If your code reader shows you rpm while cranking what is it reading from the ecm? Most of the guys I saw on the internet that had a factory tach on the dash not reading correctly and a no-start it was the crank sensor.
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.
8.1/LS swap no start issues, help needed
- Thread starter Kay86K5
- Start date
The one thing that has me thinking it is not the crank sensor (after all my research) is the fact that I have fuel. I have the needed ~58 psi at the rail and confirmed the injectors are firing. From my understanding, if you have no fuel and no spark, then it's always crank sensor, but if you have fuel, but no spark, then it's not crank sensor...am I wrong on that?
On the grounds, get a couple sets of jumper cables and use those to go from battery to frame, frame to engine, engine to battery and body to one of those. Temporary free grounds and see what happens. I would be tempted to take a direct to battery lead and hook it up to the howell harness instead of through the truck and see what happens. I'm of little help really and I know you've at least talked to Alex which is where I would turn, he's on vacation right now but will be back in a week or a little more.
Yeah, I text Alex a day or two ago but hadn't heard back (that makes sense that he's on vacation). I was going to see if he would take some time to come over and take a look at it with me. We have been talking quite a bit during this 8.1 swap I've been doing (he's a very smart individual!). I'll try to catch him when he gets back from vaca.
In the meantime, I may try some of the things you mentioned along with messing with the grounds some more and checking readings from the common sensors again.
One of these days this stupid thing is going to start up, it's going to be a really fun truck to drive with the 8.1/NV4500 combo in it (pic for reference of those who haven't seen it yet)
Not real sure I thought I saw that the cam sensor controls the injector firing and the crank sensor controls spark but not sure also.
I actually think it's opposite of that. Cause the cam sensor is only sending a signal to the ECM of TDC so the ECM knows which coil pack to send the fire signal to in the correct firing order (there may be another function of the cam sensor, but this is it's main function), but I believe the crank sensor is more connected with fuel. Hence why on the 8.1's when the infamous crank sensor goes out, it just cuts everything out...no spark, no fuel, no nothing.....at least this is my understanding of it all, I could be wrong.....I've been doing so much research about all this lately my head is starting to hurt
a quality scanner sure would help.
To confirm, you have not checked for a cam/crank signal at the ECM right?
I mentioned the grounds before in your build thread and I think it's worth a mention again.
To confirm, you have not checked for a cam/crank signal at the ECM right?
I mentioned the grounds before in your build thread and I think it's worth a mention again.
I just checked to make sure each sensor was getting power/ground, but no I did not check the signal at the ECM yet. Both that and the grounds will be double checked this weekend.
And yeah, quality scanner would probably fix my problem quicker....but I'm cheap
or even if someone would rent a damn scanner it would be nice.
In to hear what it is. Doesn't sound like a bad ground, or VATS. My experience with VATS is they will fire and run but then cut fuel.
No firing and voltage drop says sensor or PCM to me.
No firing and voltage drop says sensor or PCM to me.
You could use to know if codes are being set. Seeing how you a using the same setup as Larry, you should be able to use normal GM diagnostics to aid in finding the problem. That is, while supplementing the GM wiring schematic for Howells. I'll shoot you a copy of the diagnostic chart in a PM. I tried copying it here but the formatting gets lost and it's impossible to follow.
Still a scantool is invaluable to prove some things out. But the chart is probably a better starting point than you have had so far.
Still a scantool is invaluable to prove some things out. But the chart is probably a better starting point than you have had so far.
That would be much appreciated...thanks!!
- Joined
- Jul 18, 2000
- Posts
- 1,695
- Reaction score
- 2,115
Did you ever get this thing sorted out??
You know, I was thinking of your engine issue as I was thinking if Zoomad75’s K5.3L swap last night while helping another guy with a 8.1L swap. Zoomad is actually using my old MEFI-4 harness made for an 8.1L to run his 5.3L. To make the 8.1L harness work on his LM7 5.3L I had to swap the wires around at the CAM and Crank sensors because the 8.1L cam and crank sensors are wired different from the LS engines.
You mention you have a crank but no start but have solid fuel pressure (62 psi), I believe you confirmed you have the correct power wires going to the ECM (KO VS. Bat Pwr) and you feel it is something related to the crank sensor. With that, it would be well worth checking the wiring at these to sensors. Even though you have a quality Howell harness, they are humans too. They build a ton more LS harnesses than 8.1L harness so they could easily of accidentally wired the CMP and CKP backwards to the usual LS fashion. You may want to open those connectors and verify they have the 12 volt reference voltage going to the correct pin at the sensors themselves. See the diagram below…. If Howell did pin the sensors wrong that is an easy fix to push the terminals out of the connectors to swap them around. It would be nice it this was your hot ticket. Anxious to hear what you find.
