Weird Things Happen Every Day

by Mike Souza (ATRA Memeber)
Gears Magazine December 2011

Once upon a time, when a car came in with a transmission problem, there was a really good chance there was something wrong with the transmission. Batteries? Tires? Tailgates? Those were someone else's problems... they certainly had nothing to do with the transmission operation. 

These days? Well, weird things happen every day. Like transmission problems caused by low voltage, poor cable connections, or loose or dirty grounds. And to make things even weirder, the battery may still start the engine without being jumped or the terminals cleaned. 

So how do you deal with transmission problems caused by the battery? Start by cleaning and tightening the battery terminal ends before chasing electrical codes. Even if the terminals look good and the vehicle starts without a problem. And if it looks like this one (Right, below for mobile readers) don't even think of going any further until you clean the terminals and check the voltage. This particular instance was a 41TE where a shop worked on it for two days for solenoid codes.        

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Battery Weirdness

Here are five examples of cars with battery problems that affected transmission operation:

1. A Saturn equipped with a TAAT transmission had a complaint of hard shifts. There were no codes stored in any module. The battery voltage was low, but the engine started without any problems Charging the battery corrected the hard shifts. 
2. 2004 Saturn Vue with the MDRA Honda-type 5-speed transmission had a slip or delay on forward engagement. It sometimes fell out of gear at stop or defaulted to second-gear starts. The wrench light on the dash would light sometimes, but there were no codes present. Checking the battery revealed it would develop only 11.9 volts; replacing the battery fixed the transmission problem.
3. Dodge or Chrysler vehicles cycle in and out of lockup or 4th gear without any codes stored. This symptom appears most often at light throttle, between 35 to 45 miles per hour. There's a list of 22 or more issues that can cause this complaint, but the most common is loos or dirty battery terminals and poor ground connections. 
4. Ford trucks equipped with a power takeoff (PTO) unit ; the PTO stops working after a transmission rebuild. There's no problem with the battery voltage or connections; the problem was cause when the battery was disconnected during repair, causing the computer to lose its memories. TO correct it, you'll need to drive the vehicle for at least 7 miles at speeds over 50 MPH for the computer to relearn PTO function. 
5. Late model Mercedes vehicles with a weak battery or a replacement battery that won't provide adequate current for the starter. This can cause pump bushing failure because of the additional starter draw through the transmission bell housing. 

Tire Weirdness

And the weirdness doesn't end with the battery: Weird things can happen to the transmission because of tire problems, too. Problems such as low pressure, wrong size, or mismatched brands can cause a range of "transmission" problems. 


For example: A BMW equipped with either a 4L30E, or ZF 4- or 5-Speed transmission won't upshift to or past 4th gear after driving on the highway. The transmission receives commands to only 3rd or 4th gear, with no codes stored. 
This particular vehicle had only 42,000 miles, and the right front tire was just changed because it went flat. The spare was new and had no mileage on it. The difference in tire size caused the computer to think the vehicle was in a high speed turn. The curve recognition software would downshift the vehicle to a lower gear and wouldn't allow the transmission to upshift past that gear. This is a safety feature to prevent the driver from losing control of the vehicle on high speed turns. The quick check for this problem was to swap the tire from the right front to the right rear. 
There' a list a mile long of all the issues tires can cause on 4-wheel drive vehicles. The most common complaints are clunks noises, and binding on turns, to name just a few. Use a stagger gauge to check each tire's size when working on these vehicles. An alternate check would be to mark each tire at the bottom with chalk, then roll the car straight until each tire makes 10 revolutions. All 4 tire marks must be within 1/4" of one another. This method might not yet yield accurate results, as even a slight variation in wheel direction can affect the tire position.

Another quick test: If you have a sand or dirt road nearby, drive off the pavement and onto the sand or dirt road. if you notice a sudden jump in rotation, you're probably dealing with a tire size or pressure problem.

Lighting Weirdness

Probably some of the weirdest conditions can be caused by problems with the vehicle's lights. And these conditions can be the most difficult to diagnose.

For example: Some early model Ford trucks may develop harsh shifts; at the same time, the odometer stops working. These vehicles are quipped with a programmable speedodometer/odometer module (PSOM) located inside the instrument cluster. No need to connect the scan tool to this vehicle: just look up at the dome light If it isn't working , the fuse for the PSOM may be blown. A short in the interior lights can cause a circuit problem.

Another problem with Ford trucks can occur with the taillights If someone replaced the original, incandescent, factory-style bulbs with light emitting diode (LED) bulbs, the PCM will think the brakes are being applied. The torque converter will cycle in and out at 25 to 30 percent throttle opening. The same conditions will occur if the cab-mounted brake light is blown, or changed to an LED bulb.  


Hyundai vehicles equipped with an F4A40 or 50-series transmission may stop moving forward when the backup lights are on. A Short in the taillight circuit can back feed into the reverse light range sensor circuit. The TCM is programmed to failsafe to neutral when a signal of reverse and drive occur at the same time. On this vehicle there were no codes stored in any module, and the problem wasn't identified until the transmission was replaced with another unit.

So you have these weird transmission problems that people spend days on, only to find that they're something simple... and completely unrelated to the transmission. Knowing this, maybe you'll remember to check more than just the fluid level.

Here's a set of basic checks that should help you isolate some of the weird problems:

• Fluid level
• Battery voltage (12.4V minimum)
• Battery ground
• Battery cables and terminal ends
• Body grounds
• Interior light function
• Exterior light function
• Instrument cluster gauge function
• Instrument cluster warning lamp function
• Diagnostic trouble codes stored in all modules
• Range/inhibitor switch function (check for start in each detent)
• Any aftermarket accessories installed (interior or exterior)
• Manufacturer and any aftermarket technical bulletins or recalls

And with that, you'll have a leg up on discovering those weird problems...before the have you chasing ghosts.