Renix CPS Testing and Adjusting

CPS drill mod

CPS location

Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS itself as shown in Figure 2 as Connector A, crank the engine over. It won’t start with the CPS disconnected.

You should get a reading of .5 AC volts.

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from NAPA or the dealer.

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

A little trick for increasing the output of your CPS is to drill out the upper mounting hole to 3/8″ from the stock 5/16″, or slot it so the CPS bracket rests on the bell housing when pushed down. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

Another little tip to save tons of aggravation is to stick a bit of electrical tape to your 11mm socket and then shove the bolt in after it. This reduces the chances of dropping that special bolt into the bell housing. CPS bolt socket

Revised 8/8/2016

54 thoughts on “Renix CPS Testing and Adjusting”

  1. Hey Guy, thanx for your tips!
    Question: Do you think running a ground wire from the upper mounting hole to the engine or body would improve the signal from this sensor?

    1. maybe put it in line with the stator/distributor ground to improve the overall sync? All this assuming that those little wings on the CPS are designed to ground..

    2. No. That sensor generates an AC voltage signal on it’s own. Tip 7 will help with generating a stronger signal.

      Which Tips have you completed by the way?

  2. My cps voltage fluctuates between 3.85 and 4.10 even in a no start situation. Change or not change?

  3. Will my cheapo Harbor Freight meter be able to do this ac volts test? The AC VOLTS section of the dial only shows 750 and 200 positions.

    1. Yes, that’s a safety setting internal to the meter. 750VAC will lose the resolution that 200VAC offers. It will work, but a cheap unit might not be very accurate. Is it an analog or digital meter?

      1. Its digital. Cen-Tech #90899 So put it on 200 ACV position and try that? Ok thanks alot, I will let you know how it works.

  4. Cruiser, thanks for all the help.

    I measured my CPS and was only getting .3 AC So I replaced it from autozone and got .2, I bought a new one from NAPA and I am still only getting .2 . What should I do next? fuses? I’ve checked battery, alt, starter.

    – new plugs, cables, ignition coil, oil pressure sender, map sensor.

    -just did all that rebuild work that we talked about in the other post and now getting crank but no start condition. 89 XJ Laredo 4.0 auto.

    -only thing is transmission has a small leak..

    Thanks

  5. Yes, if its getting spark the CPS is good.
    Check fuel pressure and IAC motor, AIT, CTS and O2 sensor

  6. OK, so I am picking up 1.2 amps from the CPS. My Comanche is currently in a no spark condition. Is it possible for there to be too much voltage from the CPS? Also any other suggestions on troubleshooting the no spark condition? I am working my way through the tips !

    Thanks!

    1. You should be measuring AC voltage while cranking. I’ve seen 1 AC volt with no issues.

      If you have an 87 or 88, keep in mind the signal must travel through the C101 connector and that can reduce it significantly before it gets to teh ECU. Tip 2 and 27…….

      1. Yeah, come on out and visit !

        So I am still in the crank all day, no start condition. I have recently eliminated the C101 connector, cleaned up the ignition module and replaced the coil. I guess next is the ECU harness, and then maybe I start tracing back from the Spark plug wires for spark?
        Any other suggestions would be appreciated.

  7. OK, I will try testing it again. It seemed pretty strait forward to me, is there something people commonly miss when testing??

    Thanks

    1. OK, will do. I recently tried replacing the coil, my distributor cap and rotor are fairly new. I did bend up the button and shined everything up with some sandpaper and still nothing. I plan to hook up a spark tester to the coil next to see if I am getting anything from it.

      I’m not sure how related this could be, I recently replaced my fuel pump, new fuel pump relay, etc. I am no longer hearing the signature groan when the key is in the on position. I have bypassed the ballast resister, refreshed tail light ground and added a ground as you recommend. I DO hear a slight sound from the pump at the very end of cranking, but that’s it. I plan to borrow a fuel pressure gauge, I guess I will be looking for pressure during cranking as well as just key in the on position. So two questions … Should the pump be active just with the key on?
      I am wondering if somehow my lack of spark could be related to some issue with my fuel pump/pressure?

