oil pressures switch vs oil pressure sending unit vs fuel pressure switch

[mlm0]

I am having a problem of long cranking time on cold start on 89 vette. I have read a large number of articles on how the fuel system and oil system interact on start up. One of the problems I have read about is a bad oil pressure switch sending a low voltage to the ECM and not letting fuel get to the engine. I don't seem to understand the following Is the oil pressure switch attached to the fuel pressure switch and is that location next to the brake booster. Is it one unit or two that I replace Where is the oil sending unit located. Is that the one on the bottom of the block. It seems that when reading about the oil pressure switch and sending unit, some are getting them confused when referring to them by name, which has me really screwed up.. Is there someone out there you can give me the straight info. I think what I need to do, is replace the oil pressure switch and not the sending unit. I have read the FSM, Hanes, and Chilton manuals , but still screwed up.. Boy, do I feel stupid

 

[Cliff Harris]

When you turn on the ignition switch the ECM turns on the fuel pump relay for 2 seconds. This applies 12 volts to the fuel pump which pressurizes the fuel rails in preparation for starting. The reason it only runs for 2 seconds is to keep the fuel pump from running continuously if you turn on the ignition but don't start the engine. If you turn off the radio and A/C fan you should be able to hear the fuel pump run for 2 seconds when you turn on the ignition. The ECM will turn on the fuel pump when it sees reference pulses from the distributor. This will happen when the engine is cranking or running.

There is a backup for the fuel pump relay. This is the oil pressure switch, which applies 12 volts to the fuel pump when it sees more than 4 - 6 PSI (don't remember exactly) of oil pressure. It will see this much pressure when the engine is cranking.

If you are experiencing long cranking it could be because there is a problem with the fuel pump relay circuit and the oil pressure switch is taking over. The way to see what is really happening with the fuel pump is to put a fuel pressure gauge on the fuel rail.

This switch is located on the back of the block next to the distributor. There were two versions. The early one has a separate oil pressure sender (for the instrument panel gauge) and the oil pressure switch. The sender has one terminal and the switch has two. Later on GM combined the two into one sender/switch unit. This unit has three terminals. I believe the second version started in 1989.

 

[mlm0]

thanks for reply, and the info. I have replaced both relays, but it still cranks long only when cold. My question would be this. If the fuel pressure switch is bad, meaning that it takes more pressure then the 4 lbs of oil pressure before it lets the rest of the system do its thing, would that cause the long cranking. 4 lbs of oil pressure is not much, and should happen very quickly, but it the pressure switch takes 15 lbs because its off, that would take longer. Does my thinking make sense?.. Where does the oil pressure switch get its info from. Is that from the oil pressure sending unit mounted on the block somewhere? thanks in advance, Mike

 

[Cliff Harris]

Where does the oil pressure switch get its info from. Is that from the oil pressure sending unit mounted on the block somewhere? thanks in advance, Mike

On your car there is a combination oil pressure sensor and oil pressure switch (started in '89). It is located on the back of the block next to the distributor on the driver's side. It has a three pin connector on the top.

The oil pressure comes up through a passage in the block vertically directly from the oil pump. It's been that way since 1955.

It does take a while for the oil pump to prime and build up pressure at cranking speeds. It has to fill the oil passages with oil before the pressure builds up enough to close the switch. I've never done a test so I don't know how long it would take to reach 4 PSI. I did a search and I was not able to determine where that number comes from. It could be an Internet myth that somebody put out there and then got spread everywhere. There is no mention of the switch pressure in the FSM (that I could find). It's also not listed in the Corvette parts book.

Here's a link to a pic (hopefully they won't move it for a while...):

http://htsmall.ecklerscorvette.com/assets/corvette/images/size/265x265/sku/48052.jpg

 

[mlm0]

does it take a special tool to remove it?

 

[Cliff Harris]

does it take a special tool to remove it?

I doubt it. I have an '86 and it has a different setup. I have never had to mess with mine, but I have heard that you have to take the distributor out to get access to it.

From looking at the picture in the link it looks like you need to get to the bottom area of the sensor where there is a hex for applying a wrench. The top part is probably plastic.