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The Gentleman's Express: Tech-Tips from the JIOC

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Transmission Road Test Check

Author: Autoweek, 08-31-81

Like the gourmet chef who sniffs even the finest cut of meat prior to cooking, a good mechanic uses his olfactory bulge for more than just an eyeglass rack. Every time you check the fluid in your transmission you should sniff the end of the dipstick as well as making sure the fluid level is between the full and add marks. Foul smelling automatic transmission fluid is an early warning symptom that can tip you off to impending trouble.

If caught in time a simple change of automatic transmission fluid, a new filter and pan gasket can go a long way toward preventing major transmission repair expense. If neglected to the point where the transmission fluid turns a dark mustard color and a sniff of the dipstick causes your nostril to curl in protest, it is just a matter of time before you can expect major repairs that probably will require removal and complete disassembly of the transmission.

The best test of how well a transmission is behaving can only be done on the road. For purposes of demonstration, let’s talk specifically about the General Motors Turbo Hydramatic 350. Over 22 million have been built to date, so chances are your transmission is either a 350 or one that shifts very much like the 350. Start your road test for a transmission performance from a dead stop on a quiet, straight road where you aren’t likely to run into too many lightrunners or haywagons. Once you’ve allowed the engine to warm up, gently accelerate using a minimum throttle.

The first shift, that is called the 1-2 shift, should occur at approximately 9-14 mph. Continue your normal acceleration until you feel the 2-3 shift at about 15-22 mph. Then, while you are coasting along at about 35 mph, move the shift selector to the L2 or S position, depending on which model car you are driving. You should feel a change as the transmission drops out of third gear into second. Shift back to D, and again you should feel a slight change as the tranny shifts back up to third. This is more of a free rolling feeling than an obvious shift change. In the L2 or S position the tranny will provide what is called overrun braking, a sort of built-in braking system that will help to hold the car back on hills where excessive braking could overheat the brakes.

Again rolling along at about 35 mph with the selector in the D position, floor the accelerator pedal. You should feel the engine roar as the transmission downshifts and the car bolts ahead. If you don’t get this ‘detent downshift’ to occur after a few tries it may mean the transmission has not upshifted in the first place and you have been driving in second gear, a condition that, if left uncorrected, will cause premature tranny wear and knock a massive hole in your gas budget. The speed at which the tranny shifts is directly related to how hard you depress the accelerator pedal. For example, the 1-2 shift will not occur on a floored accelerator until the car hits between 45-55 mph. The 2-3 shift won’t happen `till you’re heading down Boot Hill highway at between 65-86 mph.

If you find the car stays in low gear and are sure the fluid level is where it belongs, there are some checks you can make before surrendering those hard earned bucks for a tranny overhaul. Make sure that none of the linkage has jammed or become disconnected, and then check the vacuum supply to the transmission modulator. The modulator looks like a metal inline gas filter and is screwed into the right side of the transmission. A vacuum hose is connected to it. For the tranny to upshift engine vacuum must be present in the hose. Check this by disconnecting the hose at the modulator and, using a vacuum gauge, check for the presence of a vacuum. Traces of oil in the end of the hose indicate a leaking modulator that eventually will empty the transmission of its fluid and may cause spark plug fouling combined with blue smoke from the tail pipe.

From time to time transmissions are overhauled and even after a complete tuneup the car has no power, yet there is obvious transmission slip. Should this happen to you, consider the possibility of a clogged exhaust pipe. There have been enough cases of the inner shell of an exhaust pipe distorting to block off the exhaust gases, and a contaminated catalytic converter will do the same, that it is worth checking whenever transmission work is considered. When checking your car over, always take a look at the condition of the coolant in the radiator. An oil foam present in the radiator neck may mean the transmission oil cooler located in the base of the radiator is leaking. This will allow coolant to enter the transmission and ruin the clutches. It may also cause engine overheating and destruction of the radiator hoses.

A few extra moments of thoughtful checking of the transmission dipstick and the fluid condition may save you a costly transmission overhaul, or at least a breakdown on the road. So to know what’s cooking in the transmission, always sniff the dipstick!

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Contents copyright 2008, 2009 - Jody F. Kerr

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