Cps no spark slow crank

[mbred0609]

Alrighty here it goes. I figured out the no spark without asking any questions on the first renix, but #2 is giving me some issues.

 

First off, I swapped the known working components from the 4.0 I just got running. Cps, coil, cap/plug wires, icm. Took me about 3 or 4 days of research and swapping parts around until I found a working combo. Once it was running I felt like I might have a handle (albeit a small one) on what to look for. So I swapped all these parts to another manche I want to check the motor on.

 

Now I'm getting .2v on the cps I was getting .54v on the other vehicle and back to no spark. I tried 2 other cps I have laying around to verify. 1 being a stock I believe as it says renix on it and 1 being a nib ntk. The renix comes out as .1 and the ntk shows .2. I'm aware its a crap shoot with these even new and even from reputable brands. But the fact I know the last one I tried worked as of yesterday seems odd to me.

 

I'm also aware they need to see 300 rpms. It does have a really weak crank imo. I'm guessing starter, but in fairness I have no idea how long this jeep sat before it got a battery today. 

 

So I guess my first question is will a slow crank drop the voltage coming out of the cps or since I'm showing voltage does that meaning I'm cranking fast enough.

Secondly any idea why the known good cps shows such a difference between 2 vehicles.

 

I am a renix noob, so please bear with me. I am a pretty quick study and a half decent at finding the information i need imo.

 

I know this is long but I figure i should mention. Cruiser tips 1-5. 

1,3,4,5 complete

2 is on the short list and now that I'm typing this I'm thinking i should at very least tackle the cps wires... 

 

[DirtyComanche]

Slow crank reduces the voltage.  There is effectively a minimum cranking speed to make a suitable signal for the engine to run.  You need to address your slow crank before doing other things.  If you have a jump pack or an extra battery try giving it some beans with that.

[mbred0609]

2 hours ago, DirtyComanche said:

Slow crank reduces the voltage.  There is effectively a minimum cranking speed to make a suitable signal for the engine to run.  You need to address your slow crank before doing other things.  If you have a jump pack or an extra battery try giving it some beans with that.

That is what I was thinking might be the problem. I appreciate you confirming my suspicion. I tried to get better voltage with a jump pack and also with a charger on start mode. Barely made a difference. I also made new cables from some 1/0 and 4g i had laying around to no avail. I plan to swap out for a better starter and go from there. But today was a junkyard day so I'll keep you posted. 

Thanks again.

[mbred0609]

I replaced the starter with a better-ish starter and am now getting spark. Still no start but one step closer. 

[pizzaman09]

Getting spark is a big step forward, congratulations!

[DirtyComanche]

If you're now getting spark you can try misting some gas into the throttle body and see if it will fire off.  You may get it to go that way.  Otherwise it's move on to checking timing, fuel pressure, that the injectors are firing, etc.

[mbred0609]

On 3/29/2026 at 3:53 PM, DirtyComanche said:

If you're now getting spark you can try misting some gas into the throttle body and see if it will fire off.  You may get it to go that way.  Otherwise it's move on to checking timing, fuel pressure, that the injectors are firing, etc.

Yeah even after finally getting spark, it just didn't sound quite right when turning it over, and kept backfiring through the throttle body. Pulled the head and have multiple seized valves. I pulled that motor and put it off to the side for now. It will eventually get rebuilt and put back into this jeep.

Hopefully Sunday I'll be swapping the parts jeep motor into this one for now. We'll see how the weekend goes, but I'm highly considering running new brake lines before the swap. Everything is pulled and ready with the exception of the parts jeep motor, and even that its only 4 more bolts. I spent all week replacing seals on the backhoe (cherry picker) so fingers crossed my weekend doesn't go to sh*t.