        1. Glad to see you’ve done all those Tips.
          Definitely check the fuel pressure. No spark will not cause no fuel pressure.
          I have personally seen numerous fuel pump replacement failures if Bosch pumps were not used.

          1. OK, thanks. The brand of the new pump is air-tex. I was looking for a bosch, but also needed the sending unit. I guess if I have to crack back into that bad boy I will just swap the pump itself out for a bosch…

          2. Alright, here’s an update we are back up and running!!

            So, after replacing the fuel pump I was getting no spark and no fuel pump groan. I jumped the relay plug with the key on to find that the pump did in fact work. So the fact that the relay was not receiving a signal for the fuel pump, as well as the ICM not receiving signal for spark lead me to ordering an ECU from JC Whitney. Plugged it in and fired right up!

            Thanks again for your advice and of course the tips!

          3. Also, for the record, I went with a 1990 ECU in my 88 comanche. I am feeling some extra get up and go for sure, but then again I did eliminate the C101 so who knows maybe both are to thank!

  8. I have a 1990 Comanche. The cps is located in the cutout on the firewall and is hard to get to. I got some different tools (shorter) and it looks as if I’ll be able to get to ths cps now. I hope.

  9. My 87 XJ was knocking real bad. Flex plate replaced along with crank sensor while it was apart. Ran great before, now it will idle but not rev up. Loud rapid backfiring from exhaust and no power in mid to upper rpm, due to running awful. Ordered 2 flex plates from one store and they were different than what came out of it. I ordered 3 more from another store to see if manufactured wrong, they all had the holes stamped too far away from the ring gear. Ordered another one from another mfg. after talking with technician, and measuring the distance from the ring gear to the openings in flex plate. This one matched. Needless to say we compared the old one to this one closely. The mechanic put the new flexplate in and said it runs good…wrong. Still sounds like a machine gun when you accelerate enough to make it downshift. I read all the comments about bad sensors so I get a NAPA sensor, same problem. Remember, this Jeep ran great until I had the flexplate fixed. We tested voltage and were all over the place with .1 to .35 and sometimes to .6 We drilled out the holes to 3/8″ and got .3 to .6. Took it out and removed 1/16″ off the bottom of the sensor bar, now we are getting a steady 2.0 volts. One sensor would allow motor to idle but it would not rev up at all. So I now have the sensor that reads 2.0 volts in it. WHAT HAPPENED? Still backfires like a machine gun.

      1. Mr. Cruiser54,

        Thank you for maintaining this website!

        I have this same problem with a cracked flexplate replacement. My 1988 Jeep Cherokee Laredo 4.0L ran great since 2003 until I took it upon myself to replace the cracked flexplate. The flexplate had cracked recently due to a transmission bolt backing out. A previous owner had replaced the engine and the E12 bolts were not even in it. Please note that other than the engine replacement many years ago, there have been no other modifications made to this Jeep.

        Anyway, in my backyard, I replaced the flexplate with an Omixada flexplate. That flexplate was defective because the 120 degree shutters were out of place by about 2 inches. I didn’t realize the importance of that fact at the time. As a result, the engine would start and idle fine enough, but once the accelerator was pressed, there was lots of backfiring but the engine rpms were robust.

        The second flexplate from ATP Z-205 was analyzed more closely, and it seemed to be a winner. Also, according to many posts, I found that the flexplate should only bolt onto the crankshaft one way. However, this one starts after coaxing with the accelerator and will idle at about 200 rpm. Very disappointing. After testing the fuel pressure which is good, I tested each cylinder’s spark with a spark tester. This is what I found:

        Cylinder 1 = No spark
        Cylinder 2 = Good Spark
        Cylinder 3 = Medium Spark
        Cylinder 4 = Medium Spark
        Cylinder 5 = Good Spark
        Cylinder 6 = No Spark

        I laid a spark plug on the block coming straight from the ignition coil. It sparks consistently and good spark.

        Since it will only fire well on two cylinders, it barely moves under its own power.

        At this point, in your opinion, do you think I need to get a used flex plate, or do you feel that there is some type of new electrical problem that I need to investigate. After the second flexplate, I bought and installed a new CPS and modified it like your page says to get it closer to the flexplate. No change. Like I mentioned, it does run but very poorly.

