99 Suburban Transmission Shifting Problem

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

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??)

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

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

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

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

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.

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

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

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

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/

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.

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

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

Todd
99 k1500 sub

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.

Just did this R&R on my 05 because the TCC wasn't locking up properly and it was throwing a 2761 code. After checking the connections and current from the switch through the PCM and down to the tranny the problem persisted so we changed the inner harness and both TCC solenoids. After a little driving the shift points have settled down (relearned) and the CEL went out by itself. So, it looks like the solenoids and harness did the trick.

Any time a tranny is rebuilt there is NO excuse for not changing ALL those solenoids. They are not very expensive and they always seem to be an issue later. It is a PITA when you're under the tranny and it's dripping fluid, but on a tranny removed and disassembled or upside down it's a breeze. I guess you have to demand it and have them give you the old solenoids (not that they couldn't hand you some other old ones). I think they total about $200 tops.

Just did this R&R on my 05 because the TCC wasn't locking up properly and it was throwing a 2761 code. After checking the connections and current from the switch through the PCM and down to the tranny the problem persisted so we changed the inner harness and both TCC solenoids. After a little driving the shift points have settled down (relearned) and the CEL went out by itself. So, it looks like the solenoids and harness did the trick.

Any time a tranny is rebuilt there is NO excuse for not changing ALL those solenoids. They are not very expensive and they always seem to be an issue later. It is a PITA when you're under the tranny and it's dripping fluid, but on a tranny removed and disassembled or upside down it's a breeze. I guess you have to demand it and have them give you the old solenoids (not that they couldn't hand you some other old ones). I think they total about $200 tops.

your right on with the solenoids being replaced at rebuild. those components need replacing with over 100,ooomi and the transmission fails., not much of an increase in cost when the tranny is apart.

It is ALL about money the job can be done from a couple of hundred dollars,,,,to thousands of dollars,,,,I in a lot of cases prefer the used component vs new because it is tried and true,,though some components must be replaced,,it is a judgement call based on the techs past experience,,and limitations the customer has put on the bottom line,,also have seen new parts fail in short order,,,cheap no name brand and high priced top of the line !!!!wish it was that simple,,after all,,,that is why they sell new cars,,,if they were perfect,,,why would you need a warranty ????

It is ALL about money the job can be done from a couple of hundred dollars,,,,to thousands of dollars,,,,I in a lot of cases prefer the used component vs new because it is tried and true,,though some components must be replaced,,it is a judgement call based on the techs past experience,,and limitations the customer has put on the bottom line,,also have seen new parts fail in short order,,,cheap no name brand and high priced top of the line !!!!wish it was that simple,,after all,,,that is why they sell new cars,,,if they were perfect,,,why would you need a warranty ????

new parts or new vehicle does not mean it will last...even with a new vehicle warrantee these new GM trucks have many owners wishing they never got rid of their old truck ...with transmission and engine erraticly working, the dealer response is they all do this ...