Welcome to Chevrolet Forum - Chevy Enthusiasts Forums
Welcome to Chevrolet Forum - Chevy Enthusiasts Forums
You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, at no cost, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!
I can not remember ever running into this code before and since I do not work on the weekends I can not look in alldata to help you . I have seen may GM trucks with trans code caused by a bad ignition switch. I am going to PM you some info it is kind of long so I will not post it. However if you do not think it is too long then add it to the thread. It is information from IATN a very useful tool in repairing problem cars.
ASE Master Tech, L1 Advance Engine Performance certified
I will not answer repair questions by PM, if you would like my input you can PM me and ask me to reply to your public thread.
Thanks for the info, This explains the possibilitys well, I felt it could benifit others so I am posting it.
Originally Posted by MDTAHOE -via PM to me
Here check this out.
1997 GMC Jimmy, Electronic, 3-2 shift P0785
Tom from VermontNov 23, 2005 Replies
Hi Group, This vehicle has an intermittent Service Engine light with a stored P0785, when this occurs it has 3rd gear starts. Usually occurs after a cold start with short trips around town. Connections have been checked on the transmission and pcm. Grounds have been checked and VD tested. I have monitored the power feed on trans fuse as per bulletin#01-07-03-002c with no loss of power when the problem occurs. With a volt meter connected on the white #13 c1 wire on VCM when a 3-2 shift is called for the voltage drops near 0 but seems to stay low until the vehicle is cool again. I have serviced the trans and replaced the 3-2 shift solenoid but did not correct the problem. We have also amp flow tested the solenoid at .459 amps. The scanner shows the 3-2 downshift solenoid is activated at all times again only once in a while. I do find I can command the solenoid on and off with a scanner but the operation seems slow when watching the voltmeter. At this point I believe I am looking at a VCM problem but hope with your help I am not overlooking something else related. Any help would be great. Drop me a note with any questions or thoughts. Thank you
FIX posted on Dec 19, 2005
Thanks to all that replyed to this post. After performing a number of tests to rule out a bad ignition switch and other related faults I installed a new VCM, and had it flashed by the local dealer. After a number of test drives everything seemed fine. It's been gone 2 weeks now and the customer called to say everything is fine with the vehicle. Thanks again for all the help. I hope to return the favor in the future.
Bill from Massachusetts Nov 23, 2005 5:49:00 PMThere is a GM bulletin about adding or relocating a ground inside the PCM on Blazers around your year, but I'm not at my shop right now to get specifics. Contact your local Gm dealer, they might be able to look it up for you. Nothing else comes to mind, you seem to have covered all the external electrical bases already. Good luck.
Carl from Florida Nov 23, 2005 I have seen this before replace ign switch it has a singluar power supply to the soelnoids. Alot of times it is (the ign switch failure ) caused by a low charging alt or voltage leaking battery or internal shorted battery. The volts are there(until the load opens the circuit enough) but loses amps/then volts to run the soelnoids.The draw causes the soelnoids to "fall off " and cause 785. The Ign fails from the same draw, its the weekest link in the chain. If you have a Vat 40 or 60 then: ALT at least 14.1 , amps at least 60 % of 105 amps Battery hold a load for 10 sec @9.5 v then "bounces back to at least 12.3 in 10 sec" if not (no matter what alldata or mitchell says causes concerns with other systems).
Matt from Oklahoma Nov 23, Hi Tom, I believe Carl to be correct. I too have seen this same problem and it was the ignition switch. But to be sure here are the test you need to perform. This can tell you for sure if it is the PCM or the ignition switch.
1. Clear the code and install fused B+ to the Pink wire terminal E of the transmission and see if the code resets. 2. If the code does not set, move B+ to the TRANS fuse instead and recheck for the code to reset. 3. If it still does not set, replace the ignition switch. 4. If supplying B+ at terminal E of the transmission does not help, amp-flow test the 3-2 shift solenoid at terminal 13, the White wire at the Blue PCM connector. 5. If amperage is good, replace the PCM if connections are OK.
Miguel from Texas Nov 25, 2005 Measure resistance between ground and each solenoid teminal. If resistance is more than 250k ohms, check for short to ground in transmission wiring. Inspect wiring for poor connection at PCM and transmission 20-pin connector. Bent, backed out or damaged terminals should be check. Weak terminal tension. Check for a broken wire inside insulation. Diagnosing for intermittent short or open, move wire harness while observing scan tool for value change.
