1990 Transmission goes to nuetral over 2k rpm

[mentel]

I have a 1990 Lumina with the following problem:

Whenever your driving, and have even a little bit of load on the trans, as soon as you get over 2k rpm, the tranny feels like it is trying to shift to the next gear, but then kicks into nuetral. When you let off the gas, it will engage to the next gear, and if you baby it, you can eventually get up to speed, but as soon as you put down the gas to accelerate, it happens again. This happens during shifts in all gear, at all speeds. It engages smoothly from park to drive or nuetral.

I have removed the pan, and there doesn not appear to be any parts in the bottom of the pan, nor shavings, or particals.

Anyone know what could be causing this? I am going to change the filter/oil, but other than that I am at a loss.

[jeffcoslacker]

I think yours is old enough to still have a vacuum modulator. It would be down low on the side of the tranny, facing the radiator, a small metal can with one vacuum hose running to it. It may have a green or red or other color srtipe around it. If the diaphragm inside is blown, or if the hose is cracked, it will cause this. If the hose appears to be intact, pull it off the unit and see if tranny fluid drips out of the hose. If so, the diaphragm is blown, and you need a new one. They are less than $15, and there is a single bolt bracket that holds it in place, like a distributor.

[jeffcoslacker]

The modulator uses engine vacuum to tell the valve body when to initiate a shift, and if the vacuum is low, like when you open the throttle, and there is a leak, as soon as the shift begins the load changes on the motor, changing the vacuum at the modulator, and the tranny becomes confused, not wanting to upshift until you close the throttle and a strong vacuum returns. So you get that where you have to feather the throttle to get it to shift, and it wants to jump outta gear when you accelerate.

[mentel]

I'll check that out and report back.. Thanks.

[mentel]

Nope. I removed the modulator and it does not have oil coming out of the vacuum port, and it seems to hold a vacuum just fine. Any other ideas?

[jeffcoslacker]

Did you check the hose at the motor end? They had a habit of cracking or swelling at the point where they connect to the motor. Usually a small piece of hose at the end of a steel line. The lines fitted together, one inside the other with a short piece of vacuum tubing to sleeve them and hold them together. When that tubing got old the two lines would just seperate and come apart.

[mentel]

Ok, I opened up the filter, and saw this inside (see pic). Anyone know where the white material comes from?



I would like to add that sound you get when your power steering fluid is low comes from the engine compartment, even tho the power steering fluid is full.

[350TPI]

My tranny is doing the same thing except i cant find the vacuum modulator.I have been looking for about half an hour.Can i see it from the top after removing the breather or do i need to get under it?I have done both actually with no luck.I also have a 90 model that i was ready to take to the junkyard and hasnt been driven in many months.Any help very much appreciated.

[jeffcoslacker]

Originally Posted by 350TPI

My tranny is doing the same thing except i cant find the vacuum modulator.I have been looking for about half an hour.Can i see it from the top after removing the breather or do i need to get under it?I have done both actually with no luck.I also have a 90 model that i was ready to take to the junkyard and hasnt been driven in many months.Any help very much appreciated.

Look at the back of the motor(toward the tranny) there will be a line small steel line coming off the back of the intake manifold, that goes down toward the tranny down near the pan, on the front (toward radiator) It is possible that yours doesn't have one. It was right around there when they did away with the vacuum modulator. But if it has one, you should be able to see it from the top, although it is down low on the case. Look for that steel line, that will lead you to it. Maybe 1/4" line or smaller diameter.

I don't know what the white plastic is. The only plastic I ever remember finding in them was the remains of the dust plug, which gets shoved through into the pan at the factory when they drop the dipstick tube into them, but that was on RWD cars. I imagine the stick comes installed on these. I have a friend that was a service manager at an AAMCO franchise, I'll try to ask him.