Fuse block or pcm- fuel pump

Tiny
OMCL1004
  • MEMBER
  • 2000 OLDSMOBILE ALERO
  • 157,000 MILES
Trying to figure out whether my fuse box is bad or pcm is bad. Replaced fuel pump (turns out old one was still good) [live and learn] Anyway, My f/p relay is good but the socket it plugs into is not showing constant power on pin 87 with key on as its my understanding that it should. I thought it got its power from the f/p inj fuse but that fuse tests goon AND works. Now also, I have no power for 2-3 seconds on pin 86 when key is turned on or cranking. Pin 30 (grey wire to f/p) shows continuity and pin 85 shows good ground. So, is it more likely my fuse box or the pcm? Ps: I actually havr the car running to get me by till payday till I can replace fuse box or pcm. I have a jumper from 30 to 87 and then a hot wire from battery clipped to that for power and connected to a switch.
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Monday, August 26th, 2013 AT 12:29 AM

16 Replies

Tiny
OMCL1004
  • MEMBER
Ps: This is the only way I can drive my car the way I have it wired, which is fine temporarily, but the only problem ( and not a big one) is that the key will not turn off the engine this way. The only way to kill the engine is to flip the f/p switch I installed and then turn off and remove the key. Just out of curiosity, is pin 87 oconnected to or powered by the coil thus keeping the car running when key is turned off?
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Monday, August 26th, 2013 AT 2:07 AM
Tiny
HMAC300
  • EXPERT
Try getting something in the slot to clean it it may be just loose and clean relay add dielectric comppund to it andsee if that works.
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Monday, August 26th, 2013 AT 7:01 AM
Tiny
OMCL1004
  • MEMBER
Well, I already used a small pipe cleaner and electronic spay. Also, I took the fuse box apart and found no loose or burned wire
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Monday, August 26th, 2013 AT 7:30 AM
Tiny
OMCL1004
  • MEMBER
So, does it sound more like a bad box or a bad pcm?
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Monday, August 26th, 2013 AT 7:33 AM
Tiny
HMAC300
  • EXPERT
If the wire in the box gets power then its' the fuse box
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Monday, August 26th, 2013 AT 7:49 AM
Tiny
OMCL1004
  • MEMBER
Well, it is getting power provided by the battery but pin 86 isnt getting any power to the relay coil which means either bad pcm or broken wire between pcm and fuse box, which I saw none?
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Monday, August 26th, 2013 AT 9:29 AM
Tiny
HMAC300
  • EXPERT
The connection can always go bad to the pcm instead of a pcm if it sits low put some dielectric grease on the connectors then plug back in to the pcm. Then make sure that the pcm is putting power out andi'd bet its 'either a poor connection or connector on pcm. Pin 86 wont' get any power because it's the ground. What engine is in this so I can trace this down the relay ONLY works when the key is first turned on for a couple of seconds then when car is starter the pump runs off of the oil pressure sensor pin 85 comes from the pcm and pin 30 goes to the injectors if this is a2.4L pin 87 is hot when running you are trying to guesstimate your way through this without giving US all the info that is required, like engine size for one. PCM's don't go bad that often although not to rule it out.
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Monday, August 26th, 2013 AT 9:38 AM
Tiny
OMCL1004
  • MEMBER
Thank you sir for replying and bearing with me. And yes, there is a certain amount of guessing on my part, but only because I have asked 3 different "experts" so far about my problem and have gotten 3 different answers. This is a 2000 Oldsmobile Alero with a 2.4 L engine. I'm sorry but I thought I had said that before. Some say pin #85 is ground and pin #86 is hot coming out of the PCM. Others say the opposite. Some say pin #87 is always hot with the key in the "on" position. You seem to be telling me that pin #87 is getting it's hot from the oil pressure sending unit (which would only be hot once the engine achieves oil pressure, ie; during cranking or running) That makes more sense to me at least over the others. Also, some say pin # 30 is the grey wire leading straight to the fuel pump. You seem to be saying that it powers the injectors. Unless, you meant that it powers both. If I understand you correctly, you seem to be indicating that 86 is always ground to the coil inside the relay and 85 supplies hot from the PCM to the coil for 2 seconds which closes the contacts between 87 and 30 and primes the fuel pump. Where I'm getting confused is this: With the key in the "on" position (and not cranking or running) then how is 87 getting power from the oil sending unit to power the fuel pump on for 2 seconds? Are you saying that THAT power comes from the PCM itself via pin 85 for 2 seconds and then once cranking or running the coil opens back up and then the power comes from the oil sending unit? ( In other words the little coil inside the relay is only energized for 2 seconds to prime the pump and then the rest of the time it's open as long as the engine is running and has oil pressure?) But wait, what would be holding the contact between 87 and 30 closed then? I thought the little coil did that. Now, I've confused myself even more. I thought I was on the right track for a minute at least. Could you do me a favor and run it down for me step by step as for example:
