Corroded Transmission Dipstick

Late last Summer, the automatic transmission on my 96 Jimmy was losing about a quart of fluid a month. After the weather cooled down, the problem went away. I noticed at the time that the upper part of the dipstick was corroded. The transmission did not seem to be shifting properly today (it would kind of thunk at times) so I checked the fluid. The level was OK, but the top part of the dipstick was corroded much more than before.

Also, the check engine light has been coming on periodically (once or twice a year) and the code indicated something about the throttle position. It happened again yesterday, but this time when it was reset, the light came on again right away. I don't think these two problems are related, but maybe they are.

Anyone have any ideas what may be going on?

What's the deal with the throttle position? Is this something that can be fixed by replacing a part, or do I need to take it in to a shop?

Have your radiatorand integral tranny cooler pressure tested. You might be leaking coolant into your transmission.

Trottle Positions Semsor TPS is about $20 at most parts stores. Easy to change.

Clean the throttle plate.

I've dealt with the exact same problem many times. Here's the deal... The dip stick gets rusty and crappy and the particles find their way down into the valve body inside the transmission. This is a well known problem that these transmissions have defective valvebodies in them. You getting a check engine light because the computer is sensing tranny slippage from the hard shifts. If you check the code history I'll bet its the code for slippage. The fix is to replace the actual dip stick itself and get a engine brush and run it through the dipstick tube to clean it out good. The next step is to replace the valvebody with one that has the revisions and updates done to it. AC Delco has a revised valve body available but is quite expensive. I suggest to my customers they buy them from Valvebodies Express or Trans Star in Philly Pa. You will be very happy after doing this as it will feel like a whole new animal. Expect to spend 300 bucks on the valve body or 800 from the dealer. But thats what your problem is. Nothing to it but to do it

I've dealt with the exact same problem many times. Here's the deal... The dip stick gets rusty and crappy and the particles find their way down into the valve body inside the transmission. . . . The fix is to replace the actual dip stick itself and get a engine brush and run it through the dipstick tube to clean it out good. . . .
I have the same problem on my '95 4x4 with the dip stick getting rust and particles on it. However, I don't have a problem with the transmission, which was dealer replaced or rebuilt on warrantly in 1996.

If I replace the dip stick will the new one get rusty too?

In short Yes, you will have the dipstick problem again unless you take action to prevent it this time around. The corrosion happens for two reasons. The first being the constant hot cold cycle the transmission goes through and the air pocket inside the dipstick tube developing condensation is most likely the source of the moisture and the second reason is just simple neglect from us truck owners. Most people rarely check the trans fluid level regularly and most only if they think there may be a problem. I have adapted my maintence to hopefully prevent this by regularly checking the fluid and keeping a thin coating of a rust preventive on the stick like WD-40 or something like it. This is also a good idea because the O-ring at the top of the locking style dipstick found on many of GM's cars and trucks from these years becomes frozen in place from drying out and almost never seeing the light of day usually. I have had some sticks stuck so bad before that I've had to soak them in WD-40 just to get them to come out. I now religiously keep the O-rings oiled to prevent this also. The other primary concern of course isn't just whats on the dipstick itself but also the rust that has developed inside the dipstick tube as well. I wouldn't recommened disturbing this rust unless you can have the trans pan off and run a engine brush through it to remove it. Otherwise if you mess with it, the rust will only contaminate the fluid. Next time you have the trans serviced you can have this attended to at that time. I would say just replace the whole dipstick assembly but on these trucks its a difficut job and is not easy as it is on most trucks.