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1989 Crew Cab Tow Rig Build

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Ok, question for any of you that watch this thread. So this weekend I confirmed that my fuel pressure is dead on and my injectors are firing. I also confirmed that there is power getting to every single coil pack and the ground is good there, however, I still am not getting any spark at the plugs. When testing the wires at each coil pack, there is the 1 wire that is supposed to be the "fire" signal from the PCM, and I have nothing there. So essentially what I think is going on is the PCM doesn't recognize the motor is turning over so it's not sending the fire signal to the coil packs at all. I know these 8.1's, especially in the early years, were notorious for eating up crank sensors, that's why I changed mine out with a brand new one before installing the motor. Also, I replaced the TPS with a new one and cleared the code that was being thrown for that last week, now no codes are being thrown at all.

I have a buddy who has EFI live who said in the next couple weeks will be willing to swing by and hook it up to see what we can find, but has anyone had an issue like this before? This harness and tune is literally identical to @Larry with his K-10 and Polar Bear burban which has proven to work really well, and I'm having a hard time believing my brand new crank sensor is junk (even though sometimes we all know that can happen). I'm going to call Howell today and pick their brain as well, but just trying to get as much info as I can to try and get this thing fired up.

It's so freaking close it's not even funny. Literally everything is hooked up and working except the PCM is not sending the fire signal to the coils...that's it. Any help/info would be appreciated.

Thanks! :waytogo:
 
Do you still have the old sensor you took out?

Was wondering why we didn't have vid of the first firing!
 
No, I don't have the original sensor. When I pulled it out it seemed like it was the original sensor and was a bugger to get out, so the clip portion of the actual plug got broken getting the plug out, so it was tossed in the trash
 
There should be a signal coming from that sensor, wonder if it could be tested by some probes while cranking. Or if there's some other test procedure for it.
 
I don't know 8.1L engines but if it is like any other crank sensor you should be able to test its output with a digital mult meter. A little google searching should come up with the test procedure.
 
JoshHefnerX said:
There should be a signal coming from that sensor, wonder if it could be tested by some probes while cranking. Or if there's some other test procedure for it.
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Truckman4life said:
I don't know 8.1L engines but if it is like any other crank sensor you should be able to test its output with a digital mult meter. A little google searching should come up with the test procedure.
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I did test the plug for the crank sensor and saw it had good power/ground to it, but did not test while cranking. I guess I could try that or just wait until my buddy comes over with EFI live (then you can see it all on that software anyways). I still have plenty to do like install the lift. So maybe I'll start working on that until he has some time to come over...we shall see
 
I really doubt you have a crank sensor issue plus you are not going to want to replace that sensor as the engine will need to be lifted out and away from the firewall in order to remove it. Yeah, that is the bad thing about installing a 8.1L into an older vehicle….servicing the crank sensor requires the engine to be moved forward a few inches (believe me, crank sensors are not easy to change in GMT800 trucks either) as it sits too far under the firewall to pull the 8" long sensor out.

Here is what I would do….

1) Check the simple stuff like the fuses in the Howell fuse block. You've probably already done that.

2) There are two main power wires for the Howell harness. One is key on and the other his hot all the time. Are you sure the key on power is actually hot when the ignition is in the crank position? I suspect you have an issue with the ignition feed more than anything as that will prevent the coils from firing. The key on power to the ECM should be the chassis wire that used to go to the coil on the old engine.

3) If you suspect the circuit from the coils to the ECM then open the harness and tracing the circuit. I found the harness I used on the Suburban had many less than sterling splices in it. I even had to redo two of them.
 
Larry said:
I really doubt you have a crank sensor issue plus you are not going to want to replace that sensor as the engine will need to be lifted out and away from the firewall in order to remove it. Yeah, that is the bad thing about installing a 8.1L into an older vehicle….servicing the crank sensor requires the engine to be moved forward a few inches (believe me, crank sensors are not easy to change in GMT800 trucks either) as it sits too far under the firewall to pull the 8" long sensor out.

Here is what I would do….

1) Check the simple stuff like the fuses in the Howell fuse block. You've probably already done that.

2) There are two main power wires for the Howell harness. One is key on and the other his hot all the time. Are you sure the key on power is actually hot when the ignition is in the crank position? I suspect you have an issue with the ignition feed more than anything as that will prevent the coils from firing. The key on power to the ECM should be the chassis wire that used to go to the coil on the old engine.

3) If you suspect the circuit from the coils to the ECM then open the harness and tracing the circuit. I found the harness I used on the Suburban had many less than sterling splices in it. I even had to redo two of them.
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Yeah, I already checked the fuses and they are all fine still. And I did splice the key on (ignition) wire to the wire that went to the coil on the old engine. Confirmed there is power when key on/cranking, but it can't hurt to double check this connection to make sure it is really good. I will double check the harness for the coils and make sure everything is fine there on the pin-outs too.

I was talking with Alex last night and we agreed as well that it can't be crank sensor sense I have fuel right now and the injectors are firing. When we looked at a GM diagnostic it said if you have no spark and no fuel, then it's probably crank sensor. But if you have fuel but no spark, next best place to look is cam sensor sense the cam sensor is sending info to the ECM about TDC so the ECM knows what coil to fire at what time. So I figure it can't hurt to change that sensor sense it's cheap and easy to get to on the timing cover.
 
