97 sub blows fuse ecm b with fpump relay removed

[hartzell]

97 5.7 suburban stopped died this morning. the ecm b fuse is blown and blows on contact with a new one. Removed the fuel pump relay and still cant get a new fuse in without blowing on contact. I assume that would mean its not the fuel pump circuit thats blowing the fuse. Where do I begin from here. The ignition seems to have been periodic cutting out, just for an instant. Runs fine for a hundred miles, then might instantly cut out from time to time then gets smooth again. Every once in awhile it might backfire as you drive it but then like someone throws a switch it clears up and runs fine for another hundred. I bought new wires,plugs,coil, cap and rotor to install today (probably needs it anyway-bought truck used 20K ago , wires look origianl because of #'s on them , 130K on truck) and started it up this morning to warm it up and it died after about 30 seconds and now I find the fuse blown as described above. what from here???? Can a bad ECM cause what ive been experiencing and now the blowing of the fuse???MG

[jdl]

If there is an oil pressure switch in the fuel pump circuit, you will have to account for that switch. Might be located at the rear of intake manifold in block.

You can use a testlite across the fuse terminals, if it burns bright with everything turned off, it's going to ground somewhere. You could unplug the pcm, see how the testlite reacts? If the lite no longer burns? If the lite still burns, then you have a short to ground on that circuit and I think the pcm is ok. If you think it is a pcm problem, you have to be sure it wasn't a sensor or circuit problem that caused the pcm problem, or it might cause the same problem in the new pcm.

[j cAT]

Originally Posted by hartzell

97 5.7 suburban stopped died this morning. the ecm b fuse is blown and blows on contact with a new one. Removed the fuel pump relay and still cant get a new fuse in without blowing on contact. I assume that would mean its not the fuel pump circuit thats blowing the fuse. Where do I begin from here. The ignition seems to have been periodic cutting out, just for an instant. Runs fine for a hundred miles, then might instantly cut out from time to time then gets smooth again. Every once in awhile it might backfire as you drive it but then like someone throws a switch it clears up and runs fine for another hundred. I bought new wires,plugs,coil, cap and rotor to install today (probably needs it anyway-bought truck used 20K ago , wires look origianl because of #'s on them , 130K on truck) and started it up this morning to warm it up and it died after about 30 seconds and now I find the fuse blown as described above. what from here???? Can a bad ECM cause what ive been experiencing and now the blowing of the fuse???MG

expose wires to pcm check for damage if none disconnect pcm connectors see if it still blows fuse... check connectors that go to pcm for green/black leakage of current... as you where having some ignition issues prior.. i would not do any replacing of parts until this is addressed....

[hartzell]

thanks for the info to begin with but I am trying to figure out which module is which. The PCM is which one? is it the main big box module (about the size of a cigar box) located on the drivers side wheel well next to the underhood fuse box. also, what is green/black leakage of current. I'm more of a old airplane mechanic so i'm learning my way around here on 4 wheels. Also is the VCM and the PCM the same thing diff names???

[j cAT]

Originally Posted by hartzell

thanks for the info to begin with but I am trying to figure out which module is which. The PCM is which one? is it the main big box module (about the size of a cigar box) located on the drivers side wheel well next to the underhood fuse box. also, what is green/black leakage of current. I'm more of a old airplane mechanic so i'm learning my way around here on 4 wheels. Also is the VCM and the PCM the same thing diff names???

pcm is the cigar box driverside below fuse box the green and black discolorations are indicating a leakage path for electricity to flow.......basicly pcm/vcm/ecm computer that controls the operation of electronic systems installed......

[777stickman]

Looking at my '98 C/K manual I can only see 2 things the ECM B fuse powers, although there may be more. The ECM B fuse is "hot at all times".

1. Powers the fuel pump thru the "energized" contacts of the fuel pump relay.
2. Supplies 12v power to the VCM/PCM.

Since you have removed the fuel pump relay and it still blows the next thing to check is the VCM.

There are 4 connectors at the VCM. The one that has the ECM B feed is the one labled "white" (top fwd). Pull this connector off and install a new ECM B fuse and see what happens.

If the fuse blows then there is probably a worn spot in the wire shorting directly to ground. If it does not blow, wiggle the harness around and if still does not blow then remove the fuse and reconnect the "white" VCM connector. Reinstall the fuse. If it blows then it's most likely an internal short in the VCM.

Hope this helps and please post back what you find.

Cheers from another old aircraft mechanic...........Steve

[hartzell]

glad to hear from you steve. I had read some of your other posts and was hoping to hear from you or old dave I think his name was. I just got back to my computer. I think I just finished doing some of what you described, although I think I did it the long way. In this order, I disconnected the fuel pump relay - install fuse & it blows / disconnected the connecter at the fuel tank (3 prong .....I believe it would be the right one...it's coming from the top of the tank..... gnd*pump*fuel gauge... I would guess) - install fuse & it blows / unplugged all 5 connectors from the PCM - install fuse & it blows.
Reconnected all of the above and came back in to check AF.

So, I would assume at this point its not the fuel pump circuit past the pump connector or the PCM. Is this correct? If so what the hell's next outside of needle and haystack scenario. I am remembering that this is why I decided to start flying airplanes instead of wrenching on them. Thanks MG

[777stickman]

OK, I just went out and looked at the PCM on my '98 Sub. There are only 4 big connectors to it and you said you disconnected 5?? Are you sure we're on the right page?

