Write a value through OBDII port

I am new here and I hope i am in an appropriate Forum for my inquery. From what ive read it seems reasonable.


B.L.U.F


P1345


I want to reset the CaMshaft Position Retard Angle PID in the EVAP EGR Data. Really I want to make the SMOG god happy and save some gas money and not have to limp up grades. I'd also like to stop buying distributors for this '99 K1500 5.7L Suburban but that won't happen.

I am an Electrical Engineer and I have a PLC in my garage i use for random things . I figure i could program the PLC to send a string of data through my Dash OBDII port that opens the bus and sends data using the SAE J1850 VPW protocol to reset CMP Retard to Zero or whatever value is needed.


So my question is if anyone could tell me the correct sequence of hex to send to simply write one value to that register? Also any other thoughts on the situation. It seems there is no consensus among among internet people about 5.7L Vortec distributors, I assume because the older ones had turn the thing timing and the newer ones didn't even though some new ones came with a round hold down bracket that allowed a few degrees and aftermarket dist. Had a rectangular that allows nothing. I am thoroughly fed up trying to find the truth of the matter but a little pain to spike the pleasure is good so if you guys have any really good explanation/advice/rants I would appreciate the input.


i have the protocall timing as follows:




SAE J1850 VPW
Feature Description
BUS + Pin 2
12V Pin 16
GND Pins 4, 5
Bus State: Bus idles low
Maximum Signal Voltage: +7V
Decision Signal Voltage: +3.5V
Minimum Signal Voltage: 0V
Number of bytes: 12
Bit Timing: '1' bit -HIGH 64uS, '0' bit -HIGH 128uS, Start of Frame - HIGH 200uS


Thank you

Disclaimer: I have no experience with your specific PCM, only OBDII PCMs in general.

That said, with all the OBDII PCMs I'm aware of, it's not possible to just 'reset' a PID by shouting data at a register. If you want to alter how much retard angle there is from EGR operation, that's something that would have to be done in the actual operating code, which is likely stored in an EEPROM. That being the case, you can't just change one value, you have to erase the EEPROM and write it all over again. This is far more complex than just the protocol you listed, as it generally requires a security handshake to allow switching the PCM into programming mode so that it will accept the erase command and then accept new data to write.

If there's something wrong with your vehicle, I'd try to actually fix it rather than praying you can change data in the computer to hide the problem. Best of luck.

Quote:

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Originally Posted by NomakeWan

Disclaimer: I have no experience with your specific PCM, only OBDII PCMs in general.

That said, with all the OBDII PCMs I'm aware of, it's not possible to just 'reset' a PID by shouting data at a register. If you want to alter how much retard angle there is from EGR operation, that's something that would have to be done in the actual operating code, which is likely stored in an EEPROM. That being the case, you can't just change one value, you have to erase the EEPROM and write it all over again. This is far more complex than just the protocol you listed, as it generally requires a security handshake to allow switching the PCM into programming mode so that it will accept the erase command and then accept new data to write.

If there's something wrong with your vehicle, I'd try to actually fix it rather than praying you can change data in the computer to hide the problem. Best of luck.

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NomakeWan,

Thanks for replying, I wasnt sure I was going to get a reply.

I didn't think about the possibility of active security keeping me from writing values. I guess I would need to read all the values out of the eeprom then make the change I want and then write them all back. Which is just alot more yelling at a few more registers really.

I was trying to not spend money on a GM Tech II and instead tinker/fabricate with the things I have in my garage for free . If I can make something with parts and tools I have I can't get myself to pay for it until I wasted countless hours , probably a fault of mine. I'm actually really interested in some of the things you guys are doing here so I might just break down and get some special gear for programing different tunes.

I don't give much credence to prayer or sweeping things under rugs.
I believe I have fixed the problem. The issue is whenever the distributor in these trucks gets reset or a new Crankshaft position sensor installed this value to be set or you end up with the CKP to CMP coralation P1345 code . But there is no way to turn the distributor body whatsoever in some vintages.. Like I said the internet is completely full with guys arguing about how they can turn their distributor and have you need the scanning tool to set the value. That's why I was hoping maybe one of you guys here had some good input on the situation. and I would welcome anymore input from anyone on anything that Ive said. I always reserve the right to be dead wrong.

Hello,
Just a little feedback/info for others that might come across this. Those "distributors" do not affect ignition timing since that is controlled by the crank sensor and then the ecu. The aftermarket parts can be full of manufacturing errors, even ac delco quality control has been lacking the last few years. The cam sensor inside that distributor is a hall effect sensor so you can almost eyeball the location. Elongating the hole in the distributor hold down clamp with a die grinder and carbide is an option to gain some more adjustment. I am pretty sure you can cut off the non adjustable hold down clamp and use a standard hold down clamp.

