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Need some help to solve a P1645 code, might be a bad PCM


the4quads

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I did a search about this problem and it looked like it would one of two problems. It could be either a defective Evap. purge solenoid or a break in the wiring. One of the members found the wire broken internally just behind the connector by the valve. One of my first tests involved opening up the lock part of the PURGE solenoid connector and pulling/tugging on the wires to check if the wire was broken inside the insulation. After that I back probed with my fluke to check for continuity. The terminals are like new- clean and shiny.

The wire felt good so I ordered a OEM valve from Rockauto and installed it. I cleared the code and a second or two later the "service engine soon" light popped on.
I probed the Pink wire and it had battery voltage so maybe the Dk.Green/white wire is damaged somewhere. According to the FSM, that wire goes to #24 terminal on the PCM. I removed the multi-wire connector from the PCM and did a continuity test (three times) while wiggling the wire and I had no interruption of continuity.
I believe that the 1645 code is a electric problem not a vacuum leak.

I've read on some other forums that the Output driver module (ODM) inside the PCM might be bad. That device is what grounds various circuits including the EVAP solenoids. I would need a Tech II to trigger the PURGE solenoid and check for a ground signal at the solenoid terminal.

A friend of mine is also a mechanic and has access to Alldata. I explained to him everything I did so far and he ran down the possible causes. It seems that I covered all the common possibilities. There where two Techs. that ended up changing the PCM to solve the problem.
In case I have to change the PCM, is there a vender or member here that could program the replacement PCM for my car?

My last test will be in two parts. One is to remove the connector at the PURGE solenoid and jump the two terminals together. I'll check for continuity between the load side of the fuse (fuse removed) thru to the #24 terminal by the PCM. The PCM wire connectors will be both disconnected. Second will be to put the fuse back in, connect the PURGE solenoid connector, ignition on and check for battery voltage at the #24 terminal by the PCM. If both check out, I'll presume the PCM has a bad component in it.
If I buy a PCM with matching numbers, would that work or at least let me drive the car to the dealer for final reprogramming?

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I've been reading scores of pages trying to find a solid answer to my problem. I found a link on one of the members posts about a company that might be able to reprogram a replacement computer. I just sent them a email asking about that service.

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I would think you could get a friendly dealer to help with their Tech 2. I think Logan keeps an up to date tech 2 but the logistics of sending a PCM somewhere to be programmed seem daunting compared with just visiting a Cadillac dealer.

Bruce

2023 Cadillac CT4-V Blackwing

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Hmmm......maybe. It will be much faster and cheaper to find a shop with a GM Tech 2 for some testing first.

The GM Tech 2 is a bi-directional that can turn the different solenoids on and off. So...for example....you can command the vent solenoid on and off with the Tech 2. If you can hear the vent solenoid click when commanded 'on' and 'off' with the Tech 2....you know the PCM is ok.

Sooo...I would try that before condemning the PCM.

Could be a bad driver in the PCM. A used PCM will not work. Most of the major modules used are VIN specific. The VIN can only be entered one time into a module. GM has a service bulletin about NOT swapping modules for testing due to this issue.

I attached a pic of a GM Tech 2.

post-2-0-44345600-1365817107_thumb.jpg

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Thank you for the help.

I called my local "friendly" dealer and explained everything to the service manager. His response was that I have to bring the car in and let them diagnosis the problem. I asked how much a remanufactored PCM is and it's $368 plus labor and diagnosis time. A rough guess ranged between $650 to $800. That doesn't include my lost hours.

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If the Tech II triggers the PURGE solenoid, does that mean the PCM is good? If I understand correctly, a used PCM cannot be reprogrammed with a different VIN? This is getting more interesting by the day.

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You can go thru a whole GM vehicle....and command different valves, solenoids, and relays on / off for testing using a GM Tech 2. You can even turn on and off fuel injectors, fuel pumps, cooling fans, blowers, egr valves..etc..pretty much anything can be tested quickly.

So yes, if the Tech 2 can turn on and off the purge solenoid...the PCM is ok.

The VIN number is locked in to the PCM. A salvage yard PCM should be a no start.