Wayne from Connecticut Nov 25, 2005 Tom, The 97 jimmys had a very common problem with shift solenoid problems. If you can't find anything else, the ignition switch could be the problem. 12 volts goes through the ignition switch and to the shift solenoids.As the switch gets old it becomes intermittent(12v) and solenoid problems appear. I have fixed 2 in recent months with an ignition switch...both were intermittent. wayne hackney Barry Klein Automotive Brookfield connecticut
Peter from Ontario Nov 25, 2005 make sure the input & output speed signals are there if one quits default is third , check ecm pins for corrosion good luck
Originally Posted by MDTAHOE
Here is another set
1997 Chevrolet Pickup, Electronic, 3-2 SOLENOID CODE PO785
By: Greg BaraksBy: Greg from TexasJun 26, 2002 7:42:34 AM3 Replies
Unit sets PO785 15 seconds after start-up. 3-2 Solenoid can be commanded on & off W/ tech2. Voltage @ PCM Terminal 13 (sol control) is B+ when off, and drops to .21 when on. Monitored voltage upon start-up, and remains @ B+ ( Does not change when code sets.) Checked Voltage @ Trans Fuse #20 and it does not go below 10.5 on start-up. It remains @ B+ through code set. Is this sytem failing some sort of self-test upon start-up? Trans operates fine otherwise. Thanks in advance....Greg
Related Repair History
Replaced 3-2 Solenoid.... No change
FIX posted on Jul 9, 2002 5:20:20 AM
Thanks for the responses, Both of which confirmed my suspicions that the PCM was at fault. I did not personally repair this vehicle, as I do not have re-flashing software to program the PCM and had to send the customer to the dealer. I called him 10 days later to find out how it went, and was pleased to hear that the PCM fixed his problem. It was done under the 8/80 emissions warranty, so he wasn't out anything. I may have lost my rear on this one, but I kept a customer. Thanks again, Greg
CancelReply 1Report Abuseok Harold from Maine Jun 26, 2002 10:48:38 AMHi Greg There is an early and late solenoid,differant resistence.If it hasn't been touched,there is a buletin on ign sw too,and the fure you already checked.Does it loose the PND light off and on.Switch if it does. Hope this helps Harold Field TRIPLE R TRANSMISSION 112 Basin Road New Vineyard, ME 04956 Phone: 207-652-2396 Fax: 207-652-2605 E-mail:
CancelReply 2Report Abuseok Frank from Oklahoma Jun 26, 2002 3:53:42 PMMonitoring the voltage at pin 13 during start up, better yet, don't start the engine, just do ignition on. The voltage should go to battery voltage, now watch it very close, the voltmeter will flicker during the self test. I use a LED that I've configred for test like this. If the circuit doesn't drop momentarily during the self test or just befor code set, then the driver circuit in the PCM is bad. I chase my tail on one of thes for half a day once, finally decided it was a bad PCM, replaced it, problem solved.
Now, if the voltmeter or LED flickers during the self test, that means the compter is commanding the self test, but doesn't like the current draw it sees, so you'll need to check that with an ameter.
CancelReply 3Report Abuseok Gary from New Jersey Jun 26, 2002 5:49:54 PMI have had this code show up several times after checking all wires solenoids fuses ect. Running my own power and ground to trans turned out to be bad ecm (PCM) 3 gm cars in one week try a know good pcm. Will the code set as soon as you put it in drive
ok i bought it with it in safe mode only 2nd and 3nd shots the 2-3 shift code when shifting from 2nd, replaced 1-2 and 2-3 soleniods and trans wiring harness, we have it hooked to scaner at trans shop and can get all soleniods to snap but the 2-3 will not. then checked the harnesss from the trans to the pcm same resistance as all the others, then checked voltage at soleniod 2-3 no voltage change when actavated with scaner but is constantly grounded from the pcm 4th pin on the red connecter at all time except with key off and the constant voltage to the solenoids with key on is above 10 volts, we figured the pcm is grounding the 2-3 solenoid keeping it on so we got a used pcm got it reflashed and same we think we got a pcm with same problem idk wat else now
My 99 tahoe has 1-2 -2-3 -3-4 then when i come to stop it stays in third gear. I have replaced all the shift selinoids in trans, put in new igtnition switch and coloum sitll not working. I think its the vcm. i am going to replace this weekend hope it works. if any one has any suggestion please respond to Timwachowski1@aol.com could use all the help i can get. I have already invested $500 and still not working. thanks
I am finally getting time to address this code I have with this sonoma , I am going to go through the possibilitys that MD Tahoe addressed, I will post as I go and hopefully pinpoint it, it shifts fine and runs fine, just still throwing the Shift solenoid code P0785.