It's copyrighted material so I can't post it here but these are documents to study if you want to learn, this is stated in the SAE J1850 document.
SAE J1113—Electromagnetic Susceptibility Measurements Procedures for Vehicle Components
SAE J1211A—Recommended Environmental Procedure for Electronic Equipment Design
SAE J1213/1—Glossary of Vehicle Networks for Multiplexing and Data Communications
SAE J1547—Electromagnetic Susceptibility Measurement Procedures for Common Mode Injection
SAE J1879—General Qualification and Production Acceptance Criteria for Integrated Circuits in Automotive Applications
SAE J1962—Diagnostic Connector
SAE J1979—E/E Diagnostic Test Modes
SAE J2012—Diagnostic Codes/Messages
SAE J2178—Class B Data Communication Network Messages
SAE J2178/1—Class B Data Communication Network Messages: Detailed Header Formats and Physical Address Assignment
SAE J2190—Enhanced E/E Diagnostic Test Modes

Look for SAE white paper J2190 ENHANCED E/E DIAGNOSTIC TEST MODES to understand more about the packet structure. The best el cheapo laptop scanner I have seen is the OBDLink SX and OBDWiz software ~$30. It will allow you to watch packets as well as send packets(not recommended but can learn some things without crashing stuff).

Have fun :)

IN v6s vortec the distributor is not adjustable, you can`t turn it at all. On v8 vortec the diss can be adjusted some degrees.

You can`t reset the angle in pcm, you can only monitor it through obd2 port. What you need is to find the pid for it. It is different between years.

When you have the chance to monitor the angle you can fine adjust the diss by turning it[v8 only]. I think it was at 2000 rpm, fully warmed to target 0*.

V6s are not adjustable.

I suggest you first to change to chain. Stretched chain is the main reason for this error.

We're still using L31's in our fleet. I believe I recognize some of the issues the OP is describing.

Some aftermarket V8 distrbutors are built so movement is limited once the clamp is installed. There are two raised areas on the base of the distributor which will contact the clamp if the distributor. When needed I have trimmed the end of the clamp to allow some movement.

It is not necessary to have a scantool which can enter special test mode adjust the distributor angle. The cam sensor angle updates automatically on startup and when the throttle is fully depressed as long as CTS is above a minimum value. IIRC this value is close to is about 206 deg F. Using a generic scanner or a laptop with a tool like Tunerpro RT would allow viewing the angle while updates would occur with repeated startups or throttle snaps.

Frequently I find a replacement distributor has been installed incorrectly which can prevent one from setting the cam angle within the window. I do not know if that's the case here, but when the old distributor is removed the oil pump driveshaft ends up incorrectly positioned for the distributor to be dropped back in the same position. The secret to fix this without special tools is to lift and drop, lift and drop the distributor while "walking" the rotor counterclockwise one tooth at a time until the distributor ends up in the right position.

I have not found a way to "fix" the distributor once and for all but I have found a way to make them live longer. We have substantial troubles with moisture collecting under the cap so I will epoxy a 90 degree vacuum fitting with a small orifice (around .020-.030"). I will then connect that fitting to a ported vacuum source. This causes air to flow through the distributor wnen the vehicle is under load which tends to remove moisture and results in fewer cap and rotor or complete distributor replacements.

I have disassembled pickup engines and found timing stretch that seems excessive, and I have read many forum posts that indicate timing chain stretch was an issue. We have never had problems with excessive stretch causing codes in our vans. I feel the heavy weight of the vehicle really doesn't allow rapid acceleration of the crankshaft and this may help the chain last longer. Chain stretch can be estimated by manually, gently, turning the crank back and forth while feeling for tension on the cam. Anything approaching 5 degrees means chain replacement is definitely warranted.

I've noticed my cam sensor is retarded 12 degrees according to the scanner. I don't know what the threshold is for setting the code, but it's obviously higher than that. If you've exceeded the threshold, I'd say you have a mechanical issue that needs to be corrected. As others have said, timing chain stretch can cause this problem, or perhaps the distributor wasn't timed properly the last time it was installed. Either way, you will need a scanner of some sort to properly time the distributor. An alternative to get you through emissions, that I don't recommend, would be the guess and check method of moving the distributor a little bit, clearing the code and seeing if it returns. Repeat until the light doesn't come back. Mark your starting position and If it starts running bad, you probably went the wrong way and you need to stop and back up. I would expect that most vehicles built in 1999 would have high enough mileage and enough timing chain stretch to justify replacement. If you do replace the chain, be sure to order a new timing cover. They are not meant to be reused.