97 sub blows fuse ecm b with fpump relay removed

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

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.

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....

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???

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......

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

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

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.

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....

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?

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.

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.

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:banghead:

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.......

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

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

if the fuse to pcm continues to blow and the wire to /from is ok then something in the fuse block is shorting out..... did you look under fuse block......any loom that is missing or damaged like melted? loom is the plastic that covers/protects wires from chaffing/heat......

I've inspected the fuse block from top, bottom, and every which way at least 10 times. Nothing that I can see wrong. no luck at all. unplugged the oil press switch and fuse still blows. So with All components on the ECM-B circuit ----the oil press switch, fuel pump, fuel p. relay removed & the vcm/pcm unplugged the fuse blows on contact. do I have to check the wiring all the way to the back fuel pump connector and is there an easier way to do this other then actually removing all covering and inspecting visually? I'm seriously about to loose it now.:banghead: :banghead: :banghead: :banghead:

You haven't got dual fuel tanks by chance do you? If so there is another balancer module to deal with......other than that, how does the wiring look from oil pressure connector back to loom, or within the connector itself?

Nope, just the 42 gal large tank. As you know it's hard to see the oil press switch and wiring. I unplugged the connector and it still blew fuse (everthing on the ECM-B is still unconnected also). I can see into the connector and it looks good and clean inside with no discoloration. I have to use a mirror to see the first foot of wiring fromthe connector down to where it starts to run along the top of the bell housing and it looks good. ????????? I was so hoping that would be it. I'm popping a beer.......I don't know what else to do. MG

a little more info...... Im not sure if this helps, should I have continuity across both sides of the ecm-B fuse thats blown/open. Does this just mean I got a problem and not help with where. I also have continuity between the orange wire at the white PCM connector and both sides of the ecm-b fuse slot. Does that tell me anything. Also, there are 3 wires going to the oil pressure switch and I found continuity between the orange and pink at the connector end, is that normal? in dire straights here!!! MG

a little more info...... Im not sure if this helps, should I have continuity across both sides of the ecm-B fuse thats blown/open. Does this just mean I got a problem and not help with where. I also have continuity between the orange wire at the white PCM connector and both sides of the ecm-b fuse slot. Does that tell me anything. Also, there are 3 wires going to the oil pressure switch and I found continuity between the orange and pink at the connector end, is that normal? in dire straights here!!! MG


ok now you have continuity across blown ecm fuse and every thing is disconnected...... start removing the other fuses one at a time until this changes then you'll know what circuit is cross connected....to the ecm circuit....

manoman......this is getting even worse. I just spent about 2 hours (ontop of the est. 8 ) just pulling wiring out of protector covering and looking for something that might be the cause (can't feel the tips of any of my fingers any more). Decided to try another ECM-B fuse and it didn't blow. hooked everthing back up seperately (oil press switch, fuel pump, f.p. relay, vcm/pcm and nothing would blow the fuse. I wiggled each item around many times over trying to get it to blow; it wouldn't. I'd unhook that item and do the same with another, nothing. So, I hooked everything back up and put the top of the engine stuff back on and started it. Ran it about 20 minutes on about 6 starts. Fuse won't blow. Haven't drove it yet, but so far no ignition cutout either. I'll drive it around in the morning and see what happens. The only thing worse then what I was dealing with is too not find the problem, the truck start working, so i'm waiting to get stranded eveytime I leave the house on a trip to the west texas hunting grounds. wow. Thanks for all ya'll help fellas. MG

Since that is a "battery hot" circuit - the ability to blow that fuse due to a dead short is always there- whether the truck runs or not---If I read your posts right- you were blowing the fuse as soon as you inserted it--the ignition was NOT on- so if you are concerned that there is a partial short- something that is (drawing some current)- you can do battery draw tests-on that circuit alone- to help rest your mind that the short is not longer present- you can attach your ammeter (or DVM in amps mode) across the ECM-B fuse slot and watch your current draw as you plug in and unplug components- this will tell you if the short you had full time is now just partially present (10 amps, 6 amps, 2 amps etc)- not enough to blow the fuse but enough to drain your battery overnight- it would also lead you to which component is failing (drawing current when it should not be)-- or if no component shows current draw- you are back to the wiggle test on the conduits etc... it is tough- but would lead you to peace of mind- if there is only 20ma draw- which would be the normal PCM draw- I would strongly recommend you try this testing- if you want resolution to the problem...good luck..

bizarre, so now the ECM-B fuse mysteriously stopped blowing. There is no am draw across it at all. However, now the ING-I fuse has begun to blow after the engine is started.????? I don't have repair manual,so I just ordered a shop man. cd so I can have the schematics. Any ideas???? I know that the MAF is on that circuit, so I unplugged that and the fuse still blows. At any rate, the ECM-B isn't blowing anymore, but now the IGN-I is. MG

If you are referring to the ENG-1 fuse (you have it as the IGN-1)- now you must look at the EGR valve- all the oxygen sensors- the cam and crank sensors the Evap purge-- I would have that fuse box unbolted and laying upsidedown- looking for a loose piece of metal- a paper clip- a test lead- a needle- something someone has dropped into it trying to test- that is now contacting the metal ground somewhere.....it jumped or moved from the ECM-B circuit over to the ENG-1 circuit.........good luck

I just dropped the truck off at friends house who has electrical schematics. I'll start on it tonight. I thought it was labeled the ING-I circuit, is that the one that has the MAF on it. I had the fuse black unsecured out of the box and turned upside down several times checking for exactly what you saying. There's nothing though. I've blown it out and probed through each wiring row. Nothing is crossing over. This morning the battery intermittantly acts like its week on startup. The batt. voltage is the same everytime. 12.40-12.50 but one time it will turn it over like you would expect, the next it barely turns to it over one stroke at a time, then the next it back to normal cranking. the cables are good shape, good connections, positive and ground to battery, ground to body, positive to fuse block. does this give you any ideas????

bizarre, so now the ECM-B fuse mysteriously stopped blowing. There is no am draw across it at all. However, now the ING-I fuse has begun to blow after the engine is started.????? I don't have repair manual,so I just ordered a shop man. cd so I can have the schematics. Any ideas???? I know that the MAF is on that circuit, so I unplugged that and the fuse still blows. At any rate, the ECM-B isn't blowing anymore, but now the IGN-I is. MG

you have disturbed the bad ecm feed wire so that it does not blow...... this is unfortunate that as you where messing with wires no one was observing the ecm feed circuit for changes in resistance....... now you have another circuit blowing fuses....carefully follow your schematic wiring and if possible have assistant keep watch on the resistance on the ING-1 fuse/ENG-1 fuse for changes......you are making progress... make sure battery is charged use a charger to top off battery or next you'll need alternator........just a thought you put all the wires back like they were ? if not a wire may have gotten damaged like touching exhaust manifold etc.......

thanks I'll following your instructions later tonight. However, I don't understand the altenator thing. Batter reads 14 v while running and 12.4-12.5 once charged off a batt charger. so I would assume that means the altenator is working fine. Is this a bad battery?