LM7 Lost OBD and fuel (I think!)
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LM7 Lost OBD and fuel (I think!) 2004 LM7/4L60E with factory harness and underhood fuse block. ECU has VATS removed. Swapped into a 68 Suburban.
Engine was running (poorly and without pedalcontrol). I added an aftermarket fan controller (put the temp probe in the back head hole, wired into a blade with 12V on ignition and a terminal to 12V always hot, fan ground is a common frame ground. Now I have crank no start and the obd2 port has gone dead. The OBD port has 12V going to it, and the check engine light rigged into that system illuminates, but nothing on my scan tool. Doesn't even light up. I get a quick pulse of voltage to the fuel pump (less than a second coming to the wire) when I move the key to run, but nothing going to it on crank or staying on while in run.
It feels like a broken wire to me, but I'd love some extra brain power on this one. It was running! (poorly - but I found a vacuum leak and have the TAC module on the way (I've already replaced MAF and pedal position sensor, my TAC module is coming on Tuesday, I think) but it always started immediately. Now lots of crank, no start.
Or am I obsessing over fuel when it might be spark? I JUST DON'T KNOW
(which is why I have retired for the evening with a cold malted beverage. don't want to screw it up in a rage blindness)
Engine was running (poorly and without pedalcontrol). I added an aftermarket fan controller (put the temp probe in the back head hole, wired into a blade with 12V on ignition and a terminal to 12V always hot, fan ground is a common frame ground. Now I have crank no start and the obd2 port has gone dead. The OBD port has 12V going to it, and the check engine light rigged into that system illuminates, but nothing on my scan tool. Doesn't even light up. I get a quick pulse of voltage to the fuel pump (less than a second coming to the wire) when I move the key to run, but nothing going to it on crank or staying on while in run.
It feels like a broken wire to me, but I'd love some extra brain power on this one. It was running! (poorly - but I found a vacuum leak and have the TAC module on the way (I've already replaced MAF and pedal position sensor, my TAC module is coming on Tuesday, I think) but it always started immediately. Now lots of crank, no start.
Or am I obsessing over fuel when it might be spark? I JUST DON'T KNOW
(which is why I have retired for the evening with a cold malted beverage. don't want to screw it up in a rage blindness)
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An update:
The OBD port was a simple wiring issue (the 12V had come out of the back of the unattached for now port.)
I checked spark by putting a screwdriver into the plug boot (when it was unplugged from the plug) and got no spark to ground while cranking. I have unhooked the fans so that is out of the equation. I don't see any blown fuses.
Is it possibly a Crank Position Sensor?
The OBD port was a simple wiring issue (the 12V had come out of the back of the unattached for now port.)
I checked spark by putting a screwdriver into the plug boot (when it was unplugged from the plug) and got no spark to ground while cranking. I have unhooked the fans so that is out of the equation. I don't see any blown fuses.
Is it possibly a Crank Position Sensor?
TECH Regular
Joined: Sep 2007
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From: Kansas City, MO
Use a volt meter or test light and check all of the power and grounds to the PCM, injectors, and coils. I’m guessing you somehow shorted one of these circuits to ground when you hooked up the fan to power and blew one of the fuses. Or the fan itself has too low of electrical resistance. But I’m assuming you never got to the point of the fan working?
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OK, I'm still trouble shooting, but wanted to document what I've found so far:
1. I haven't had a chance to check all of the injectors (I am my only hands, so I have to rig a phone to record my multimeter while I crank) but here's what I found on Cyl 2 when I put the probes into it:
key on gives 6 .1 V, on crank it drops to 4.71 and seems to stay constant.
2. PCM is talking to OBD so it seems ok, right? Or are there other grounds that might be a problem? Should I check every pin marked GND on the schematic out of the PCM? (looks like blue 1, blue 24, blue 40, blue 64, blue 67, green 1, green 40)?
3. Still no spark when I pull the plug and crank it.
4. Showing the following codes:
P2135 (Throttle/Pedal Position Sensor / switch A/B Voltage Correlation)
P0120 ( Throttle / Pedal Position Sensor / Switch A Circuit)
P2135 a second time
P0120 a second time
Appreciate any insights.
