hey guys,

Ive got a classic FAST setup on my 540. take a look at this tps screen shot. i was cruising at about 1600 rpm. throttle barely open (23 is max idle). most of the time, it hangs around 24-25, but every once in a while (every second - every 7 seconds), the tps signal spikes to 32 or 39. Is this some really slow electrical noise? Do i need a bypass capacitor or something to clean up the signal? some of the spikes last 2-3 second...pretty good sized cap?

other thoughts?

thanks

what all does the wire run near?

most of the sensor wires are near each other. distributor pickup is shielded. all sensor grounds go back to the ecu.

any thoughts?

Most TPS sensors use a simple wiper-based variable resistor which is not exactly the most precise electronic device ever created. Then you put into a high heat environment with lots of operational cycles. Oh and just for fun toss in a bit of vibration. This is exactly the type of result you can expect, if not early in the TPS lifecycle certainly later.

Fortunately this is well known and the engineers factor it in so you usually do not have to do anything about it. Rather than put a capacitor or something on the hardware to smooth out the signal, you can instead put some smart signal processing into the software. When the signal is sampled the new sample is cross checked with the previous sample. There are numerous ways of cross checking that can be used:

A) the effective value can be an average of the last N samples
B) the effective value can be limited to a maximum Z change from the prior value
C) samples more than X different than prior sample can be ignored as spurious noise

And those are just some simple possibilities, more is possible if you delve deeper into more complex math and statistics. More is also possible if you start to look at other signals. For example if RPM is unchanged and MAP is unchanged than a radical change in TPS is very unlikely. These types of secondary relationships are often used for built in diagnostics.

I would not worry about this noise on the TPS signal unless you start to see the system reacting to it in some way (which means the noise is overcoming the standard built in filter processing). For example changes in other signals that are in synch with the TPS spikes: changes in injector pulse width and/or AFR spikes in particular.

Hey Steve

Can you see my attachment? The TPS signal spikes and then the target AFR/O2 correction flutuates and then my actual AFR changes. all the while, my rpm is fairly constant. my target AFR table is fairly smooth, as is my fuel map. But there does seem to be a correlation between these TPS spikes and the O2 correction. And, thats what im trying to prevent...

thanks

Those "spikes" don't look like electrical phenomona to me. They look like mechanical noise. Is the trottle assembly bolted down well? Is the TPS sensor mechanically stable?

Ron

Hey Steve

Can you see my attachment? The TPS signal spikes and then the target AFR/O2 correction flutuates and then my actual AFR changes. all the while, my rpm is fairly constant. my target AFR table is fairly smooth, as is my fuel map. But there does seem to be a correlation between these TPS spikes and the O2 correction. And, thats what im trying to prevent...

thanks
I can see it... but deciphering all the labels and correlating and picking out particular lines not so well ;)

But you obviously read and understood my comments perfectly :thumbsup:

Ron,

im sketchy on the electrical noisy theory too. seems much to slow to be electrical. i was going to put ona heavier return spring and see if that dampens any flapping i might have. ill check all the other associated mechanicals as well

Steve,

the tps sensor is the 4th up from the bottom. its the spikey one. the yellow near the top is O2 Correction and the red is the actual AFR. you can clearly see the relationship...

thanks

Here is a good example of 'electrical' issues... notice the huge spikes...

http://img96.imageshack.us/img96/7091/fastlog.jpg

Here is a good example of 'electrical' issues... notice the huge spikes...

http://img96.imageshack.us/img96/7091/fastlog.jpg

Damn that's ugly...
What was the problem?
Ron

Damn that's ugly...
What was the problem?
Ron

Not sure, can you bring a scope up to Sac? hahaa....

I'm going to pull the MSD and coil out of the picture the time I check for RPM. I can't get a clean cranking signal to the XFI. You can see the huge spikes and that causing major havoc when the ECU is trying to read the cranking RPM.

It would be sure nice to know what is causing this. I just ordered a new MSD box and I'm going to try that. The dist modules keep frying for some reason. I'm starting to think these modules are way too sensitive. They say to make sure your coil is unhooked when you set the crank/cam sensor, which I did. However, I have left the coil wire unplugged to make sure I'm getting spark when I'm cranking. I wonder if this is what's causing them to go on the fritz and short out.

This is my only theory at this point. The guys @ FAST don't know either and can't be bothered to help and just say it's your module that doesn't work. Buy another one....

Not sure, can you bring a scope up to Sac? hahaa....

I'm going to pull the MSD and coil out of the picture the time I check for RPM. I can't get a clean cranking signal to the XFI. You can see the huge spikes and that causing major havoc when the ECU is trying to read the cranking RPM.

It would be sure nice to know what is causing this. I just ordered a new MSD box and I'm going to try that. The dist modules keep frying for some reason. I'm starting to think these modules are way too sensitive. They say to make sure your coil is unhooked when you set the crank/cam sensor, which I did. However, I have left the coil wire unplugged to make sure I'm getting spark when I'm cranking. I wonder if this is what's causing them to go on the fritz and short out.

This is my only theory at this point. The guys @ FAST don't know either and can't be bothered to help.

I have had similar issue with FAST in the past...maybe in 3 weeks I could come up..let me know.

Ron

you know, i was playing with my IAC PID terms tonight, just idling in the driveway and the car died a few times. was idling realy nice around 13.5-14.0, 1050 rpm, 190 degrees and then boom, just dies.

im thinking this is ignition. reading what i read on chevytalk and seeing you also using msd stuff, makes me wonder about my own msd distributor

keep us posted

I have had similar issue with FAST in the past...maybe in 3 weeks I could come up..let me know.
Ron

Thanks Ron. I'll let you know.

I pulled out the MSD for now and checked the LED's looks like they are triggering ok with out the MSD & coil, LEDS on XFI light up, Banks A-D flash, points light is firing. Next step is to check while cranking and see what happens. Hopefully later today.