With the pump relay removed the problem is not downstream of the relay but could still be a chaffed power wire shorting to ground.

The 2 top plugs on the PCM are labled "white & black" on the unit itself. Are these 2 of the 5 plugs you disconnected? I could only find 4 on mine. The PCM is just fwd and at the same level as the underhood fuse block with cooling fins on the top of it.

[j cAT]

Originally Posted by hartzell

glad to hear from you steve. I had read some of your other posts and was hoping to hear from you or old dave I think his name was. I just got back to my computer. I think I just finished doing some of what you described, although I think I did it the long way. In this order, I disconnected the fuel pump relay - install fuse & it blows / disconnected the connecter at the fuel tank (3 prong .....I believe it would be the right one...it's coming from the top of the tank..... gnd*pump*fuel gauge... I would guess) - install fuse & it blows / unplugged all 5 connectors from the PCM - install fuse & it blows.
Reconnected all of the above and came back in to check AF.

So, I would assume at this point its not the fuel pump circuit past the pump connector or the PCM. Is this correct? If so what the hell's next outside of needle and haystack scenario. I am remembering that this is why I decided to start flying airplanes instead of wrenching on them. Thanks MG

this is good news for you as it is not a major expense .... you have most likely a bad chaffed wire shorting out to frame or other ground point ... get a good drop light and look for bad wire from fuse panel to computer... also if you can this could be under fuse panel where wire is damaged....

[hartzell]

thanks fellas........I was just eating dinner with wife pretending to be paying attention to whatever she was talking about all the while still thinking about this still. I guess I'll starting chasing wires. Not a big ticket item might be the case, but time is sometimes more expensive. The pre 98 must have diff PCM. The big 4 are there black, white, blue, red I think were the colors on the connectors but there's also a smaller one in the middle. Anyway, I sure appreciate your help fellas, I'll follow up with what I find. It sucks to start the day trying to track down one problem and end up spending the whole day working out an entirely different one that happened to show up same time. Anyway, changed out the plugs, wires, rotor, cap, coil, ICM or ECM whatever it's called over there by the coil. I guess once I get this ECM-B fuse blowing thing figured out I can see if any of this fixed the cutting out ignition. thanks again.

Oh, if one of you has a minute, can you explain the black/green electrical leak thing for me. I've gotten the image that it has to do with the PCM. This is new to me, what is it and where will I see it if it's happening?

[alexmac]

i would take a look around the ignition switch area for burnt wires if you have an alarm or remote start theres lots of brackets for wires to short out on. might also smell like burnt hair werever the short is.

[777stickman]

The wire coming from the ECM B fuse is orange. It goes to a splice in the underhood fuse panel and then to the fuel pump relay and to pin 21 of the VCM white connector. Maybe trace it back from there to the fuse.

Good luck to you.

[hartzell]

alright fellas,....ive checked the power (orange) wire from the PCM - white all the way back to the underhood fuse that keeps blowing on contact. Nothing exposed ,chaffed or burnt along that run. With the f-pump relay removed and the fuel pump connector at tank unplugged do I need to remove and check all the fuel pump wire between fuse block and pump connector....or is that eliminated since the relay is removed??? Im really at a loss here and don't know what else to do. Are any of yall certain that there's nothing else on that circuit other than the PCM and Fuel Pump??? Thanks MG

[brcidd]

Don't you guys forget that the fuel pump relay is in parallel to the oil pressure switch circuit - and that the fuel pump hot wire may be arcing inside this sending unit/fuel pump switch-- I had a 454 C/K Dually do this so bad that the engine bucked and carried on- until I unplugged the sending unit/switch assembly- my big hint was that the oil pressure gauge would fluxuate- while this happened. I pulled out the sending unit/switch and two terminals were melted- from the internal short-- The 454 engine runs really hot and melts those units down by the exhaust manifold- since that time I have seen two other 454's do the same thing.....just something to have you check... you'll know it is in parallel if your sending unit is a three terminal unit- 1 terminal for the gauge and 2 terminals for the fuel pump switch......The sending unit on a 5.7L I believe is behind or to the side of the distributor........unplug it and see if your fuse blows.....

Also- since your truck has been running badly- this may be the culprit that caused that as well- my dually ran like crap intermittently- and once would barely get me home-- then I had problems with the battery running down overnight (pulling amps)-- then I did amp draw tests- found the Ecm-B fuse was the 1-2 amp draw- and looked at the circuit- and pulled the oil pressure sending unit/switch- to see the melted terminals-along with the oil pressure gauge fluxuation- the switch is fed by a battery hot at all times- and would pull some amps that tried to run the fuel pump- even though the truck was shut off- there was arcing going on inside the switch- even when the truck was running- that was shorting to ground- thats why it ran so bad- the fuel pump was not getting enough current while running and too much (any at all is too much) when shut off---- I threw that sending unit/switch assembly over the barn roof when I finally figured it all out.......

[hartzell]

thanks for the additional info. I just got back from a friends house where I was using his computer program to look at wiring diagram and found what you just told me. I'm crossing all fingers, toes, and anything else I can that op switch is problem. Let you know more in a little while. MG