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cps fix

cygnus58 said:
the link provided only mentions sensors mounted on distributors.
Maybe they are just that.
The FSM for Renix shows the waveform (I stand corrected) as a sawtooth.
Either way, put a cap across a variable AC signal, it will load it at these moderate frequencies.
Click to expand...
The waveform shown in the VRS link provided earlier looks exactly like the waveform from the CPS and the Tranny output speed sensor. If you're looking at the picture in the Jeep Renix Fuel Injection Manual, the sawtooth pattern is labelled "magnetic field", so the inducted voltage is due to the rate-of-change of the field - you get the spikes at the tips of the sawtooth.

In any case, I agree that the cap can only reduce the voltage spikes as the cap will act as a 1st order low-pass filter. Adding the cap in parallel makes it a low-pass filter with a cutoff freq of about 80kHz if you assume the nominal sensor resistance of 200 ohms. The rep rate for the pulses is about 5000 pulses/second but the frequency content of the spike should be a lot higher. My guess is that it's simply cleaning up a noisy sensor signal - perhaps due to a cracked magnet in the sensor. The computer undoubtably has a cap on the signal lines anyway.

The voltage peek of the spikes is dependent on the engine speed too. They grow proportionally bigger with engine speed. When I was building a box to modify the tranny sensor signal to use an 89 TCU with a 99 trans, I measured the voltages from zero to nearly 30-volts at 60 mph.

I'm not a EE, but I'm a reasonably competent electronics tech and I happen to do a lot of work with acoustics, filters, amplifiers, and data acquisition systems.

If a 3-cent cap fixes your problem then by all means be happy. It might not work for everyone else and I highly recommend having a spare with you in case this bandaid stops working.
 
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lawsoncl said:
The waveform shown in the VRS link provided earlier looks exactly like the waveform from the CPS and the Tranny output speed sensor. If you're looking at the picture in the Jeep Renix Fuel Injection Manual, the sawtooth pattern is labelled "magnetic field", so the inducted voltage is due to the rate-of-change of the field - you get the spikes at the tips of the sawtooth.

In any case, I agree that the cap can only reduce the voltage spikes as the cap will act as a 1st order low-pass filter. Adding the cap in parallel makes it a low-pass filter with a cutoff freq of about 80kHz if you assume the nominal sensor resistance of 200 ohms. The rep rate for the pulses is about 5000 pulses/second but the frequency content of the spike should be a lot higher. My guess is that it's simply cleaning up a noisy sensor signal - perhaps due to a cracked magnet in the sensor. The computer undoubtably has a cap on the signal lines anyway.

The voltage peek of the spikes is dependent on the engine speed too. They grow proportionally bigger with engine speed. When I was building a box to modify the tranny sensor signal to use an 89 TCU with a 99 trans, I measured the voltages from zero to nearly 30-volts at 60 mph.

I'm not a EE, but I'm a reasonably competent electronics tech and I happen to do a lot of work with acoustics, filters, amplifiers, and data acquisition systems.

If a 3-cent cap fixes your problem then by all means be happy. It might not work for everyone else and I highly recommend having a spare with you in case this bandaid stops working.
Click to expand...

Yah - I just "notch" a spare and toss it in the glove box. I've also got an 11m/m "stubby" wrench in there - I can work with that if I really need to (but the 11m/m flex socket on a 30" extension is just so much easier!)

Service life for the CPS - at least for RENIX - seems to be 150-180Kmiles, so I'm not overly worried about it. I've had more issues with the cabling managing to rest against the header and burn through/short out than with the sensor wearing out - I can fix that just by crimping on new wires. Put some "fire sleeving" over the wiring, and alles gut.

Got any more information on your transplant mod? Not only am I interested personally, but it sounds like the sort of thing that should probably be on RENIXPower (groups.yahoo.com/group/RENIXPower) as well...
 
