but I can't exactly tell by the drawings where it is or what I need to remove to access it.

 

Which one?
If stock,yours should have a 3 wire combined sender unit that includes the OP gauge sender and OP switch for the pump
Jumpering the red and orange wires on the connector should make the pump run with the key off

 

My car has a horizontal sending unit with 3 wires going to the connector (I noticed it looks like it's leaking some), and a 2nd more vertical (and newer looking) sending unit with 1 wire going to it, and I assume this is the switch for the "check gages" light. But when I disconnected the single wire connector, my engine wouldn't start and was hard to start after I reconnected it. Are you saying that jumpering my red and orange wires will send power to the fuel pump relay to run the pump with the key off? Would this tell me the relay is operating properly?

 

I can't stand it... Answering multiple posts here.

There are two oil pressure/switch combinations, depending on the year. Earlier C4s have two separate units: an oil pressure switch (two terminals) that is a backup for the fuel pump relay. The other device is a single terminal oil pressure sensor (NOT SWITCH -- THERE IS NO SWITCH FOR THE LOW OIL PRESSURE LIGHT). It sends the oil pressure to the dash. The computer in the dash decides when oil pressure is low and turns on the warning light. These two were combined into one unit with 3 terminals in later C4s -- I don't know what year that happened.

When the ignition is first turned on the ECM turns on the fuel pump relay for two seconds. That's to prime the fuel rails for starting the engine. The fuel pump relay turns off so the fuel pump will not run if the ignition is turned on but the engine is not started. When the oil pressure switch senses 4 PSI of oil pressure, it adds a redundant backup path to turn on the fuel pump.

It sounds like your fuel pump relay is not working. Check to see if the fuel pump runs for 2 seconds when the ignition is turned on.

Check the error codes from the ECM/PCM to find out why your SES is on. The relays on 1990 and later cars are mounted on the firewall at the base of the steering column. You need to take off the kick panel to get access to them.

 

Thanks, got it, I moved the wires around on/pushed in on the 3-wire connector and noticed that the fuel pump now comes on for a few seconds when I turn the key on like it is supposed to (didn't hear that before and thought it was strange). So now I was able to perform step #2 in the FSM (disconnected 1-wire pressure switch connector), started the engine, and, sure enough, the check gauges light was gone. So, according to the FSM, the switch is probably bad (pressure gauge shows plenty of pressure although they still suggest checking with a mechanical gauge).
Phew, finally.
Lessons learned...........when performing this test, (1991 FSM page 8A-82-20, Chart #26 CHECK GAUGES INDICATOR ALWAYS ON), be sure you can hear the fuel pump run for a few seconds when you first turn on the ignition, or you probably can't perform the 2nd step of disconnecting the oil pressure "switch" that sends the signal to the "check gauges" light because that switch is needed to sense the 4psi to send a signal to turn the fuel pump on and run the engine if the fuel pump relay isn't working. Hope this helps someone else someday.
Cliff, thanks for hanging in there

 

As already observed
The OP switch runs the pump direct from the battery , independent of the pump relay.
The only thing the two have in common is they both share the same
power supply and their outputs go through the same pump fuse
Typical C4 pump circuit shown ( your colors may vary )

]

 

As you note , you have two separate sender units,
the 3 wire unit is as described above that does the analog gauge and runs the pump
as well the 1 wire ( faulty ) unit for the digital OP gauge which is why you got the check gauges light

3 combos actually
'90+ got the 1 wire unit circled for the digital OP gauge as well as the 3 wire combo unit

LT1 shown

 

Thanks, that's great. I get that now. Maybe I pulled on the harness when I replaced the injectors?
I"ll know to keep an eye on this connector. I probably need to replace the sender so it stops getting oil on the wires, too. Might as well just change both sender and switch.

 

The item you are testing has no relevance to the fuel pump running , you could put it in the bin and the car would still run

The FSM states the engine must be running to perform the check gauges test.
If the engine won't start for ANY reason you can't do the test

 

I think it does if the relay isn't working because of a bad connection at the sending unit next to it. Once I got the relay to operate the pump, I was able to start the engine with the switch disconnected.

 

What you may or may not have done while playing with your car is unique to you.

Check out 8A-50-4 , it shows the 1 wire unit you are testing wired back to the CCM for the check gauges ,
no connection to the ECM that operates the pump relay

 

I installed a new oil pressure sender located on the DS under the distributor; it is a 2 prong connector. However it did not solve the erratic pressure readings. I then removed the sending unit and screwed in a mechanical oil pressure gauge which read normal at all operating temperatures. However, I was astonished that the dash gauge still read erratic readings even though I had replaced the sending unit with the mechanical gauge. Where are the readings to the dash gauge coming from? Is there another oil pressure sending unit on my car; where is the other one? I couldn't find any useful information in the shop manual.

 

On your '86 there should be a "bell shaped" sender with a single wire connector. That is the sender for the dash unit.

Something similar to this:

http://i481.photobucket.com/albums/r...o/DSCN0676.jpg

 

The 2 prong part is the oil pressure switch that backs up the fuel pump relay. The single terminal part is the oil pressure sender that displays the oil pressure on the dash.

WVZR-1's pic didn't work for me, so here's another one:

 

Would you use the hex end by the connector for removal? I was told these are common sources for a leak in these cars. Any truth to that before I go digging in there?

 

Not all of the "bell senders" have a hex at that end BUT you would still prefer to use the hex of the sender at the fitting.