99 Suburban Transmission Shifting Problem

[Crosst]

There is a problem with my transmission where it seems to be in what is called sometimes as a "limp home" mode. Seems it works fine for a bit and then goes to 2nd only and starts there never shifting up or down. Maybe it is 3rd. I don't know. Anyway it doesn't shift. Reverse is good. I pulled a transmission code p0785 which seems to be "generic shift/timing solenoid malfunction". I noticed the fluid is just a little bit low. The transmission only has maybe 25k or 30k miles on it.

What is this likely to mean?

Thanks
Todd
99 sub

[777stickman]

P0785=3-2 shift solenoid electrical. This means that there is an electrical issue feeding the 3-2 solenoid or the solenoid itself.

1. Replace the "Trans" fuse #20 10a in the I/P fuse block even if it looks good.
2. Inspect the elec connector at the tranny for bent, pushed back pins or corrosion. (Corrosion in Tuscon??)

[Crosst]

Thanks. I will do that. This problem happened for the first time on the last 10 miles or so of a trip in the snow where I used 4 wheel drive for at least 100 miles in and out of Flagstaff.

Todd
99 k1500 sub

[trannyman52]

along the same train of thought ,,,a shift solenoid,coil failure (short) will give the same problem...easy enought to get to,,,would replace them both if you go that route

[Crosst]

Thanks Guys

I replaced the trans fuse, but the same result. Also topped the ATF up. No change. I start the car and it maybe goes a few mile normally shifting and then it will clunk on an hard upshift and it will then go to limp mode.

The solenoids can be accessed via removing the pan?
Is there a possibility for a little DIY help on that?

Thanks again

Todd
99 Sub k1500

[Crosst]

Also, I still have to get it up in ramps to check on the electrical connector on the trans. Maybe there has been corrosion from my 50 miles trailing a snowplow on the az 17 few weeks back.

Todd
99 k1500 sub

[j cAT]

Originally Posted by Crosst

Also, I still have to get it up in ramps to check on the electrical connector on the trans. Maybe there has been corrosion from my 50 miles trailing a snowplow on the az 17 few weeks back.

Todd
99 k1500 sub

I would check the connector for damage and being properly installed. then drop the pan and check that the connections to the solenoids are proper..

this transmission is new , or an old rebuilt transmission ?

If rebuilt the solenoids will be old, as well as many many other components.

[Crosst]

By the connector, do you mean the plastic box the wiring is plugged in to? I went to clean the connections, and they were stuck. When I pulled on the wire connectors, the plastic flat box attached to the trans cracked apart. Looks like I will replace that. Looks like the linkage goes though it too.

Any recommended parts sources?

Thanks
Todd
99 k1500 sub

[j cAT]

Originally Posted by Crosst

By the connector, do you mean the plastic box the wiring is plugged in to? I went to clean the connections, and they were stuck. When I pulled on the wire connectors, the plastic flat box attached to the trans cracked apart. Looks like I will replace that. Looks like the linkage goes though it too.

Any recommended parts sources?

Thanks
Todd
99 k1500 sub

when the transmission repair is done too many times the connector/wiring gets damaged.

scrap yard

[Crosst]

The "plastic box" is the neutral safety switch which I have now replaced. It wasn't the issue. Where is the electrical connection to the trans that would control the operation?
Thanks

Todd
99 k1500 sub

[777stickman]

Should be a round connector. 12-13 wires on the right (pass) side of the tranny for a 4L60-E (M30), or on the left side for a 4L80-E (MT1).

As far as DIY, check out this site for a subscription.

http://alldatadiy.com/

[j cAT]

Originally Posted by Crosst

The "plastic box" is the neutral safety switch which I have now replaced. It wasn't the issue. Where is the electrical connection to the trans that would control the operation?
Thanks

Todd
99 k1500 sub

the transmission harness runs along the top of the transmission front to the rear. then this branches out . the harness connector to control the transmission is round and plugs in on top of the transmission on the passengerside. the smaller connector on that side is the vss sensor.

park neutral switch and back up is on the driverside.

[trannyman52]

shift solenoid issue ,,,if you are going to buy "stuff" you don't need,,,,or things you think you need ,,,,,,replace them first,,if you can't find at like auto zone ,,,available on ebay or tranny shop,,,worst--dealer,,then keep replacing stuff you don't need..they are to 2 square ones on back of the valve body,,,check all internal connectors that they are seated

[Crosst]

Thanks for the help. I had the solenoids replaced and all is well now.

Todd
99 k1500 sub

[j cAT]

Originally Posted by Crosst

Thanks for the help. I had the solenoids replaced and all is well now.

Todd
99 k1500 sub

with this correcting the problem , this clarifies what type of transmission repair was done prior to this failure. transmission tech did a cheapo job.