5-90 said:
Got any more information on your transplant mod? Not only am I interested personally, but it sounds like the sort of thing that should probably be on RENIXPower (groups.yahoo.com/group/RENIXPower) as well...
Click to expand...
The issue I ran into was that the 97 and later (might be 1998) AW4 rear speed sensor gives 4 pulses per rev instead of the earlier 1 pulse/rev. Basically, it kept trying to upshift at anything above idle. You can pull off the rear of the AW4 and swap the rotor and use the matching sensor, but I didn't have a donor. My solution was to create a box that went in between the 99 speed sensor and pass every 4th pulse. I still have some pictures and the schematic around here somewhere.

The later AW4 also has a front speed sensor that you can ignore if using the older TCU, but it does mean you can't use an older AW4 with a newer TCU.
 
I'm not an EE either, but I can run a Spice analysis.

The "right" cap value across the coil will create a parallel resonant circuit -- causing the voltage 'seen' across the indutor to increase.

Give the man a cookie....haters!
 
If I get bored this weekend, I'll drag the oscope outside and look at the CPS signal with and without the cap. I have trouble believing it's a LCR resonance thing as it's not a continous waveform, but rather spikes. Resonance would also be frequency dependant.
 
lawsoncl said:
If I get bored this weekend, I'll drag the oscope outside and look at the CPS signal with and without the cap. I have trouble believing it's a LCR resonance thing as it's not a continous waveform, but rather spikes. Resonance would also be frequency dependant.
Click to expand...

Not a EE either (but a Chem. Eng.) but thinking in terms of time domain, how fast does that Cap discharge as compared to the coil? Is it possible the cap is raising the entire baseline voltage, and thus raising the apparent peak (spike) voltage?
 
Both of them are slotted in my case. I don't remember if the second one I modified was slotted (elongated towards flexpale) beforehand.
This just seems so hit and miss without the EXACT clearance.
Does it have to be that precise - damn really?????
And i've read in your link of the newer ones having a cardboard tip that comes off.
Looks like the gaps in the flexplate are large enough to miss the metal when mounting on of those.
 
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lawsoncl said:
If I get bored this weekend, I'll drag the oscope outside and look at the CPS signal with and without the cap.
Click to expand...
Well I guess oldman, cygnus, and I need to eat our hats. Here's what the CPS signal looks like at idle on my 89 MJ 4.0 auto. The frequency and amplitude get proportionally higher as the engine is reved. It is spikes, but they are spaced close enough together that it looks like a sawtooth.


CPS_Signal.jpg


I add a 0.01uF cap across the sensor and the voltage increased roughly 5% from idle up to about 3k rpms. No distortion of the waveform, just increased amplitude. Stepping up to a 0.1uF cap increased the voltage roughly 20% at idle. I only reved it to probably 2k, but in hindsight I should have looked at higher rpms too. A 1.0uF cap cut the amplitude in half at idle and the engine started stumbling.

So I'm thinking it's a tuned LCR circuit and the extra capacitance is boosting the voltage at starting where the rpm and signal strength is lowest. This might also explain why beefing up your battery cables and getting better starting rpms helps too.

-Chris
(looking for the hot sauce to put on the ball cap)
 
cygnus58 said:
This has been an interesting thread.
Anyone care to check this out?
http://autorepair.about.com/library/a/1i/bl534i.htm
I had no idea, a "grommet" was essential in CPS placement.
Apparantly, part no.83100068 keeps the CPS approx. .025 from the flex plate.
This would possibly explain some long drawn out problems I have been having.
I need to check this out.
Click to expand...

I don't think the grommet has anything to do with the actual mounting of the CPS. I've replaced a couple (regular unslotted, both Renix and Mopar), and they came with new grommets (a snap-on plastic dust shield), which do help in holding it in place as you install it. I'm guessing the call for a new grommet might be because the slotted CPS doesn't properly clear the original. The grommet does not come between the cps and the bell housing, but snaps over the hole in the bell housing.
 
Great scope shot BTW!
I so desperatley need to try scope mine. I am having MAJOR no power/stalling/won't go past rough idle issues right now.
Have two CPS and neither will work right yet I modified both.
Hopefully the grommet will help me get the distance right.
 