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Do you offer the service to reprogram a used PCM? I did see some remanufactored units for sale. Are the Reman. units programable.I really appreciate any help here. I can't go for the big $$$$ the dealer wants.

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Thanks guys. I will follow up on finding somebody that has one and knows how to use it. My son knows a bunch of mechanics he went to Penn State Tech with. Maybe I'll luck out. I'll post the results as I get them.

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I wouldn't go for a PCM until you fully diagnosed the problem. It's the least likely to cause the problem and most expensive part to replace.

If you can hook a DVM to the ground terminal of the solenoid and watch the voltage, it should read battery voltage except when the PCM wants to open the solenoid, when it should read less than 1 Volt. If the meter needle (or readout) doesn't twitch when the car starts and does its self-test, then you probably have a wiring problem or possibly a shorted solenoid.

Ask another dealer about checking the EVAP exhaust solenoid. It doesn't have to be a Cadillac dealer; all of them have Tech 2 OBD interfaces and can check the EVAP solenoid in a few minutes.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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I will do the test this afternoon. I always had good luck with my GM cars and trucks. I hope my luck holds out.

Nice CTS-V you have there. I settled on a 06 GTO. Got over 211,000 miles on it since new. (June/2006) So far no major problems.

The Seville is my wifes and we bought it from a friend who's Dad owned it. Her Dad passed 11 years ago and the car basiclly sat in the garage and only was used a few times. It had 21,048 miles on it.

On the way home the "service suspension sys" was on. Based on reading several posts about the different tests to perform, it pointed to a defective CVRSS module. I was able to repair the CVRSS module by removing a bad SMD resister and soldering a new 1K 1/4 watt in place of it. I'm thankful that there are some electronic repair experts on these forums, saved me some big bucks. I hope I can return the favor someday.

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I've been helping out with 1992-2002 Eldorados and 1992-1999 Sevilles and Devilles using my 1997 Factory Shop Manual set for years and I know the FSM pretty well, and the transmission manual to some extent. There are far better DIY people and experienced mechanics on board at Caddyinfo but I have found that if reading the schematics and working with codes and symptoms, and troubleshooting electrical problems is the issue, I can contribute pretty well. Also, my first car was a pre-war GM product and I'm fairly knowedgeable about older cars and mechanics in general. For example, when you take the top off a Holley 4150/4160, there is a little brass splash shield that will jump out, and if you aren't ready for it, you can lose it or not be able to figure out how it goes back.

Now, I've got the FSM for the CTS and CTS-V on DVD, and the DVD has, I have found, all Buicks from the 2003 through 2011 model years and nearly all GM makes for the 2011 model year, plus lots of other stuff going back into the 1990's, such as the overhaul manual for the 4T80E (although I have that one in paper as part of the 1997 Unit Repair Manual). I'm slowly learning the manuals on that DVD, too, although I'm concentrating on the 2011 CTS-V for obvious reasons.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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OK, after back probing the DVM to terminal #24 by the PCM and the Batt. post, I get a ground signal as soon as I turn the Ign on. I wanted to make sure the signal was more than a few milliamps so I hooked up a 164 bulb (licence plate bulb) to both terminals of the disconnected connecter of the PURGE solenoid and it remains lit as long as the Ign is on. I believe that means the PURGE solenoid is powered open full time.

The car runs good and no other codes are registered. I do not know if the solenoid being open causes a lean condiction because it acts like a vacuum leak. Now for the $$$$ question-Does it lean towards a defective PCM? I found a shop near my place of employment that has a laptop program that he says will turn on and off the Purge solenoid. If he can't toggle the solenoid on and off, than I guess the next step is the PCM.

Your input is much appreciated.

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From the 1997 FSM page 6-612:

Circuit Description

The PCM contains chips call Output Driver Modules that provide 7 switchable ground outputs for operating solenoids, relays, telltales, an other devices. Each of these ground outputs has an internal fault line that feeds back to the PCMs microprocessor. When the output if OFF, the output driver should receive the 12 volts fed to the solenoids, relay, etc. When the output is ON, it should receive 0 volt since it is grounding the output. If it receives 0 volt when the output is OFF or 12 volts when the output is ON, it sets the fault line low. DTC P1645 is used to monitor the fault line of the EVAP purge solenoid.

