My 1999 Buick LeSabre, with 86000 miles on it every now and then the transmission will bump. I have changed the filter in the fluid, and that helped some what. I have no check engine lights, and sometimes you can drive the car all day and it won’t do it and other times it’ll do it 10 times in a row. I have noticed when the bumping occurs the tach goes up a small amount and then drops right back down to where it was. And I have noticed when I drive in 3 it does not do it as much. When I drive in the D it does it quite a bit. the car other than the bumping runs perfectly, I have been driving the car in 3 for about five hundred miles now, and with the age of the car I really don’t want to rebuild the transmission until it is absolutely necessary. If anyone has any suggestions I would appreciate it and as always thank you for your time.
Since the bump occurs in D but not in 3, that indicates a slightly rough engagement of the torque converter clutch. I would suggest changing the fluid again, and adding a bottle of “Lucas transmission fix” to the fresh fluid.
Try the torque converter trick that Old Mopar Guy suggested. first.diy repair since the transmission has to be lowered to access the solenoid. If that doesn’t cure it then it’s probably a shift solenoid. Seems to be common to these cars. It’s not really a DIY since the transmission has to be partially removed to reach the offending part. Kinda pricey, too. If you’re keeping the car for a few more years, maybe a tranmission rebuild would be a good plan. Take it to a transmission specialist for the best advice.
Good luck with the old girl.
Thanks for the. Help
Does it hurt to drive the car like that, my plan was to just keep driving it until the transmission crapped out, and then look for a used one or have it rebuilt. But if it’s a selenoid would that make it worse and more costly if the solenoid just totally craps out?
It depends. If the bump is harsh engagement of the converter clutch, and it’s due to low fluid pressure caused by a dirty solenoid screen, the only harm would be converter clutch failure. Might cost you 1 or 2 mpg in highway driving, no big deal. If on the other hand, the bump is caused by harsh engagement of the direct clutch, it could ultimately result in total failure. Just keep it in D3 and you should be ok. If you take any long expressway trips, shift it into D over 50 mph.
What if the failure time was while turning in front of a semi-truck ?
I see your point
thank you for your information, I have noticed today,put over a hundred miles on the car and it only made the bump two or three times seems to be getting better after the transmission oil and filter change I did put a can of Lucas transmission fix in it when I change the transmission oil I did today what you suggested shifted into d on the highway and kept it in three while driving in town. And the bump seems to be getting less severe I’ve noticed. One thing I forgot to mention in my original post was, the car sat for about 6 to 8 months of no driving, and before it sat I notice no bump at all ever. The bump started right after I started driving it again after it’s sat. It took me 6 to 8 months to figure out that the car needed a crankshaft sensor and a new ECU which I put in along with the crankshaft sensor. Once again thank you for your time and help I do appreciate it
It’s been a while but they used to say you can eliminate the lock up converter as being the problem by unplugging it as a test. I believe it is the four prong connector under the hood. I don’t think it is a good idea to leave it unplugged but by putting it in 3rd, I believe you are cutting the lock up out as well as OD. They tell me the 4th gear clutch is very thin and prone to failure but actually I’ve had more problems with torque converters and once they go, the trans has to be overhauled to get all the particles out of it. Running in 3rd though will heat the transmission up more.
Thank you for the information, it does still act up even in 3, but it’s not terrible, drove it today about a hundred miles just two or three minor bumps. If the transmission has to be rebuilt anyway, I may as well drive it till it gets a little worse. And when it gets really bad that I don’t feel safe, make the decision to get a rebuilt, or a used, transmission. Or send the old girl to the junkyard. As always thank you for your time, and thank you for your reply I certainly do appreciate it
An unstable Throttle Position Sensor signal can cause the torque converter to unlock and lock resulting in this type of RPM change. The TPS signal can be monitored with a scan tool.
A malfunctioning brake light switch can also cause this but then the cruise control would not work.
Thank you so much for replying, yesterday while driving the car I did notice when the when the car bumped I had the cruise control on it immediately disengaged the cruise control. Might have just been coincidental but that’s something for me to think about anyway and watch. But it didn’t do it every time because I had to cruise control on another time and it bumped and it didn’t kick the cruise control off. I did put my cheap scan tool on to look for codes, in the car currently shows no codes. But my scan tool is admittedly very cheap. As always I do want to thank you for responding, your information is extremely helpful and thank you so much for your time, I really do appreciate it.
I hope you guys aren’t getting sick of this post, but I’ve noticed a new wrinkle. I noticed when I start the car drive out my driveway about a quarter of a mile and when I go to open my gate I put the car in neutral I have noticed the engine idles down then there’s a audible click and the engine revs back up to normal idle speed. Now I must assume if this does that while I’m driving down the road of course I wouldn’t hear the audible click what could that be something electrical that’s causing my problem. But just yesterday I took it to advance and had them put the scan to want it and there’s no codes the check engine light is not on and I have changed the transmission pull fluid and put Lucas trans fix in and that has helped but has not eliminated the problem any ideas on click sound I hear and all this happens in less than a second. And as always thank you for your time and help.
That click and change in RPM sounds like a solenoid that opens the throttle to compensate for the AC compressor coming on. Does this also happen with the AC and defroster turned off?
Yes. It does, and I’m wondering if at speed that could be causing the bumping that I feel in the transmission
The Converter will engage in D3.
Thank you for your response,and your time