        Your advice is much appreciated. I started this process a year ago, and with the number of hours I had devoted to replacing the flexplate twice, I had to put the problem away for awhile to remain sane. This was my daily driver until the flexplate replacement, and hunting season is coming soon.

        Thank you!

          1. I have tried to test the CPS but my meter doesn’t read that low of ac volts. I can try the ICM/coil refreshing too. Thanks for the reply. Holy Moly is right!

          2. Back to follow up on my flex plate dilemma! After my last post, I ordered and received a very good looking used original 1988 Cherokee 4.0L flex plate from the rockitopauto eBay store out of Glen Flora, Wisconsin.

            It turns out the ATP flex plate was made pretty sloppy. The 120 degree openings were in the right place. However, most importantly, the ATP’s measurement from the outer ring gear teeth to the inner ring that the CPS reads was 1/16” more than each of the originals. That is, the originals measured 7/16” from the outer ring gear teeth, and the ATP measured 1/2”.

            Therefore, even with modifying the CPS to get closer to the flex plate inner ring, the ATP ‘s inner ring was still too far to get a reliable signal to the ECU. Thankfully, the original from eBay fired right up! I was truly relieved.

            After putting everything back together and replacing all of the spark plugs for good measure, the Cherokee sounds just like it did before I started all of this a year ago. I hope this follow up helps someone. I have read many threads online dealing with flex plate replacements that didn’t get resolved. In retrospect, the Omix ADA plate might have been o.k. if the 120 degree openings had been in the right places.

            Also, yes, the flex plate bolts onto the crankshaft only one way. And, yes, you can replace the flex plate in your back yard without pulling the transmission all of the way off. Use the two 12”-14” pieces of 7/16” threaded rod with nuts to replace the transmission to engine bolts method and a floor jack under the crossmember. It can be done because I did it three times by myself with this exercise! It ain’t fun, but it is possible.

            Thank you Cruiser 54 for all of the tips that I will be working through shortly!

  10. Cruiser, I just finished going through 3 CPS units from NAPA. My old one was bad( quit on me while driving in traffic) . Two standard units,one premium. one standard gave .1vac, the other gave .2vac, the premium wavered from .2 to .3 vac and once gave .4vac. None would start the engine, the premium started once but the engine didn’t rub well at all. The guys at NAPA said none of that had ever happened before and it sounded like there was maybe another problem. Have you got any ideas as to what the problem might be? This is a 1990 Jeep Comanche Eliminator. I would appreciate help asap sir.

    1. Sorry for the late reply. The website is supposed to notify me as soon as a comment is made. didn’t do it til today.

      I just went through the same thing with a pile of new CPSs that came with a Comanche I bought.
      Have you done the drill mod to move it closer to the flywheel?

  11. I don’t know if my last post got thru so here goes again. I have tried from NAPA two CPS units, got .2 and .1 VAC, then a premium CPS and got .2-.3, one time .4. The store guy said it sound like I have another problem. With the CPS unplugged, what else could it be? This is a no start situation. Help!

    1. Have you ever tested at the ECU to see if it’s getting the signal?
      Check at C1 and D1 terminals.
      Also look around your CPS connector and see if there is one just like it nearby.
      There was a factory fix to put another CPS patch harness in and I have seen where the mechanic didn’t cut out the old one. Then, someone down the road accidentally plugs his cPS into the dead connector.

  12. Cruiser, if the cps is disconnected, is there anything else that could affect the voltage reading. I have gone through 3 sensors from NAPA and none recorded more than .3 vac. The guys at NAPA said they had never seen one bad out of the box like that and it seemed that there was another problem. What else could it be?

  13. I put the premium CPS from NAPA in my Comanche, it has the holes drilled out. It still shows .1 vac. that’s the third one. Seems to me there is no quality control anymore. All they need is a test stand at the factory to test each one as it comes off the line. This is really crazy, you could go through a lot of money and time getting one that works.

    1. I modified the CPS as your tip and someone else’s tip said and it still wouldn’t start. Then I looked at the coil, all contacts were corroded and dirty. cleaned them all, used Oxgard on them and it fired right up several times in a row!

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