(1) Turn key on and PCM supplies power to coil via pin #?
(2) coil closes contact between pins 87 and 30
(3)?
(4)?
(5)?
And etc; etc. I would greatly appreciate it so I can get a 100% clear understanding of how it works exactly and then I can do further testing with my multimeter and know EXACTLY what I'm testing and how and why and then I can save money by not buying parts I don't need. Thank you so much in advance!
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Tuesday, August 27th, 2013 AT 12:42 AM
Tiny
HMAC300
  • EXPERT
See pic, 86 is ground 85 from pcm 87 hot when key turned on 30 goes to injectors and pump. See pic
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Tuesday, August 27th, 2013 AT 7:28 AM
Tiny
OMCL1004
  • MEMBER
Thanks! It looks like my fuse box is the culprit. The female socket for spade 85 of the relay is very loose and shows no power with key on for 2 seconds.I have the fuse box out right now and am going to test the green and white wire inside it and see if there's power there when I turn on the key. If there is then I'll be certain. Those fuse boxes won't let you take the tops off to fix a loose connection like that because of the 3 threaded female studs I guess. Do you know if the top will come off if I keep prying or is it made to not be taken apart? I'm afraid I might break it and then be in trouble. Dealers no longer sell those boxes either so I'll have to try one from a salvage yard if I can't fix this one. Do you know if a box from a v6 engine would work the same as my 4 cyl? I guess I could always just take out the wires going to the f/p relay and wire it up on the firewall or something. I now know the green and white wire is #85 and the grey wire is fuel pump and ground is probably black. I see that # 87 comes straight off the f/p- f/inj fuse, but don't see what color wire that would be. Do you know off hand?
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Tuesday, August 27th, 2013 AT 8:24 AM
Tiny
HMAC300
  • EXPERT
We don't have a color either it doesn't give it and not sure a v6 box will work. The 87 should be the only one that is hotwith key on or look on bottom of box where relay is plugged in.
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Tuesday, August 27th, 2013 AT 9:00 AM
Tiny
OMCL1004
  • MEMBER
Well I checked both green/white wires inside the box (there are 2) and neither one shows hot for 2 seconds when I turn on the key. I guess they (it) is bad inside the computer or somewhere under the dash between the computer and the fuse box. I will check that next. Can only one wire inside the pcm be bad and everything else be good? What are the odds? Will it hurt anything for me to continue driving it the way I have it jumped? Everything works and runs normally, except the key won't kill the motor. I have to kill it with the switch I installed.
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Tuesday, August 27th, 2013 AT 1:40 PM
Tiny
OMCL1004
  • MEMBER
I guess the engine continues to run with the key turned off because the pcm is still recognizes RPM's. Am I right about that? But I thought the rpms came from the ignition coil and when you turn off the key, it cuts power to the coil? I have pin 30 & 87 wired straight to the battery, but neither of those should be connected to the ignition coil, should they?
Btw, (completely off subject) but do you know the easiest way to disable my automatic headlights? I'm not talking about the drl lights (I disabled them a long time ago) thanks
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Tuesday, August 27th, 2013 AT 1:59 PM
Tiny
HMAC300
  • EXPERT
I can offer no further suggestions I really thing you should have a pro look at this before you screw things up completely. I have on idea what you have done but from the sounds of it you are screwing up compeltly anyhow an d will burn something up for sure. Good luck
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Tuesday, August 27th, 2013 AT 3:01 PM
Tiny
OMCL1004
  • MEMBER
Ok, thanks for trying anyway. I'll just have to go to the gm shop and ask them I guess
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Tuesday, August 27th, 2013 AT 3:55 PM
Tiny
SMITH2UP
  • MEMBER
I have same exact problem. Did you find the problem and fix?
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Friday, February 27th, 2015 AT 8:36 PM

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