1. I haven't had a chance to check all of the injectors (I am my only hands, so I have to rig a phone to record my multimeter while I crank) but here's what I found on Cyl 2 when I put the probes into it:
key on gives 6 .1 V, on crank it drops to 4.71 and seems to stay constant.
2. PCM is talking to OBD so it seems ok, right? Or are there other grounds that might be a problem? Should I check every pin marked GND on the schematic out of the PCM? (looks like blue 1, blue 24, blue 40, blue 64, blue 67, green 1, green 40)?
3. Still no spark when I pull the plug and crank it.
4. Showing the following codes:
P2135 (Throttle/Pedal Position Sensor / switch A/B Voltage Correlation)
P0120 ( Throttle / Pedal Position Sensor / Switch A Circuit)
P2135 a second time
P0120 a second time
Appreciate any insights.
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Found a blown fuse that was preventing the MAF and PCM from getting their 12V on ignition. Great. Replaced. Still no spark. I'm a bit over my skis here. From the fuse block how does the power get to the coils? Is that all of the pink wires labeled "Ignition 1 voltage"?
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Ok.
All of the PCM grounds show 12 v when tested.
I get a cycling 1.2 volts on the PCM to coil connector (the PCM side) when I crank. But no spark on the plugs when I pull them and crank. I am much confused. Is there a ground for the coils separate from the ground in the harness to the coils? (both of those are showing good continuity to the negative post on the battery.
All of the PCM grounds show 12 v when tested.
I get a cycling 1.2 volts on the PCM to coil connector (the PCM side) when I crank. But no spark on the plugs when I pull them and crank. I am much confused. Is there a ground for the coils separate from the ground in the harness to the coils? (both of those are showing good continuity to the negative post on the battery.
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TECH Regular
Joined: Sep 2007
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From: Kansas City, MO
The coils and injectors should have 12v available voltage on the pink wires when the key is on and during cranking. This is if you have the negative lead of the voltmeter on battery ground and the positive lead probing the terminal of the injector or coil connectors. That voltage comes from the fuse block.
You also need to make sure you have large gauge ground wires from the engine to the body, body to frame, and frame to engine. The body, frame, and engine all need to be grounded together.
You also need to make sure you have large gauge ground wires from the engine to the body, body to frame, and frame to engine. The body, frame, and engine all need to be grounded together.
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I add to my trouble shooting chronicle. I have 12V to the pink wire to the coils with IGN on, but when I crank, it drops to 9.7. This seems a likely problem, what could be causing it? I'll go through and check the big battery ground to block and the starter connections as well?
Last edited by Mr. Moeschberger; Feb 20, 2025 at 11:23 AM.
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Replaced positive battery cable, now I'm dropping to 11.4/11.5 on crank. Volt drop test on the negative side was .2, to the starter on the positive cable was .1, the starter solenoid seems to be .2, but I did see a surge of 1.4 when I turned the key OFF.
Any ideas?
Any ideas?
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An update to the many updates!
I believe I may have found the problem. After a lot of checking and poking and proding, I think that the Crank Position Sensor wires lose continuity when they get plugged in. So if I checked them connector to ECM with the connector unhooked, I had continuity, but when I plugged it in, it was losing it. I finally ran a jumper so that I could check continuity with it plugged in and there wasn't any.
But I got it hooked up and showing continuity and then it cranked and burped and ran on what felt like 1 or 2 cyl before dying, but it threw an injector circuit open code (which is new) and I found out that while checking the injectors I broke a wire there.
Feels like I'm a lot closer than I was and that I was chasing a voltage drop that, while a problem, was not the primary root of all evil here.
I believe I may have found the problem. After a lot of checking and poking and proding, I think that the Crank Position Sensor wires lose continuity when they get plugged in. So if I checked them connector to ECM with the connector unhooked, I had continuity, but when I plugged it in, it was losing it. I finally ran a jumper so that I could check continuity with it plugged in and there wasn't any.
But I got it hooked up and showing continuity and then it cranked and burped and ran on what felt like 1 or 2 cyl before dying, but it threw an injector circuit open code (which is new) and I found out that while checking the injectors I broke a wire there.
Feels like I'm a lot closer than I was and that I was chasing a voltage drop that, while a problem, was not the primary root of all evil here.