LB7 cranks for a couple seconds then stops

[toadgarten]

I have a 2003 (LB7) with 230,000 miles. I just tried starting the truck early this morning without success after starting it many times two days ago. The truck would engage the starter but after a few seconds, the starter would stop. I had done this repeatedly several times with the same result.


I scanned the prior posts, but could only find a hint that it could be a crankshaft position sensor? Any thoughts?

[DmaxMaverick]

Welcome aboard!

It could be the crank sensor. If it is, there will be a stored code for too high/low or missing reference signal. You'll have to scan for the codes. If the key is on and the engine is not running, the SES lamp is useless, because it will always be on "normally".

Before going through all that mess, try swapping the starter relay with another of the same (exact matching numbers -different numbers are not compatible). They are in the underhood fuse panel, and labeled on the cover. Relays can fail and act just like you describe. They're a lot less expensive and easier to replace than the crank sensor, as well as being free to test.

[toadgarten]

I heard great reviews and thank you for the quick reply.


I swapped the starter relay out with the "parking lamps relay" (same number as starter relay) with the same results. Cranks for a couple seconds and stops.


Any thoughts on purchasing a code reader? Would this be the next logical step?

[DmaxMaverick]

The first thing you'll want to do is check ALL the electrical connectors. Every sensor and every gang. Check for any evidence of rodents, as well. Verify you have good battery voltage that stays with you during the cranking episode. If it drops below about 9.5V at any time during a start, the PCM may kill the process.

You don't need to go to fancy with a code reader (as in, cheap). 2003 is pre-CANbus, so anything compatible through 2005 will work. You can use the google for decoding, if you get one that only spits out the alpha-num codes.

Non-running codes can be unreliable, meaning, codes that are stored that were generated before a complete warming cycle happens. One thing you can try is, get it started and let the PCM run all the diag's. I understand it isn't cranking long enough to start it, so you'll have to work around that. (what I would try) With the key on, you can (old school) jumper the starter solenoid. That's large pole connected to small pole at the solenoid, which will cause the starter to crank (in case your experience doesn't go back that far). Harbor Freight and any auto parts store should have a special tool for doing just that. It's simply a pair of alligator clips wired to a hand-held momentary button/switch, usually long enough to reach across the engine bay and into the cab. Any time the button is pressed, the starter will crank, as long as the batteries are connected, regardless of any other condition.

If it doesn't start at all, even with extended crank time, the PCM should be telling you what's missing, in any case.

[toadgarten]

I looked at all wiring and seemed to be normal along with battery voltage during crank. I installed the remote starter switch. Plugged in the code reader. Turned the ignition to running position. Let is crank for about 20 seconds two times, but it would not start or even seem like it was going to start. Code reader had one fault - P0335 "Crankshaft Position Sensor A Circuit." I see there could be multiple issues making this happen, even bad wiring. Is there anyway to diagnose this further? What would be the next step?

[DmaxMaverick]

That's fairly conclusive, so not much left to do but fix it. I'd check all the wires and connectors first. Make sure a pin didn't get pushed in, or a rodent didn't have a snack, or some things along those lines. You could get the service manual and follow the flow chart for testing, or just do what the dealer would do: replace the sensor and cross your fingers.

[Kennedy]

Definitely sounds like the crank position sensor. They wil just stop cranking like you describe if signal is incorrect. Dig back in old bulletins I seem to recall there was a deal with the reluctor screws coming lose and backing out to where it lost signal. Could also be wiring.

Worth noting: We had an LB7 engine built with a LBZ core that had this issue. The LBZ reluctor has a different count and the ECM saw this and acted the same.

[toadgarten]

I appreciate all the information.

Is there a minimal way to verify the reluctor screws condition? Through the sensor port?

I have never have changed the starter. Is it worth changing this out when changing the sensor? When should I expect a starter to go bad?

[DmaxMaverick]

Starter problems are few and far between. If you have no issues with your current starter, you're just as likely to replace it with one that won't last as long. Mine is original, 20 years old and still cranks like new. They aren't hard to replace, if you have to in any case, so no real gain bundling the job.

To check the reluctor bolts, use a small screwdriver or similar to check for movement of the wheel through the sensor hole. If it isn't solid, it's suspect, and likely needs replacement. That's a whole 'nother can o' worms, so cross that bridge IF you come to it.

[toadgarten]

Checked reluctor for movement - detected none. Replaced crankshaft position sensor - same result. Starter cranks for a couple seconds and stops. Hooked up remote to starter again and came up with the same diagnostic code "Crankshaft Position Sensor A Circuit."

Is there a way to bench test the crankshaft position sensor? What would the next logical step be? My fingers were crossed but to no avail!

[DmaxMaverick]

Next logical step: Back to basics: Chase wires. Communication is broken, somewhere. Trace back ANY repair or maintenance that may have been performed within a generous window of the failure. If nothing comes to mind, start at the sensor and work your way to the PCM. An accurate diagram and connector pin-out may be helpful, if not necessary. Start HERE (https://www.alldatadiy.com).

[toadgarten]

Back to the basics: I disconnected the CKP sensor harness connector. I turned the ignition on, measured the 12v reference at 11.67v., measured the low reference value at 0.045v and the CKP sensor signal at 0.165v (slightly lower than the suggested at least 1v). I detached the ECM connector to run checks on all three wires. The 12v and low reference checked out fine for continuity. The CKP sensor seemed to have an open. I run the CKP sensor wire down to the Edge Juice (other posted strings suggest a bad chip will throw a code of P0335). How would I isolate this to see if this is the problem? If not, the online troubleshooting flowchart is suggesting to replace the ECM. Next steps?

[DmaxMaverick]

OK. That's a valuable little piece of information: the Juice. Disconnect it. Reconnect your original harness. Start your truck.

[toadgarten]

I bought the truck with the Juice installed. I do not have the original harness. Where would I find a harness?

[DmaxMaverick]

The module installs inline in the main engine harness (the big one, about 2" x 3"). Disconnect BOTH Batt+ terminals. Ground one of the Batt+ cables to chassis while you do this, and leave it grounded for about 30 minutes (clears electronics residual power). Remove the Edge adapter and connect the original connectors together. It is located above the driver side valve cover. Be careful and align the connectors, as the pins can be pushed out of position on occasion. It should slide together very easily. Remove Batt+ from ground, reconnect the batteries.

[toadgarten]

Excellent! Problem solved. Thank you for all the step by step help. I learned a few things along the way. Now is the Juice worth installing back on after repairs? I load firewood, dirt, rocks, slide in a truck camper or occasionally pull a utility trailer. I have intentions of running this truck as long as I can. What would keep the truck running the longest to include all other options? Thanks again.

[DmaxMaverick]

If you aren't trying to win a race, stock power is fine for most people.

If you'd like to restore the Edge module, I suggest contacting Edge and let them know what happened. This isn't the first (probably not the last, either), and they'd likely have the best answers for you.

[Kennedy]

When they worked they worked well but with limited capabilities. I have a handful of them here yet that I used back in the day but that module has been pretty much obsolete for 15+ years now. One of the key issues was intermittent stalling particularly with the wipers on if you believe that. Basically if the Juice saw an erratic crank position signal (or electrical interference) it shut down to protect the engine. To correct this they took the timing adjustment out of it.

Adjusting parameters directly in the ECM is far superior provided you select the right tuner to do the work.