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61man · 08-22-2002, 08:06 PM

I know that this isn't a pontiac but here goes.
My 1997 Silverado has a ck. engine lamp on. I went to Auto Zone and they plugged in their code reader and it is code PO78S, Shift timing solenoid malfunction. My question is where is this thing located and what does it do. The truck is a 1500, with a 350 and 4 speed overdirve transmission. Thanks for your help

61man · 08-22-2002, 08:06 PM

I know that this isn't a pontiac but here goes.
My 1997 Silverado has a ck. engine lamp on. I went to Auto Zone and they plugged in their code reader and it is code PO78S, Shift timing solenoid malfunction. My question is where is this thing located and what does it do. The truck is a 1500, with a 350 and 4 speed overdirve transmission. Thanks for your help

TIAUTOMAN · 01-08-2003, 06:40 AM

I know this a while since your post.
But I have a question. Did you get it fixed and
what did they do.
I am a Heavy line Tech at a GM dealer. I have never heard of this code. Most common in a truck of that year is a P1870 Trans component slipping
this is caused by wear in the valve body, and the TCC valve sticking intermittently.

61man · 01-08-2003, 11:38 AM

I have not had the problem fixed yet, mainly because I can't find anyone that can fix it. The code is po785, 3-2 shift solenoid malfunction. I took it to a transmission shop and they replaced the 3-2 shift solenoid, it did not help. they also replaced other solenoids, no help I next took it to a chevrolet dealer and was told that the problem was with the wireing harness inside the transmission. Almost $500.00, It did not fix the problem either, and they would not refund the money. The dealer now says that the valve body is sticking when it is cold and they want to replace it. They will do this labor free, but will charge for the part. Almost $400.00. but will not refund the money if it doesn't fix the problem. I called the Chevrolet help line and got a refund for the first part, but I'm not convinced that the valve body will fix the problem.

The transmission, when cold, will hard shift into 2, shift into third normally, but won't go into overdrive. After I drive for a few miles and get everything warm, I can shut the engine off, restart it, and the transmission will shift normally for the rest of the day, or until the thing cools for over an hour.

http://kurtspontiac.homestead.com/Pre1964Pontiacs.html

TIAUTOMAN · 01-09-2003, 01:22 AM

im suspecting that you have no overdirve, due to a "limp mode" feature in the Trans/pcm. this will in hibit 4th gear.
You are being robbed. There is a harness that plugs into the trans to test, the connectors,selnoids ect. Not that hard to diag.

TIAUTOMAN · 01-09-2003, 01:24 AM

I have to look up that code, will do tommorrow at
work. but you say p0785 3-2 selnoid elec malfunction. Hmmm.......Anyother codes?

TIAUTOMAN · 01-09-2003, 01:26 AM

The Valve body replacement is a shotgun replacement. the reman valve bodys come with all new selnoids. Are you sure the orginal trans shop replaced the 3-2 selnoid? and didnt screw you. Its a two wire circuit.

61man · 01-09-2003, 05:40 AM

Yes I'm sure, The Guy is a friend of mine and didn't even charge me, the Chevrolet dealer also told me that it had an aftermarket 3-2 shift sol. in it and they couldn't guarentee it. On another forum, a fella told me that this happened to him and that the 3-2 sol. was orig. shorted. fed a sig. back to the computer and screwed it up so that it appeared that the problem hadn't been fixed, they changed the computer and the problem went away. The computer is under warenty for 80k miles but the dealer here won't replace it, says that thats not the problem, I'm not going to spend another $400.00 for them th shotgun the problem. Thats the only code that shows up also.

http://kurtspontiac.homestead.com/Pre1964Pontiacs.html

TIAUTOMAN · 01-09-2003, 06:59 PM

The PCM is a posibility.
You say you have a buddy at the dealer. Is he a tech?
Well there is a bulliten for the PCM losing the ground to the pressure control selnoid.
there is a updated PCM or a ground install kit.
If you have a buddy there. Have him "diag it as a failed PCM, and install a new one.
Or have him install one and try it out for a few days. If it doesnt work swap em out.
Unfortunatly that is a more routine mode of diag.
The 02' and 03 are worse. Manuals are wrong, Diag flow sheets are wrong or missing info............

TIAUTOMAN · 01-09-2003, 07:00 PM

one more thing. The dealer has a tech 2.
this the hand held scanner. It will show a shorted, or open selnoid. on the Info screen for the trans.

61man · 01-10-2003, 05:44 AM

I remember that they did check for the updated ground kit, and said that it allready had it on the truck, My buddy doesn't work for the dealer, he has his own trans. shop along with his Dad who was a GM trans. mech. for 25 years, I can't get the dealer to try the PCM, they keep telling me that it's not the problem

http://kurtspontiac.homestead.com/Pre1964Pontiacs.html

Bob87@intechnetwork.com · 06-04-2006, 11:43 AM

Were you ever able to get this fixed? I know its been three years, but I just bought a 98 GMC Sierra with the same darn problem.

Thanks

Bob

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