I'm wondering if the slotted CPS has wax or cardboard on the tip like ABS sensors do. On the ABS sensors I've dealt with you pushed the sensor up against the wheel, tightened down the clamp, and the spacer material gets wiped off the first time you drive.
 
lawsoncl said:
I'm wondering if the slotted CPS has wax or cardboard on the tip like ABS sensors do. On the ABS sensors I've dealt with you pushed the sensor up against the wheel, tightened down the clamp, and the spacer material gets wiped off the first time you drive.
Click to expand...

I read somewhere here (or a link from here to somewhere) that a CPS fix included putting a pair of thin metal washers under the CPS (between the cps and the bell housing with the mounting bolts going through the washers and cps) to distance it from the flywheel. It even listed the prefered thickness of the washers as I recall.
 
Aparently Renix wasn't programmed to kill everything if a weak CPS signal tries to be counted. After my post last nite I came home today and re-aligned it closer to the flexplate.
ALL the horrendous syptoms have disapperared, as quick as they came.
The fuel additive was coincidence.
When I tried the original CPS again yesterday to nip the problem, I didn't push it down hard enough either. I have modified both as well. I think this requires a little extra push to compensate for the loss of the true holes. Must wanna climb up a few hundred microns or whatever. Read today that ideal gap is .82mm. Not like you're gonna gauge it. Maybe could attach a gauged piece of plastic with parrafin or whatever to be ripped off by the flex plate and push down Hard when installing....
 
Ecomike said:
I read somewhere here (or a link from here to somewhere) that a CPS fix included putting a pair of thin metal washers under the CPS (between the cps and the bell housing with the mounting bolts going through the washers and cps) to distance it from the flywheel. It even listed the prefered thickness of the washers as I recall.
Click to expand...

That would just space it towards the rear instead of radially inward towards the flywheel or flexplate.
 
lawsoncl said:
That would just space it towards the rear instead of radially inward towards the flywheel or flexplate.
Click to expand...

That is correct. That was the way I read the post. It was posted here in the OEM or Mod forum. Posted claimed it fixed his CPS problems and that he got the trick from somewhere else. Have not tried it myself.
 
The 87 Renix FSM says this:

Flywheel has a larger trigger tooth and larger notch followed by 12 small teeth before each TDC position. When the small tooth and notch pass the magnetic core in the sensor ...then the collapse of the magnetic field induces a small voltage spike... the ECU counts the teeth....When the a larger tooth and notch pass the magnetic core a higher flux induces a higher voltage spike....the higher spike indicates to the ECU that a positon will soon be at TDC 12 teeth later.

So there are two different peak heights for the CPS to produce and for the ECU to read.
 
i have had so much "fun" with Renix in the past few weeks.
I highly recommend it to anyone with a desire to cuss, swear, lose sleep, and crawl under yet one more time.
CPS = SATAN
my son did a one thousand one, one thousand two tonite, and we were 0-60 in about 8 seconds.
I am happy.
'88 cherokee 4.0 Auto. 178,000 miles and counting
 
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I am curious how you EEs think this tooth size difference I posted earlier (quoted below), large tooth versus small tooth, might affect the capacitor addition trick, circuit regarding the signals the ECU gets. I don't recall any discussion about the signal difference between the large and small tooth, and the capacitor trick. Does the sudden change in tooth and valley size possibly relate to why the capacitor trick works??????:rattle:




Ecomike said:
The 87 Renix FSM says this:

Flywheel has a larger trigger tooth and larger notch followed by 12 small teeth before each TDC position. When the small tooth and notch pass the magnetic core in the sensor ...then the collapse of the magnetic field induces a small voltage spike... the ECU counts the teeth....When the a larger tooth and notch pass the magnetic core a higher flux induces a higher voltage spike....the higher spike indicates to the ECU that a positon will soon be at TDC 12 teeth later.

So there are two different peak heights for the CPS to produce and for the ECU to read.
Click to expand...
 
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