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Renix Computer Demon

C

CBJeeper

NAXJA Forum User
1990 Wagoneer auto, 4.0 Renix. I have been chasing a damned demon in my otherwise excellent condition Wagoneer. Randomly, the Jeep with stop running and will then crank but not start. Past episodes were remedied by disconnecting the battery and reconnecting, or disconnecting the TPS and reconnecting. It happened today and nothing would get it to start. When this happens, the fuel injectors are not firing. The rail has pressure. After 20 minutes today, it decided to start right up.

I have performed the fillowing:

New StandardMotor Products TPS
Cleaned, checked all harness connectors
Inspected sensor ground
Added additional ground cable from engine to firewall
Added new ground cable from battery to radiator shroud
Installed new 2 awg battery ground cable and cleaned all terminations

I am baffled. I have driven a couple hundred miles with no issues and then this happens out of the blue. Any thoughts on this?
 
On the mail route one day my 95 started shutting down, would just crank. But after 5 minutes it would fire back up and run for a while before doing it again. Replaced the cps, was fine ever since.
 
Yeah, CPS is pretty common. Injectors/plugs won't fire until ECU receives signal that engine rpm has reached 300-ish rpm.
 
Agree that it's CPS. Lots of extensions, a wobbler and an 11mm socket make it not TOO miserable.

Don't use a 7/16 socket, it'll seem like it fits ok, then round off the hard to replace bolts. I mean, that's what I heard anyway...
 
Yep, lot's of extensions and a wobble end. You can drop the rear of the tranny down (supported on a jack stand) to get a better angle. Be careful to NOT DROP one of the bolts down the gap.
 
It's not that bad. Back in the old days, I replaced mine with a high-altitude CPS from Jeep, doubt if they are still available. Lots of extensions, on my stick I could see and get my hands up there. I am now doomed and the POS is going to fail on me. LOL
 

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OK, I just finished replacing the CPS. It was actually not as difficult as I had imagined. My existing CPS was reading .18 to .2 volts AC. I purchased a NAPA Echlin p/n CSS980. I installed it and immediatley took a voltage measurement- .22 volts AC! Everything I read states that this is common and that I should take the CPS back and swap for another. This is a total PITA! Can anyone offer and insight on this?
 
Well, I had to get up and look at my printout for the Jeep Renix manual. Not a thing in there about the CPS. Doesn't seem like Jeep wanted their mechanics to know about the CPS. LOL I just replaced one when the beotch stopped for no reason just leaving the house and again the upgraded sensor. Made a difference at 6k feet.
 
Measuring the Renix crank sensor with a voltmeter is a rough guide and might indicate a weak sensor. The signal on the old crank sensors is not a nice sine wave, but rather spikes of voltage so the number you get depends highly on your meter. The spikes get larger and easier to detect with higher rpm, which is why idle and starting are the most problematic. With a generic meter, my oem cps shows about 0.25 volts. Using an o-scope, the peaks are closer to 1 volt.
 
lawsoncl said:
Measuring the Renix crank sensor with a voltmeter is a rough guide and might indicate a weak sensor. The signal on the old crank sensors is not a nice sine wave, but rather spikes of voltage so the number you get depends highly on your meter. The spikes get larger and easier to detect with higher rpm, which is why idle and starting are the most problematic. With a generic meter, my oem cps shows about 0.25 volts. Using an o-scope, the peaks are closer to 1 volt.
Click to expand...
I am using a Fluke meter. Are you suggesting that I live with this new CPS and let time determine if it no longer triggers the ECU to not fire the injectors and coil?
 
Can someone explain to me why there would exist a high-altitude version CPS? The CPS just creates a voltage pulse as it passes over a magnet(s) on the flywheel, correct? How does altitude affect this?
 
It slightly alters the timing (advanced I think) to adjust for the thinner atmosphere. Remember, there's no Mass Airflow Sensor.
 
I think a true rms meter like the fluke will show a lower voltage than a non-rms meter. The two failure modes I see for the cps are getting weak, which a voltage check would show, and becoming intermittent altogether when hot. I would let the new one ride and see what happens.

The high altitude version advances the timing slightly by offsetting the sensor on the mounting bracket, otherwise identical.
 
lawsoncl said:
I think a true rms meter like the fluke will show a lower voltage than a non-rms meter. The two failure modes I see for the cps are getting weak, which a voltage check would show, and becoming intermittent altogether when hot. I would let the new one ride and see what happens.

The high altitude version advances the timing slightly by offsetting the sensor on the mounting bracket, otherwise identical.
Click to expand...
Interesting. My non-starting, or shutting off episodes are typically when the engine is cold or not warmed up. It has happened once when warmed up. Thankfully, it has not yet happened in traffic or an intersection.
 
lawsoncl said:
I think a true rms meter like the fluke will show a lower voltage than a non-rms meter. The two failure modes I see for the cps are getting weak, which a voltage check would show, and becoming intermittent altogether when hot. I would let the new one ride and see what happens.

The high altitude version advances the timing slightly by offsetting the sensor on the mounting bracket, otherwise identical.
Click to expand...
One of Crusier's tips (CPS adjustment) essentially turns a stock CPS to a high altitude.
 
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