Vette X 3

C3'ers- I really need some help here. I'm pretty good with electrical problems, but this one defies most logic. Bought a used '82. Ran for one day fine coming from Tenn. to Indiana. Second day it quit on me and left me stranded. 20 amp fuse in battery compartment for ECM and FP was popped. Found fuel pump circuit board and relay assm. laying in passenger compartment floor on top of tire jack. Assumed circuit board shorted to ground and took out fuse. Replaced fuse and car started right up. Turned ignition key off and car keeps running-not dieseling...running! Ignition switch seems fine. Manual says fuel pump gets power two ways-from ignition switch feedinjg FP relay and from oil sender switch. With car running, I can disconnect oil sender switch AND FP relay board and the FP keeps running with key switch off. No accesssories work, but power stays on to FP.
If I pull the 20amp fuse, the car will die as the ECM and FP get no juice. If I then reinsert the 20amp fuse, the FP will not run until I turn the key to "on". If I toggle the key from "on" to "off" WITHOUT starting the car, the fuel pump will toggle on and off as it should. But once I start the car, the ignition key will no longer stop the ECM or the FP from being energized. It's like the alternator or the oil sender is feeding back power to the FP once the car is running, but as I said...pulling the 3 pin connector off the oil switch makes no difference. If I test the oil switch plug when the car is not running, only one pin of the 3 has 12 volts. Once the car is running, with the plug off the sensor still, 2 pins now have 12 volts which is likely why the pump won't quit.
Sorry for the long post, but I wanted to give someone with a clear head all the facts I have. I would tip my hat to someone who can steer me in the right direction here!

Bill
'82 CE with a "won't ceasefire" :cry

rcastor426

Wow.... that does not sound fun... sorry i can't help good luck.

neverendingproject

i think your gonna need to get a wiring diagram for this one. first thought was check the fuel pump relay for fusing, but i don't think it would continue doing it. let me think about this for a bit.

i tried looking thru a crappy azz haynes diagram to get an idea. eyes are blurry now. it does appear that one post on the oil psi switch does stay hot all the time.
i really dont have a definate answer for you. im thinking its gonna be a controlled ground circuit that shorted to a permanant ground. go to the crossfire forum and ask those guys. im sure one of them will know.


[Modified by neverendingproject, 9:48 PM 4/24/2004]

LAvetteman

I say the board got damaged when it shorted out and is keeping power to the circuit that runs the fuel pump during engine run, which it sounds like is different circuit than the one that cycles the pump during pre ignition. My guess, change the board and your troubles will be gone.

Vette X 3

Keep in mind that the fuel pump will continue to run with that entire circuit board that docks the fuel pump relay entirely disconnected and the ECM and injectors must be staying powered as well to allow the car to run and run perfectly. One leg going to the oil swit6ch should be hot whenever the key is on. But in my case, with the car running and the oil switch harness pulled off the oil switch, two legs show 13.6 and the third leg is about 8.5 volts...go figure. Let me ask one other question--- When you turn the key switch from off to just "on", how long is the fuel pump supposed to run? Seems like itr should only run a few seconds until pressure builds and then shut off unless the car is started. Mine will stay running all day in the "on position" with no start. Is THIS normal?

Bill

LAvetteman

Definetly not normal. Pre ignition fuel pump run time should be 3-5 seconds max.

My next guess would be the relay.

Vette X 3

Which relay are you referring to? The FP relay can't be the problem as it's not even in the car. Is there a relay that senses fuel pressure and turns the pump off after the 3-5 seconds? Where is it located?

Thanks much for all the suggestions. Will be a fuel system expert when this is over.

Bill '82 CE

LAvetteman

Fuel pressure is not sensed. Extra pressure returns back to the fuel tank. OK, if you are sure its not your board or FP relay, the next thing I would check is the oil pressure sender.

Or, try disconnecting the plug on the alternator while the pump continues to run during pre ignition.


[Modified by LAvetteman, 11:13 PM 4/24/2004]

Vette X 3

keep in mind that the car runs with the oil pressure switch out of the circuit (3 pin connector pulled clear off) AND the FP erelay board similarly disconnected. Also, the ignition switch will kill the fuel pump from running when the car is not running but the key just toggled back and forth from "off" to "on". Had the wife sit in the car and turn the key back and forth (but not start the car) and the pump starts and stops with the key, but once the car is actually started, it ain't quitting till the fuse is pulled. I told you this car is possessed. If I could locate where S340 is on the vehicle, I might be able to isolate whether the oil switch wiring is shorted. S340 is where the oil switch and the fprelay meet in common to feed the pump.

Bill

wombvette

The computer is the thing that fires the fuel pump for the inital start up. This is only for a few seconds. Sounds like the computer is bad, but you need to do some more test to make sure, unless you just want to spend the money.


[Modified by wombvette, 12:46 AM 4/25/2004]

Vette X 3

Hi all,
I'm beginning to suspect the ECM as well, especially since it won't stop the fuel pump upon "key on" condition after a few seconds. I checked for error codes and none are showing, but I guess it's possible that the ECM could be feeding back power via the "fuel pump sense" lead that conects the ECM to the fuel pump motor. This might also explain why the engine functions with the key off. Seems that somehow the ECM is staying latched "on" as long as the motor is already running. Wish I had a spare ECM to try before investing the $97 bucks. At this point, the seller is willing to take car back and refund my 10 grand. The car has more problems than just this.... Any more ideas would be greatly appreciated.

Bill
'82 CE

p.s. Is the crossfire site down? I have replys, but can't access the site

mrvette

Boody, I am quoting from memory here, over some years since messing with a TBI computer....bear with me.....

as with any FI computer there are typically 3 positive inputs to the machine...
two for the F Injectors, and ONE for the controll wire to the computer....

the two wires to the injectors....OR the main power input devices of the computer that controll the injectors....depending on internal wiring....is ALLWAYS positive....through a typical 7-10 amp fuse....

NOW that other 3rd wire is typically going to the 5 volt internal regulator which in turn supplies 5 volts to all the sending units....TPS, etc....and of course is the TTL logic as in any earlier computer....so if you interrupt this 5 volt source in this case by turning off the 12 volt feed to the regulator...the computer shuts down.....now, what is happening, I SUSPECT from what I read....is the wiring has been jinxed externally to the computer, and the fuel pump wire is now running the 5 volt line to the computer.....or something damn close to that....what is happening is everytning seems normal untill engine is running, then the oil line FP turn on is now also keeping the computer alive, and that's not supposed to happen....only logical sense thing I can figger is someone has something wired RONG!!1


it's not the computer, I"d allmost bet on it....

GENE

wombvette

:iagree: Thats why I said that you would need to go through more investigation to make sure. The run on symptom is not typical of a bad computer. That would be more likely a wiring problem. The fuel pump control symptom is typical of a computer problem. It could be one or both. If the car is good enough and you want to keep it, I dont think this is a deal breaker. It just needs to be sniffed out.

munday

Mine has done that too in the past.

Vette X 3

Can someone tell me what the circuit board that docks the FP relay is for? Located behind the passenger seat. This board was laying on top of the tire jack when I originally found the ECM fuse blown. Before the fuse blew, the car shut off as it should. After replacing the blown 20 amp fuse to get the car running again, that is when it would no longer shut off. Coincidence?? doubt it! Whatever took out the fuse started this whole mess!

Bill