Gotcha. The Cam input could be a culprit but before replacing anything open the connector in the Howell harness to make sure a wire hasn’t pulled out of the terminal or was poorly crimped. I really doubt you have a hard part problem but something on the wiring end. Just because the harness is new doesn’t mean it may not have an issue. I had zero issues with the Howell harness on the K10 but the one for the Burb had several issues right out of the box.

Also make sure wire trunk leading to the CAM sensor didn’t rub on a pulley or get smashed behind a bracket. That happened to the CAM sensor lead on my K10 when I was running the MEFI-4 harness. Funny thing is I drove it like that for two years with no cam sensor input to the ECM but, that is a simple speed density ECM like used on marine and RamJet engines. I guess that particular ECM will let it run without a cam feed :haha:
 
Larry said:
Gotcha. The Cam input could be a culprit but before replacing anything open the connector in the Howell harness to make sure a wire hasn’t pulled out of the terminal or was poorly crimped. I really doubt you have a hard part problem but something on the wiring end. Just because the harness is new doesn’t mean it may not have an issue. I had zero issues with the Howell harness on the K10 but the one for the Burb had several issues right out of the box.

Also make sure wire trunk leading to the CAM sensor didn’t rub on a pulley or get smashed behind a bracket. That happened to the CAM sensor lead on my K10 when I was running the MEFI-4 harness. Funny thing is I drove it like that for two years with no cam sensor input to the ECM but, that is a simple speed density ECM like used on marine and RamJet engines. I guess that particular ECM will let it run without a cam feed :haha:
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Good info. I will double check all of this. Thanks for the input Larry, it's much appreciated. It's so close it's not even funny. Just trying to track down this one little thing. I'm sure once I figure it out (probably something small and dumb) the freaking thing will fire right up :). I'll get there though, just one step at a time.

I'll try and double check all of this during the weekend to see what I can find out.

Thanks again!
 
A good way to check your ecm power and ground circuits instead of just a test light/volt meter/ohming them out is to load test them. It's really easy I've found a ton of wiring issues over the years this way. Make a simple test harness using a sealed beam headlight and pigtail from the parts store, one of the high beam only ones. Run one lead with an alligator clip to ground terminal on your battery for checking a power circuit and then the other lead to the circuit you want to test. If the light comes on your circuit is good. It puts a fair amount of amperage load on the circuit being tested(can't remember off the top of my head). You can also wiggle the wires while it's on to find a bad connection. If you are testing a ground circuit hook up the alligator clip to battery positive and then the other lead to your ground circuit that you want to test. I've found a lot of power/ground circuits that will test good on a voltmeter and light up a test light but then when the load of the ecm or whatever comes on the circuit goes open.
 
So, I think @Larry is on to something with the cranking power. We double checked what volts were being put out while cranking, and it drops from 12 volts like it should be to anywhere between 8.5 to 9 volts. From everyone else's experience, is this too much of a drop? The battery is the same one that was in it when I purchased the truck. My dad has a battery tester and everything seems to be fine with the battery. My buddy was thinking maybe the actual ignition switch in the column may be junk and worth changing out. I did replace the cam sensor too and nothing changed. Still just cranking and cranking with no fire...
 
I know on the powerstrokes and internationals I'm used to anything below 10.0 volts and the engine won't run. I'd bet it's the same for your 8.1L . Try a new battery or hook up another truck to it while cranking.
 
Truckman4life said:
I know on the powerstrokes and internationals I'm used to anything below 10.0 volts and the engine won't run. I'd bet it's the same for your 8.1L . Try a new battery or hook up another truck to it while cranking.
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X3 in the power while cranking, you can crank a powerstroke for days on 9 volts and it would never fire. Has to be above 10-11V for it to fire as @Truckman4life said. Hopefully it's something simple for you. Excessive amp draw from the starter maybe?
 
X4 on the volts. The LS swapped 3/4 ton s10 blazer me and a friend just drove yesterday wouldn't even think about starting below 10 volts. It would kick the fuel pump on, and the pink wires running to the coils would be hot, it would spray fuel like yours, but when you went to cranking on it it would not spark right. I swore the LS1 cam we used maybe didn't have the correct reluctor wheel on it for the cam sensor. Ended up being the 14ga wire we ran from the battery to the starter was obviously too small, not allowing voltage to move efficiently and causing all sorts of problems.
 
454Sub said:
X4 on the volts. The LS swapped 3/4 ton s10 blazer me and a friend just drove yesterday wouldn't even think about starting below 10 volts. It would kick the fuel pump on, and the pink wires running to the coils would be hot, it would spray fuel like yours, but when you went to cranking on it it would not spark right. I swore the LS1 cam we used maybe didn't have the correct reluctor wheel on it for the cam sensor. Ended up being the 14ga wire we ran from the battery to the starter was obviously too small, not allowing voltage to move efficiently and causing all sorts of problems.
Click to expand...

That sounds very, very similar to what is going on in my case. Fuel kicks on and injectors fire, it shows there is power on the ignition wire during cranking, but once we put the reader on it, it was dropping pretty low during cranking. I may change up the size wires for both 12v power and switched ignition just to make sure they are a large diameter to handle the power, and I'll probably just spend the money right now to get a yellow top (something I wanted to do on this rig anyways).

Thanks for all the replies guys, gives me somewhere else to start from.
 
I meant to say battery to alternator. See what the charging wire is putting to the battery when it's cranking.
 
I am going to have to look for some of those plug wire sleeves or the ceramic boots. Once a year I find my boots are cracked and dry and hard and you can see the plug through them. I have a 350 TBI motor but have the old ramshorns exhaust manifolds on and use straight boots at the plugs.
 
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