Q: I own a 1980 Mercedes- Benz with a diesel engine. It runs well but the transmission is weak. I would like to keep this car going and I’m wondering what causes a transmission to become weak. – D.T., Chicago

A: There are many factors that cause an automatic transmission’s performance to degrade over time. To help you understand what can happen to an automatic transmission, I need to explain some basic principles.

Automatic transmissions use hydraulic and mechanical systems to transmit power and torque to the drive wheels. A vehicle equipped with a typical automatic transmission will not move forward or backward without any hydraulic pressure. That hydraulic pressure is generated by the transmission’s engine-driven pump. The amount of pressure the pump generates can diminish over time due to wear.

Low hydraulic pressure obviously affects transmission performance and will lead to premature failure of mechanical components like the friction units that drive or hold various members of a planetary gear set. Also, the rubber seals inside the transmission that help hold hydraulic pressure in its proper place can harden, crack and fail. When that happens, performance is affected and mechanical components suffer premature wear, too.

Your car’s transmission likely needs a complete overhaul. Another option is to replace it with a rebuilt unit. But before you do, consider how many miles are left on the engine and chassis. You won’t want to drop big money for transmission repairs only to find out a few months later that the engine is failing.

Q: Each time I take my 1999 Ford F-150 4x4 (4.6-liter V-8) to Firestone for its oil change, I notice that they use 5W-20 oil instead of recommended 5W-30 oil. I want to know if this will harm my truck’s engine. – D.D., Park Forest

A: Don’t worry. While 5W-30 was originally the recommended oil for your truck, Ford issued a service bulletin (article No. 02-1-9) dated Jan. 1, 2002, saying it now recommends 5W- 20 oil in a number of vehicles, including your truck.

Q: In a previous column you talked about a crankshaft sensor and ignition module as possible causes of an intermittent no start. I think the same thing is wrong with my ’92 Buick LeSabre. I’m on a fixed income and was wondering how much it might cost to replace my car’s crank sensor and ignition module. Do you know? – M.F., Elk Grove Village

A: It depends on where you have the work done and the quality of parts installed. In my estimation, it will cost $200 to $300 to replace the crankshaft position in your Buick. Add another $300 to $400 for the ignition control module.

Q: We have two vehicles and we’re planning on selling one. One is a 2001 Grand Caravan (120,000 miles) and the other is a 2004 PT Cruiser (76,000 miles). We were just curious if you had an opinion on which one would be better to keep for the long haul. – J.S., Steger

A: If it’s just you and your spouse, you might want to unload the minivan. However, if you need seating for seven and/or like the size of a minivan, dump the PT Cruiser. You might consider taking both vehicles in for a thorough inspection and decide to keep the one that needs fewer repairs.

Questions for Ira Siegel? Call the Auto Advisor Hotline at (708) 633-6839 or e-mail autoadvisr@aol. com. Siegel is an automotive instructor and an automotive service excellencecertified master auto technician.