Tech Tips

The following technical tips provide repair information about specific conditions on a variety of vehicles. If you have a tough or unusual service repair, the TSS Technical Assistance Hot Line can help. Call 1-800-825-5886, prompt #2, to speak with a technical expert with the latest OEM information.

Speedometer Accuracy

1997-1999 GM passenger cars and trucks; 2004-2009 Cadillac XLR; 2005-2009 Chevrolet Corvette; 2007-2009 GMC Acadia

The speedometers used on most GM vehicles are of the Air-Core design (Fig. 13). This is a moving needle operated by a magnetic coil. Because there is no direct mechanical linkage, there is some variation in accuracy that cannot be eliminated.

Digital speedometer displays and Head-Up Displays are direct readouts of the Vehicle Speed Sensor (VSS) and their accuracy is much greater. Tire size, tread life and inflation pressure may cause some variation in the readings.

Questions on speedometer accuracy can usually be resolved by comparing the speedometer reading to the scan tool (which reads the VSS directly). Allowable speedometer variations include +/- 4 MPH at 20 MPH, +/- 3 MPH at 45 MPH, and +/- 2 MPH at 65 MPH.

Polycarbonate Headlamp Lens Damage

2009 and prior GM passenger cars and trucks, including Saturn, HUMMER H2 and H3, and Saab 9-7X 

Most late model vehicles have exterior polycarbonate headlamps lenses (Fig. 14). This material is used because of its temperature and high impact resistance.

Crazing or deformations of the lens may occur if a shop mat or fender cover is draped over the fender and covers a portion or all of the headlamps assembly with the Daytime Running Lamps (DRL) or headlamps on. This action restricts the amount of heat dissipated by the headlamps.

Do not cover the headlamps with shop mats or fender covers while the vehicle is being serviced with the headlamps or DRLs illuminated. Once heat buildup is generated by the headlamp, the degradation of the lens begins. It may not be noticeable at first and eventually may become hairline cracks. In extreme cases, the lens may deform.

These headlamps lenses also are very sensitive and a variety of chemicals may cause crazing or cracking of the headlamps lens. Contact should be avoided with all exterior headlamps lenses when working with any type of chemical, including rubbing compound; grease, tar and oil removers; tire cleaners; cleaner waxes and highly concentrated car wash soaps. 

A damaged headlamps lens may require the entire headlamps housing to be replaced. Refer to GM bulletin #02-08-42-001C for more information.

Water or Ethylene Glycol in Transmission Oil

2000-2009 GM passenger cars and light-duty trucks, including Saturn; 2003-2009 HUMMER H2; 2006-2009 HUMMER H3; 2005-2009 Saab 9-7X; with an automatic transmission

Water or ethylene glycol in automatic transmission fluid (ATF) is harmful to the internal transmission components and negatively affects the durability of these parts. Water or ethylene glycol also changes the friction of the clutches, frequently resulting in shudder during engagement or gear changes, especially during torque converter clutch engagement.

Indications of water in the ATF may include:

ATF blowing out of the transmission vent tube

ATF appearing cloudy

Visible water in the oil pan

Milky white substance inside the pan area

Spacer plate gaskets that appear to be glued to the valve body face or case or appear to be swollen or wrinkled in areas where they are not compressed

Rust on internal transmission iron/steel components.

If water in the ATF has been found and the source of the water entry has not been identified or if a leaking in-radiator transmission oil cooler is suspected (with no evidence of cross-contamination in the coolant recovery reservoir), a simple test kit is available from Nelco Company by calling 1-651-738-2014.

If water or coolant is found in the transmission, perform the following:

Replace all rubber type seals

Replace all of the composition-faced clutch plates and/or bands

Replace all of the nylon parts

Replace the torque converter

Use new gaskets and oil filter when cleaning and rebuilding the transmission

Flush and flow check the transmission oil cooler

Inspect the engine cooling system and hoses; clean and repair as necessary.

Refer to GM bulletin #08-07-30-035 for additional information.