Flex fuel sensor input

[turbo_bu]

Got a request from a guy to help him do a propane swap on 1 MB LS1 PCM. I believe he is mainly looking to increase the timing maps when the propane is on. I was thinking of using the flexfuel maps in the OS to accomplish this goal. The question I have is about the flex fuel sensor input. I'm pretty sure it is a PWM signal that tells the PCM the ethanol percentage. I don't have a sensor and am just looking to make a switch between gasoline and propane. Can I just feed the PCM a switched (on / off) signal instead of a PWM?

[DURAtotheMAX]

Can you just use a stock/OEM calibration from a CNG 2500HD?

[turbo_bu]

Hadn't thought about that ... Do you happen to have a copy of the that OS? I looked on the Tune depot, but wasn't sure which OS corresponds to CNG.

Thanks for your help,

[Chevy366]

Look at the bottom of this page there are a few suggestions to other threads that might be of use. "Similar Threads"
One year used a module and then GM switched to using the O2 sensors to detect gas content.

[turbo_bu]

Chevy366 - Thanks for the idea ... I looked at the Similar Threads but didn't see anything that talked about using a switch instead of a PWM signal for the sensor input to the PCM. I am still curious about how the PCM will respond to a switched input instead of the PWM signal since I am not trying to interpolate ethanol content. But I am also now more curious about how the CNG OS operates.

Did the CNG trucks have the ability to run on gasoline too? Or did they just have CNG tanks and injectors?

[turbo_bu]

Google to the rescue - sounds like the 2005 CNG trucks were Bi-fuel. Had injectors for both gasoline and CNG.

http://www.cngchat.com/forum/showthread.php?8482-2005-4x4-Chevrolet-2500HD-Dedicated

Does anyone have a copy of the CNG OS? Maybe a wiring diagram for what "sensor" or switch the PCM uses to determine which fuel is being used?

[Chevy366]

Quick search on the forum yielded this -- https://forum.efilive.com/showthread.php?11914-Liquid-LPG-tune-using-Flex-Fuel&highlight=flex+fuel+sensor

I see those CNG trucks around here in the larger cities, almost though about stopping and asking the city worker driving the thing if I could read the calibrations off the PCM.

Again look at the bottom of each thread for additional links, they are on every thread with a "similar" topic.

[joecar]

I would dearly love to see an untouched CNG tune file... :)

( offer the city worker a beer, and explain that you're a CNG engineering student and are curious about his vehicle )

:) :) :) :)

[DURAtotheMAX]

If someone can get me a VIN, I can get a stock file for you guys.

I looked into the description/operation, and its a pretty complex setup. Theres an additional FICM that piggybacks with the ECM, which then operates the CNG injectors. Then there are like 10 relays that switch control between the ECM (gas injectors) and FICM (CNG injectors).

Ben

[Chevy366]

Do you mean through a TechII? I was just wondering if a V2 would read the OS, you know correct seed key and all. Look and see if there are any on Ebay for sale, if so, they usually list VIN numbers, that is how I have found parts in the past. Quick look and there are 2 listed -- http://www.ebay.com/sch/6000/i.html?_nkw=silverado+cng&_rdc=2 , will one of them work? VIN -- 1GCHC24U64E382319, 1GCHC23U41F197449

I am on an Auction site that sells the larger city's vehicles but have not seen any CNG come across yet.

[DURAtotheMAX]

Unfortunately neither of those are factory CNG setups.

[Chevy366]

How about this one -- http://www.ebay.com/itm/Chevrolet-Express-3500-2005-Chevrolet-Express-3500-Base-Standard-Cargo-Van-3-Door-6-0L-/271576957372?forcerrptr=true&hash=item3f3b3f81bc&item=271576957372&pt=US_Cars_Trucks