Won't Run - Low Voltage to PCM Supply Pin at Fuel Pump Relay

shumie7

New Member
2004 Trailblazer EXT. So I cranked my turdblazer up and it barely ever so slightly stumbled and then ran fine for about 5 minutes. Got halfway up the neighborhood and it just died like it ran out of gas, but had plenty of gas.

I've been trying to diagnose this problem the right way without just throwing parts at it but I'm currently stumped so I am making my first post on here. I've searched and searched online but I couldn't find anything specifically about the voltage from the PCM to the relay being low.

I watched a few of May03LT's videos to get to where I am now.

At the fuel pump relay, my constant voltage supply pin is good, my ground pin is good. With the key on the voltage supply from the PCM is only 7 volts. Per May03LT's video, this voltage supply from the PCM should read 12 volts with the key on. Whenever I bypass the relay and straight wire it, the fuel pump kicks on and it starts up and runs perfectly fine.

It's been throwing a few different codes but it has been running fine with these confirmed codes so I haven't fixed the issues yet. I'm wondering if the issues with these codes have told the PCM to stop sending full voltage to the relay; however, this doesn't seem to be the case because it has been running with these codes already. But anyways, the codes are P0013 & P0014, exhaust camshaft position actuator/timing and also P0128, thermostat coolant temp below thermostat regulating temp.

I had a spare ignition switch which I tried but this didn't change anything.

I've checked every fuse, all are good.

I greatly appreciate any help out.

Thanks,

Ryan
 

MRRSM

Lifetime VIP Supporter
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Mooseman

Moderator
Although the FAQ is a good resource, it doesn't have anything specific to this particular issue.

I'd check all the grounds under the hood.
Electrical Ground locations

Then, check the voltages going into the PCM. You can find the schematics in the manuals available for download from my signature. Also check the continuity from the PCM to the fuel pump relay.

It isn't a good idea to keep running it with codes active, especially the P0128 as that WILL eventually kill your cat and plug it up. The other two can usually be easily addressed by replacing the CPAS. That will affect your MPG.
 

budwich

Well-Known Member
I guess the question is: how are you measuring the voltage? at the relay socket? at the PCM? .... and when are you measuring it? The voltage at the relay is not always on with key ON AND engine NOT RUNNING? It is only on for a short time in this mode.
 

gmcman

Well-Known Member
Also check your battery side terminals for corrosion or missing tabs, is your battery secure?
 
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shumie7

New Member
Thank you all for the replies so far. I have checked the grounds in the engine bay and all are good. I haven’t had a lot time to work on this thing so I haven’t been able to check the voltages into the PCM yet as well as for continuity from the PCM to fuel pump relay. Battery terminals and all that are good, just now put a more powerful battery in it out of my dually.

budwich, I’m testing voltages at the relay socket. I am just taking a small wire that has the ends stripped off of it and putting it in the different sockets. So there are 4 sockets. One has full battery voltage at all times. One is ground which tested ok. Another socket diagonally across from the one with voltage at all times is what I jump to and then it powers up the fuel pump and runs perfectly. So then the last one is PCM supply that I am testing. I was just testing this voltage by turning the key on then taking my time back to the engine bay, hooking up my little wire and my multimeter and then testing it and I was getting around 7 volts. After you mentioned about how the signal here should change after a few seconds with key on and engine off I figured I needed to test this socket differently. So then I rigged everything up to where I could test this socket immediately and had someone else turn the key on. This test was with a fresh battery. Once the key turned on the voltage was around 5 volts and then slowly over about a 10 second period went to around 8.4 then after 10 or 15 more seconds settled to around 8 volts and stayed there.

Not sure what this means but that’s what I’ve got so far.
 
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budwich

Well-Known Member
thanks for the extra details on the tests... sounds good. Not sure what to say. One question, you indicate that with it "jumpered", it runs.... is that for a long time or just briefly? You should look at the PCM connectors and ensure that pins are good / aligned / clean ... both on the cable side and the module side. Then you need to check the voltages coming into the PCM.
 
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shumie7

New Member
With it jumpered it will run for as long as you want it to. I needed to use the trailblazer to move some things around in the driveway the other day, it was probably running for 5 minutes or more before I shut it off. I hope to have some more time to spend on it later this week or weekend. Thanks for the advice.
 

Mooseman

Moderator
Very strange indeed. What about with it jumpered and running, what is the voltage like at that socket? It should be a solid 12v. If not, a continuity check from that socket to the PCM connector should be your next check. I'd actually do a resistance check, which should be 0, since it seems to be getting some voltage although inadequate.

As for the PCM itself, I would think that it wouldn't run at all if any of the 12v inputs weren't there. It's quite possible that the circuit for the relay feed is damaged internally, which would explain the wonky voltages. If after the continuity checks out, I'd replace the PCM. Should be fairly cheap from a Pick-n-Pull. Any from the same year and gearing will work after a security relearn.
huge problem after PCM update / calibration (PCM Security Relearn)
 

budwich

Well-Known Member
With it jumpered it will run for as long as you want it to. I needed to use the trailblazer to move some things around in the driveway the other day, it was probably running for 5 minutes or more before I shut it off. I hope to have some more time to spend on it later this week or weekend. Thanks for the advice.
ok... good luck in the "hunt". The "quick check" would be to disconnect the positive at the battery, then go to the relay socket and do a resistance measurement to a known ground (negative battery) on the socket pin that you are getting the "low battery reading". This test basically does a "black box" test of the cabling AND the circuitry in the PCM. Depending on the reading, it may tell you where to look next.... most likely doing the same test at the PCM connector for the same appropriate pin location which then at that point would tell you if the cabling has an issue or possibly the pcm. Careful around working with the pins / connectors to prevent damage.
 

gmcman

Well-Known Member
Sounds like either your alternator is not putting out enough or your battery or battery cables are bad. You can have a tight side-terminal connection but making poor electrical contact.

Try wiggling the two small wires going to the alternator while observing voltage.

These vehicles are not like back in the day, if you disconnect the battery while the engine is running it will likely stall out, even with a healthy alternator.
 

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