99 Venture Transmission not shifting @ high speed

[ul00173]

1999 Chevy Venture Wagon is not down-shifting when going up a hill at 70mph. As we drive up hill, in drive, the car begins to slow down because torque is not enough to move the car up the steep grade at same speed. Instead of the car automatically shifting into 3rd gear, it just continues to slow down. If we press down on the gas pedal, this does not induce the downshift properly. This is a new problem that just occurred for the first time about one week ago. The transmission appears to be shifting properly at all other conditions when not at high speed. Any suggestions?

[henry10]

I am new member and just got a 99 Venture, but I have had experience with other headache transmissions, including GM's.

Possible causes of downshifting failure are several. I would start troubleshooting with the easiest and cheapeast -- that is -- I would check transaxle's fluid level, color and smell. You should also check the wires to and electrical connections at the tranny for corrosion or looseness. Just get the easy things out of the way first.

If everything looks fine then I would get a tranny troubleshooting guide with box flow-charts and I would check for codes stored. The codes might lead you in the right direction. When a shift solenoid malfunctions, it usually sends a malfuncton code, but it may or may not illuminate the "Service Enginee Soon" light.

Downshifting is controlled by the PCM, but is carried-out by the shift solenoids. Older trannys were shift-controlled by the throttle valve cable, which was adjustable. In PCM's controlled transmission, based on pre-programmed tables, PCM takes input from various sensors and sends a signal to a specific solenoid to open or close the fluid flow. Since downshifting for you happens in other speeds (gears) it means that inputs most likely are doing their job, which means you probably have a problem with the output (i.e. solenoid and or that specific clutch/band/spring/plate/seals). In 9 times out of 10 you will have a solenoid failure before you have a mechanical failure. Which is good because solenoid repair procedures are a lot easier and cheaper than failures with clutches, springs, plate, etc, which require a re-build.

But again, you should start with the easiest and cheapest (see above), then read codes and review the troubleshooting flow-chart, then report back to the forum.

DO NOT go to tranny's shops before trying first to change the fluid AND FILTER. Do not backflush. Just drop the pan and replace the filter.

HEADS-UP: I have read that based on surveys, if you visit transmission shops you have a 80% chance that the shop will recommend a re-build, 70% comply, 10% deny. However, out of 80%, 50% should be re-builds, 30% could be fixed with partial repairs. If you end up going there, preparation and knowledge will help your pocket and grief. Did I mention that 75% of re-builds do not get done right the first time, but return for 2nd or 3rd visits or more?

[ul00173]

Thanks Henry10...

I did resolve my transmission issue. It turned out not to be the transmission after all. This is what I did: went to Transmission Repair place. They suggested a $50 assessment, but before I took that step, I went to my trusted car repair specialist that has done work on my car in the past. He said that there could just be engine operation issues that might be causing this poor performance. Engine Light was on, but always is in the Venture! After his evaluation, we opted for an EGR valve replacement, as well as fuel filter, air filter, and spark plugs. Total Cost $650 (a little high, but they have to tilt the dumb engine forward to change plugs....and I don't usually do that myself). Better than a rebuilt Transmission at $2000.

Car basically got a great tune up, and works perfectly now. Engine runs much better, and smoother, without any roughness. High-speed downshifting is great, and I guess this was because the exhaust recirculation got fixed with the EGR replacement (indicated by codes in the Check Engine Light function).

Thanks for taking the time to educate me on the Transmission.

thx. greg

[henry10]

Whew.... nice.