No power to the PCM B fuse and fuel pump relay.

Maverick6587

Original poster
Member
Dec 16, 2018
730
Sterling Heights, Michigan
You guys have the best posts I've seen online for trailblazers! You've saved me a few times. Than you all!

Initial issue was that my vehicle would crank but, not start. I almost pulled the fuel pump, but went through electrical testing.

I'm getting no power to the pcm b fuse for those 2 second it should be powering the fuel pump relay, when the key is turned on. No power when cranking the engineer either. I am getting 10 volts to the positive side of the PCM B fuse, but no ground. Zero volts at PCM when the first is in that slot. I'm just not sure if the PCM B fuse is supposed to be grounded on the other side of the fuse panel slot.

At the fuel pump relay I do have power at the battery power slot and the grounds all check out at the fuel pump relay as well.

At this point I'm thinking it's either the PCM itself or the fuel pump. Please help!
 

freddyboy61

Member
Dec 4, 2011
276
Fuses do not have positive or ground sides. A fuse is placed in series with a power source to protect the wiring if the device it powers happens to pull too much current.

I think you are not using your meter correctly. If you have 10 volts at one end of the fuse, that is not really possible. And I don't know what point you are measuring 0 volts "at the PCM".
 
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Maverick6587

Original poster
Member
Dec 16, 2018
730
Sterling Heights, Michigan
I was measuring from one side of the fuse and from the main positive connected to the fuse panel.
 

Maverick6587

Original poster
Member
Dec 16, 2018
730
Sterling Heights, Michigan
Thank you for that MRRSM. The problem is, all of the posts I see on all of these forums are showing fuse power. I have zero volts though at the PCM B fuse though, which powers the fuse pump relay. I'd rather not pull the fuel pump down, until I know for sure that the PCM itself is not at fault.

forgot to post this initially... Sorry, I do have an error code P0230.
I also have 12 volts on the green-white wire at the PCM, on C1, that should be powering the PCM B fuse.

Fuses do not have positive or ground sides. A fuse is placed in series with a power source to protect the wiring if the device it powers happens to pull too much current.

I think you are not using your meter correctly. If you have 10 volts at one end of the fuse, that is not really possible. And I don't know what point you are measuring 0 volts "at the PCM".

I'm sorry, I was measuring the fuse slot, with the fuse pulled out. That's the only way I found voltage at the PCM B fuse, is in the slot itself, not when the fuse was in place. Zero volts on either side with the PCM B fuse in place.
 

Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
I'm sorry, I was measuring the fuse slot, with the fuse pulled out. That's the only way I found voltage at the PCM B fuse, is in the slot itself, not when the fuse was in place. Zero volts on either side with the PCM B fuse in place.

If the slot feeding the fuse has power, but the fuse itself doesn't, it sounds more like the contacts may be loose, and not making proper contact with the blade of the fuse once inserted. How easy was it to pull the fuse? Did it let go easily and come out, or did it have a bite/grab to it?
 
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Maverick6587

Original poster
Member
Dec 16, 2018
730
Sterling Heights, Michigan
So PCM B fuse would only have power from from PCM itself, when the key is turned to prime the fuel pump and when the vehicle is running. That's where I'm stumped. I don't know if it's possible a bad ground somewhere, the PCM itself is bad, or maybe a bad connection at the fuel pump.

The fuse slot pins didn't seem loose when I was probing them. They didn't seem to move at all.
 

freddyboy61

Member
Dec 4, 2011
276
PCM B fuse derives its power from the battery, this is power that is on all the time. If you do not have 12VDC at either contact for PCM B fuse, then you may have a problem with the fuse box.
If you are comfortable troubleshooting electrical problems and an read schematics, download the service manuals from Mooseman's sig and look for the wiring section for you model year.
 
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Maverick6587

Original poster
Member
Dec 16, 2018
730
Sterling Heights, Michigan
PCM B fuse derives its power from the battery, this is power that is on all the time. If you do not have 12VDC at either contact for PCM B fuse, then you may have a problem with the fuse box.
If you are comfortable troubleshooting electrical problems and an read schematics, download the service manuals from Mooseman's sig and look for the wiring section for you model year.

I do have 10 volts on the left side (closest to the engine). I'm also concerned that it's 10 volts and not 12 volts. I assume it should be 12 volts.

