kenai Posted November 15, 2006 Share Posted November 15, 2006 I know this has been covered but i can't figure this out exactly. My lt1 is from a 94 vette. The oil pressure switch has a yellow wire coming out, a red and a wht/brn. According to my schematic, there is no third wire (the wht/brn) what is its purpose? It also shows that the red wire goes to my pcm 20 amp fuse hot at all times and the yellow splices into the fuel pump relay wire that is going out to power the fuel pump (what would be terminal 30 i believe). I know the PCM is supposed to energize the relay for 2 seconds prior to starting the engine. Does the PCM continue to energize the relay once the engine is started and running? Or is power now supplied to the fuel pump via the oil pressure switch? If so, these wires on the oil pressure switch seem to be to small guage to run current to the fuel pump. I guess a full detailed explanation would be great! I'd love to have this running by thanksgiving!! thanks a ton!! Quote Link to comment Share on other sites More sharing options...
MONGO510 Posted November 16, 2006 Share Posted November 16, 2006 The wires don't run current to the pump, they power a relay. the relay powers the pump. Quote Link to comment Share on other sites More sharing options...
kenai Posted November 16, 2006 Author Share Posted November 16, 2006 thanks for the reply. My schematic shows it actually bypassing the relay, the yellow wire from the oil pressure switch splices into the power wire to the pump after the relay. If the switch was before the relay then it seems to me that the PCM couldn't energize the fuel pump because there is no oil pressure. Quote Link to comment Share on other sites More sharing options...
Bartman Posted November 16, 2006 Share Posted November 16, 2006 I'm not sure exactly how it is wired, but there's some information on this thread: http://forums.hybridz.org/showthread.php?t=95610 In the thread they talk about switching from the fuel pump relay to power from the oil pressure switch once the engine cranks, but I'm not convinced. I'm just using the output from the PCM to activate my relay (the PCM uses a ground signal for this) along with an ignition hot. The relay turns on for two seconds when ignition is turned on and then turns on again once I start cranking the engine. I think the oil pressure sender is supposed to be wired in such that the fuel pump is turned off if the oil pressure is less than somewhere around 4 psi. Quote Link to comment Share on other sites More sharing options...
baddriver Posted November 16, 2006 Share Posted November 16, 2006 I've got a copy of that schematic at home, I don't remember the details, but I'll look at it. I'm pretty sure the FSM states that the oil pressure switch is used to power the fuel pump if the fuel pump relay fails. The PCM does continue to power the fuel pump relay when the engine is running, the oil pressure switch is a backup so a relay failure doesn't cause the car to die. It seems like a second relay would be a good idea, rather than powering the pump directly off the oil pressure switch. I'm not sure if a second relay was used on the original car. Quote Link to comment Share on other sites More sharing options...
kenai Posted November 17, 2006 Author Share Posted November 17, 2006 thanks for the help. I talke to some guys on another board and they told me that the oil pressure switch is only a backup in case the fuel pump relay goes bad. This makes sense with my wiring diagram. But, i still wonder why some people say that the pcm must recognize 4psi to energize the relay because the PCM doesn't even know what the oil pressure is as it has no incoming data. Quote Link to comment Share on other sites More sharing options...
Bartman Posted November 17, 2006 Share Posted November 17, 2006 I'm still not convinced we have everything figured out correctly. My understanding is that the line from the oil pressure sending unit is not a back up in case the fuel pump relay goes bad, but a safety device to ensure that once the engine is running that extremely low oil pressure will stop the flow of fuel. I don't think the PCM has to know anything about the low pressure, it knows when the ignition is first turned on and then energizes the relay for 2 seconds (by giving the relay ground). I know the PCM continues to give the relay ground as long as the car is running, because that's how I have mine wired. What I don't understand is how the oil pressure sender cuts power to the pump or relay when the oil pressure is low. In general the PCM doesn't power devices directly, it gives ground to a relay which then allows the relay to give direct power to the device. This is how the cooling fans work work as well as the fuel pump (even the MIL is activated by a ground from the PCM). P.S. - Perhaps the third wire you mentioned from the sensor is for an idiot light? Quote Link to comment Share on other sites More sharing options...
kenai Posted November 17, 2006 Author Share Posted November 17, 2006 yeah, there still seems to be some misunderstanding. I didn't know that it the PCM was giving the relay a ground i thought it was suppling the 12v to activate the relay, good thing i didn't wire it up yet i could have maybe messed up my PCM. Quote Link to comment Share on other sites More sharing options...
wheelman Posted November 17, 2006 Share Posted November 17, 2006 Bart is partially right about how the system works. The PCM powers the relay (provides a ground) for 2 seconds to prime the system and while cranking. Once the engine fires the oil pressure switch keeps the relay closed (provides a ground) unless the engine dies and the oil pressure drops below a set level (4 psi). The pressure switch is a safety device in case a fuel line is cut in an accident. The premise is that the engine stops, oil pressure drops, switch opens, relay opens, fuel pump stops, fuel stops flowing in a matter of seconds. I'm not sure about the wire colors in the corvette harness but that yellow wire should be connected to the same terminal on the relay that the PCM is connected to. The switch is supposed to provide a path to ground and power comes from the fuse. Hope this helps. Wheelman Quote Link to comment Share on other sites More sharing options...
kenai Posted November 17, 2006 Author Share Posted November 17, 2006 thanks for all the replies, but just to shed some light on the situation. My schematic shows that the pcm does infact give the 12v to the relay, not a ground. I even checked with my DVOM to make sure. When i turn the key to ON i get 12v from the Grn/wht wire from the PCM for 2 seconds then it goes away. When i crank the motor the 12v reappears. Perhaps there are different systems?? I am almost 100% confident that the oil pressure switch is just a backup incase the relay goes bad. Starting would be more difficult because the engine would have to build 4psi before the pump would get power. THis theory was confirmed by one who builds wire harnesses on another board. Quote Link to comment Share on other sites More sharing options...
Bartman Posted November 17, 2006 Share Posted November 17, 2006 Well, it sounds like there must be differences. My PCM definitely provides ground both for the 2 seconds after iginition is turned on as well as when the engine is cranking. I'm not using the Chevy oil pressure sending unit, so I can't say for sure how that was originally setup to work (my engine and PCM is from a 94 f-body). Either way it sounds like you have what you need to get it wired up. Good luck on your swap! Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.