Dear All,

Im having problems with noisy rpm signal. I wrote some of the info on another thread but I realized that his is different so I created a new one.

What are the consequences of the problem?: RPM noise near 1200 rpm causes bad injection pulses and very hard to tune, and also engine does not run ok.

What is the problem?: unknown so far !.

What have I done?:

I have checked
-spark plugs (all good in good shape BKR5E-11 with resistor)
-spark plug wires (Taylor spiro pro 8mm) new,
-ACDELCO ignition module new.
-Checked for EM interference with a AM radio and seems to be ok, maybe a bit noisy the coil cable, I have no replacement.
-Changed dissy cap and rotor cap
-change and check two coils.

All same result.

This noise makes the ecm think that the engine is on another RPM station. It very interesting that it is only visible (the noise) between 900 and 1200 rpm, but I am aware that it is always there.

Since I could not find any scope reading of the AC of the reluctor to compare, I thought it could be interesting to share to the world mine, it may or not help others, so I connected the scope meter, one of the probe on the reluctor and the other to the reference input on the ECM. With those readings I could figure out if it was a problem on the ECM, ignition module or reluctor signal or cables.

I notice that the signal of the reluctor is affected by the spark. I previously recorded the reluctor signal a long time ago but with the distrib was out of the engine and turning it by hand, and it was nice and clean, but this time i did it with the engine running to check what really goes on.

Since at idle the advance is 20 degrees, it is near halve way the reluctor signal ramp, the spark discharge causes a low on this positive ramp on the reluctor signal, and makes two lows and that is why on the WUD of the EBL sees double rpm, it sees that always jumps to 2000 RPM when it really is near 1200, some times is worst and jumps al the way to 3000 rpm.

Have any of you have an idea why is spark discharge is affecting the reluctor singal so much?

Measurements.
Reluctor signal at idle (650rpm): 1.7V peak to peak
Reluctor signal at 1300 and up: 2.7 vpp and higher
Are this ok? it seems to me that it could be a bit low, the pick up coil is maybe 2 years old so If it is bad, I might have to replace the entire distribuitor.


This issue have made me look into other options well documented here like going to distribuitorless ... I like the nordstar option but maybe any of you could help me on a good way to go, in the mean time, i have to fix this problem!

Check the video here:
https://drive.google.com/file/d/1PZ5...ew?usp=sharing

https://drive.google.com/file/d/1rJu...ew?usp=sharing

Screen shot here:
https://drive.google.com/file/d/1gmO...ew?usp=sharing

https://drive.google.com/file/d/1qfM...ew?usp=sharing



Cheers!,

TP

 

Here is an update to all,

I did a warm up test today, to plot in open loop the WB all the way to operating temperature. I notice that it is not only 1200, it is all below 1200 RPM. Maybe it is just a problem with the permanent magnet on the reluctor.
I will share later on the plots

Keep all posted on any further tests.

TP

 

Check that the star wheel on the distributor shaft is still solidly connected to it.

RBob.

 

Thanks RBob,

Do you mean this?




This is not my real distributor, I just used this image for education purpose only...

 

Here is the plot of the warm up test.
This test is, open loop idle but at the end I did rev it a little at minute 9.
It can be seen that It starts in 700RPM and goes down with CTS.
AFR with CTS seems to be good from 35 to 55 then it goes on the lean side, Ill get it better tuned with OL AFR vs CTS.. it is rich below 35 because of the noise so I need to get a clean signal first then finish tunning.. a couple of more tries and I will get it perfect.
RPM starts very noisy, then goes ok until minute 6:19. Then it goes exponentially bad causing a rich mix that when rev to 1300 (out of the bad rpm reading area) for short time you can see that it goes stoich because VE table is ok on that spot, then back to idle and messy RPM, rich WB reading. . Ill check on RBob suggestion and post any results..
Thanks all for following the post and RBob thanks for the reply!. I hope this info is useful for others.





Cheers,

T

 

Also a good time to check the magnet for cracks while you're in there.

 

Yes, the shaft has splines and the star wheel is pressed on. Sometime they come loose and start flopping around.

RBob.

 

Here is an update!,

Checked on the star wheel and it is ugly but well fixed!.. also here is a picture of the magnets.. i believe it is cracked... or it may seem to be small cracks, so it seems that faulty permanent magnet it is!. What do you think?.
Time to change the distributor ..., maybe an opportunity or perfect excuse to go distributorless!

 

Why is the shaft knocked down so far ? Its pressed on and any shimming gets done at the bottom of the distributor.

 

Hi, I believe it is an optical illusion because of the angle of the camera, it looks short but it has good length.

 

Good camera trick

 

I will replace the Distributor and make the same readings and compare. Keep all posted.
Thanks again to all for all the help!.

Cheers,
T

 

Hello to All!.

An update of further tests and results.

Replacing the distributor had the effect on making a very smooth RPM signal (I will post pictures later) but did not solve the noise at 1200 RPM.

I was really running out of ideas and then I though It must be EMI, and there was only one thing left, the coil...I took the coil out of the base (close to the distributor) and left it hanging on top of the valve cover on passenger side, this was the furthest away I could because the coil power and control cables are short...... NOISE GONE!. Checked from idle to 5000 and perfect RPM signal!!! I guess ill find a way to get it fixed to the firewall or something!. Keep all posted!

Cheers,

T

 

Nice find

 

That's an odd one. Good work tracking it down.