Did you ever get this thing sorted out??
You know, I was thinking of your engine issue as I was thinking if Zoomad75’s K5.3L swap last night while helping another guy with a 8.1L swap. Zoomad is actually using my old MEFI-4 harness made for an 8.1L to run his 5.3L. To make the 8.1L harness work on his LM7 5.3L I had to swap the wires around at the CAM and Crank sensors because the 8.1L cam and crank sensors are wired different from the LS engines.
You mention you have a crank but no start but have solid fuel pressure (62 psi), I believe you confirmed you have the correct power wires going to the ECM (KO VS. Bat Pwr) and you feel it is something related to the crank sensor. With that, it would be well worth checking the wiring at these to sensors. Even though you have a quality Howell harness, they are humans too. They build a ton more LS harnesses than 8.1L harness so they could easily of accidentally wired the CMP and CKP backwards to the usual LS fashion. You may want to open those connectors and verify they have the 12 volt reference voltage going to the correct pin at the sensors themselves. See the diagram below…. If Howell did pin the sensors wrong that is an easy fix to push the terminals out of the connectors to swap them around. It would be nice it this was your hot ticket. Anxious to hear what you find.
No, I haven't figured it out yet. I was on the phone with Howell yesterday talking through my issues. They suggested I temporarily go straight from the battery to the switched 12v wire to see if I get it to start up. They said if that worked that I could just go from battery to a relay, then to the 12v switched wire and call it good. I may try that as what we have found is that 12v switched wire is dropping voltage to below 9v while cranking which is what we believe is the reason why it won't start. I am going to try that this weekend and see what happens.
Thanks for the diagram...I will double check the connectors at the sensors. I do believe a while ago we did check to make sure the crank sensor did have power/ground on it and from my memory it was good, but that was about a month ago and I think its worth another look at it. And we checked all 8 coil connectors and the correct wires are going to the correct ports at the ECM and their is good power going to all of them....the ECM is just not sending the fire signal to each coil pack....which makes us think the overall issue is the voltage drop during cranking on the 12v switched wire.
Alex said he would come over and help after he gets back from Blazer Bash this weekend. So hopefully by next weekend we can get everything figured out and get all the wiring buttoned up on this thing. I think I'm at the point where I just need another brain or two with me to bounce ideas off of, cause I'm starting to get at a loss a little bit
The voltage drop is why I would look seriously at the ignition switch. If it isn't a known good part, how many years and key cycles old is it? Remember that it has contacts sliding inside it while moving to all of the different positions. If that is the problem, just would be easier to replace than wiring a relay. Just my 2 pennies..
Team208Motorsports
Its Magic Bitch
Hook the 12v sw wire to constant and see what happens. If it fires you know you are losing 12v sw while cranking, common problem.
If its a no go then look for tach numbers rising on a scanner while cranking. If there are tach numbers you know the crank sensor is good. These engines will not fire without a crank signal. They will not fire injectors or coils. Cam signal is used for phasing on gen 3 and will fire without it.
Beyond that its a Howell problem................
VATS is a shutdown system not a no start.
If its a no go then look for tach numbers rising on a scanner while cranking. If there are tach numbers you know the crank sensor is good. These engines will not fire without a crank signal. They will not fire injectors or coils. Cam signal is used for phasing on gen 3 and will fire without it.
Beyond that its a Howell problem................
VATS is a shutdown system not a no start.
The ingnition switch on the column is factory as far as I'm concerned. Not known if it's been switched out before or not. I guess I could change that, but for some reason my gut is telling me this is not the issue
I have my aftermarket tach hooked up right now and spliced into the wire they labeled/supplied with their harness (don't have access to a good scanner...just a cheaper one/software on my laptop but does give me some sort of real time info such as rpm's and stuff like that, just not really good real time). When I crank right now, the neddle on the tach and the computer software starts bouncing up and down a little bit, so I'm pretty sure I'm getting a tach signal based off that. That's why I don't think it's my crank sensor either (as I replaced it before stabbing the motor in the truck because of the known crank sensor problems).
Your suggestion of hook the 12v switched wire to a constant 12v power is exactly what Howell suggested too. I'm going to try that this weekend and see if it starts up. If it does, they said to run the 12v switched wire through a relay and then just to the battery and that should be good long term. I'm hoping this gets it fired up.
Team208Motorsports
Its Magic Bitch
What are you using for your IGN source? What circuit?
We spliced the 12v switched wire into the wire that used to go into the TBI coil on the old TBI 350 that was in the truck before (this is what the instructions from Howell said to do)
Team208Motorsports
Its Magic Bitch
If you are dropping that much voltage on the old coil circuit the tbi engine never would have ran. Do the jumper see whats up. What does the voltage drop to on the old coil wire while cranking?