Conditions for setting the DTC

Test Conditions

  • DTC 1640 [Driver-1-Input High Voltage] is not set.
  • Engine running at least 10 seconds.

Failure Condition

Fault line is low for 2 seconds.

So, the PCM is trying to turn the EVAP solenoid off but still sees 0 Votls, apparently. Since it switches, it isn't likely the output driver on pin number 24. See if you can hear the EVAP control solenoid click when someone else turns the key off and on.

It may be the sense line on pin 55 of the PCM, which is labeled "EVAP Vacuum Switch Input" on the schematic for DTC P1645. Pin 55 is connected to the Vacuum Purge Switch through the PPL wire, and the other side of the switch goes to ground through a BLK/WHT wire. You might put a voltmeter on pin 55 and see if that voltage switches when you turn the key off an on.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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One of the other tests I did was to disconnect both harness connectors at the PCM, turn the Ign on and flash the #24 wire to ground. The solenoid clicked each time I flashed the wire. With both leads of the DVM hooked to the solenoid connector, the only time it showed "0" volts was with the Ign off. I figured i'll leave the solenoid connector disconnected for now. That's it for today. I hope to get the shop to toggle the solenoid (or not) to test the PCM tomorrow.

To be continued.

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Ok, the shop used a Snap-on scanner to get into the computer. The Purge solenoid was being pulse width modulated at 22% at idle. He could increase the pulse width up to 100% and was able to toggle it on and off. The vent solenoid was powered up to vent. Then he checked the fuel tank pressure sensor to see if it was registering the vacuum as he triggered the Purge solenoid with the vent solenoid closed. The voltage reading slowly dropped as the vacuum went up. As soon as we removed the gas cap to readings showed the lack of vacuum.

He believes the PCM is not processing the inputs correctly and he feels replacing the PCM is the next step.

I might have missed a few steps but does it sound like a correct diagnosis? If so I will order a Reman. AC/Delco computer as a replacement.

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Ok, the shop used a Snap-on scanner to get into the computer. The Purge solenoid was being pulse width modulated at 22% at idle. He could increase the pulse width up to 100% and was able to toggle it on and off. The vent solenoid was powered up to vent. Then he checked the fuel tank pressure sensor to see if it was registering the vacuum as he triggered the Purge solenoid with the vent solenoid closed. The voltage reading slowly dropped as the vacuum went up. As soon as we removed the gas cap to readings showed the lack of vacuum.

He believes the PCM is not processing the inputs correctly and he feels replacing the PCM is the next step.

I might have missed a few steps but does it sound like a correct diagnosis? If so I will order a Reman. AC/Delco computer as a replacement.

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It sounds like the PCM is working fine to me. Was the code CURRENT during all this?

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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The PCM was bad. I ordered a AC/DELCO remanufactored unit and had it installed and programed. The drive home from work is 57 miles and no lights or codes. For some reason or other, the PCM was not processing the some input or changing some output. Maybe the software became corrupted like a desktop computer could.

Thankfully the components I replaced weren't to costly. I'm glad the Tech. I went to had some experence and saw this problem several other times. The PCM was $122 and the labor and programing was $145 plus tax. He said there were three software upgrades included in the software upload. All said and done, it could have been worse.

Thanks for your help and hope I can return the favor.

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Since the rest of the PCM seemed to be functioning, it may have been an electrical problem on the circuit in the PCM that senses the voltage on the EVAP solenoid. This can happen from back-probing the connector to the PCM with a voltmeter or ohmmeter that isn't designed for static-sensitive circuitry.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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I used a Fluke 88 to do my electrical testing so I think that shoud not have been the problem. I did the testing after the new solenoid did not solve the problem. In either case, the problem is solved.

I might have been one of only several with this problem. Does that make me special? (LOL). Anyway the wife is happy with her car, and I can move on the some of my other projects. Camper is next than electric brakes on the trailer.

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Well, I spoke to soon. The wife came back from getting the papers and the code 1645 came up again. I'm going to check out some info Cadillac Jim posted on the 14th. about voltage at pin 55 on the PCM. I have a dark cloud over me on this other wise beautiful day. The saga continues!

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