I'll take a look at Mooseman's manuals, thank you.
 

Sparky

Member
Dec 4, 2011
12,927
Make sure you have a good ground point on your negative test lead bu checking a known good 12v source (like the battery).

You're sure your fuse is ok? When that fuse blew on mine it was because the harness connector on the driver side frame rail shorted due to water intrusionand corrosion. (I think it was the pcm B fuse that was blowing)
 

Maverick6587

Original poster
Member
Dec 16, 2018
730
Sterling Heights, Michigan
Make sure you have a good ground point on your negative test lead bu checking a known good 12v source (like the battery).

You're sure your fuse is ok? When that fuse blew on mine it was because the harness connector on the driver side frame rail shorted due to water intrusionand corrosion. (I think it was the pcm B fuse that was blowing)

I was using the positive and negative points that are attached to the fuse panel. I tested a few other fuses to make sure they had 12 volts and they did have 12 volts.

I replaced both the PCM B fuse and the fuel pump relay with brand new ones.

The ground that you're referencing... Is that the on that is posted on the top of the frame, in between the strut and the firewall? I did notice that ground wire did have some green corrosion on it. I sprayed it with electrical cleaner and that took all of the corrosion away, maybe I'll unscrew it and make sure that it has a clean connection to the frame then.
 

Maverick6587

Original poster
Member
Dec 16, 2018
730
Sterling Heights, Michigan
I have access to hoist that I'd like to get my vehicle on this weekend, that's about 80 miles away. I live in Michigan and it's freezing outside. Anyway, that would make it a lot easier and faster to be able to check the fuel pump connection to make sure that connection is solid or not. As well as being able to easily view/access that wire harness going back to the pump. Just in case I need to run a new line back to the pump.

Question: If I jump a functioning fuse slot (One that I would not need) to the PCM B slot while cranking, I assume that would start the vehicle? Also, would that harm the PCM?

I only need to do this for the 80 miles to get to the hoist. I do understand I would need to pull that fuse jumper before turning the vehicle off.

This is just a last minute Hail Mary.... Assuming that there is nothing wrong with the fuel pump or pump connection.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Yes, it should work.
 

Maverick6587

Original poster
Member
Dec 16, 2018
730
Sterling Heights, Michigan
Sorry this is taking so long. This is the first time I've EVER had an electrical issue on a vehicle. what a nightmare... Normally it's just a broken part, rip and replace. Thank you Freddyboy, and Sparky for the advise and Mooseman for the diagram, total life savers! I have always though the fuse box was a ground and positive link. I didn't think twice about it because there was a red and black wire going to it, until I actually looked at their post locations. Learning a lot here... I can't thank you all enough!

So far, I had two bad grounds and a bad relay (relay #41)... The ground at G107 on the engine, causing the no power to fuel pump and the ground at G305 (near rear down on the outside of the frame), lowering my 5 volt reference voltage. G305 is what made this take so long... I didn't know that there were two grounds on the left frame rail. I had cleaned G304 (near front door on the outside of the frame), thinking that it was G305.

The fuel pump itself seems not to be working as well, but those two grounds and the relay took care of the check engine light. The relay took me for surprise as I bought a new one and neither gave power to the fuel pump power connector. I used the bright lights relay and that fixed the fuel pump power relay in the end.

I'm currently waiting on delivery of the new fuel pump, should be here in a few days. I tried a fuel pump from the junk yard, just to get by for the few days that shipping would take, that ran for maybe a second and died. lol, seems right.

Sorry for the long post, but I wanted to make sure the entire event was covered. I'll post again when I get the fuel pump in and the vehicle running again.
 

Maverick6587

Original poster
Member
Dec 16, 2018
730
Sterling Heights, Michigan
Almost forgot to come back here and validate.

The new fuel pump did work. I bought a Delphi fuel pump, because I did not want to do that again in a month.

side note:
I did burn out my transfer case from only buying two new tires. Swapped that out and now I have an vibration issue. I assume it's the motor mounts and for sure the transmission mount, I saw how bad that looked. I'll actually look at them today though to verify, but I ordered them on Amazon because it was so much cheaper, AC Delco of course.

I need this 97x to give me a break, so I can start improving instead of fixing, lol.
 

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