Your 1999 Nissan Frontier pickup truck has either an automatic or a manual transmission. The procedures for troubleshooting each type of transmission are described below. Not all of the problems you can diagnose can be fixed by a do-it-yourself mechanic. However, where the fix is something you can do, the corrective action is described. In the other cases you will need to take your vehicle to a qualified mechanic, transmission repair shop or dealer maintenance department.
Begin your troubleshooting by checking the level of the automatic transmission fluid. A low fluid level can cause the transmission to slip and the vehicle to lack power. Too high a level can cause foaming and loss of fluid. Add -- or drain -- transmission fluid to bring the fluid level to the hatched area below the "Cold" marking. If the vehicle was driven recently, the level should be in hatched area above the "Hot" marking. Drive your vehicle to see if this has corrected the problem.
Wipe transmission fluid from the dipstick with a clean rag and look at its color and smell it. If the fluid is a dark color, rather than reddish, and smells burnt, this indicates an internal problem that will require professional service.
Check for shift linkage problems. These will be indicated by the shift indicator pointing to a gear other than the one the transmission is in, being able to start the vehicle in a gear other than "Park" or "Neutral," or the truck rolling when in "Park." These conditions can be fixed by adjustment of the shift linkage cable.
Drive the vehicle and see if it does not downshift with the accelerator pressed to the floor. If this is so, the throttle valve cable (four-cylinder engine) is out of adjustment or broken; this can be fixed. A same problem with a six-cylinder engine means a faulty transmission pressure control solenoid valve or throttle position sensor. A defective throttle position sensor causes the "Check Engine" light to come on. Correcting the problem requires an expert.
Clean the transmission using a power washer. Drive the vehicle a short distance at low speeds. Place the vehicle up on jack stands using a jack. Examine all the joints and penetrations of the transmission for transmission fluid leaks. Brown fluid is probably engine oil. You can stop most leaks by tightening bolts, replacing the pan gasket or replacing the speed sensor or filler pipe seals. Fluid leaking from the vent pipe may be from a too-high fluid level or from coolant in the transmission. You can drain some transmission fluid from the transmission. The later situation means a professional transmission repair.
Test the operation of the transmission by driving in different gears. Noise, failure to shift, refusal to go into a gear, rough shifts and slippage are all matters to be dealt with at a transmission repair place.
Place the vehicle on jack stands using a jack. Look for transmission fluid leakage, particularly at the transmission oil seal. Loss of lubricant at the transmission oil seal means you need to have it replaced. Check the lubricant level in the manual transmission by removing the fill plug -- on the top of the transmission -- with an adjustable wrench. Put your little finger into the hole to feel for lubricant. If you cannot feel lubricant, add transmission lubricant using a syringe or squeeze bottle until you can. Replace the plug and lower the vehicle.
Check how the gears engage and disengage with the engine running. Problems may be with the clutch out of adjustment or in the shifter mechanism. These are problems you can correct.
Drive the vehicle in the different gears. Noise in any or all gears, or in "Neutral," indicate internal damage to the transmission. You need to have the repair done at a transmission repair shop.
