ENGINE COOLANT TESTING (USE OF REFACTOMETERS)

VEHICLES AFFECTED: 1988-1990 ALL CARS AND TRUCKS

Due to a worldwide shortage of "Ethylene Glycol" in 1988, some coolant manufacturers have started to mix other types of glycol in their coolant formulations; propylene glycol is the most common new ingredient. A hydrometer will not always provide a correct measurement of freeze protection when anything other than ethylene glycol and water is being tested. The degree of inaccuracy will vary depending on the proportion of other glycols present in the coolant.

All 1989 model GMC trucks are produced with coolant that can be accurately measured with a hydrometer; however, when the type and quality of coolant being measured is unknown, such as a customer vehicle that has accumulated mileage, use of a refractometer is recommended.

Hydrometers test the amount of glycol in a mixture by measuring the specific gravity of the mixture; the more ethylene glycol, the higher the float balls go, and the better the freeze protection. Because ethylene glycol and propylene glycol do not have the same specific gravities, hydrometer readings of mixtures containing propylene glycol give incorrect values.

As the shortage of "ethylene glycol" continues and/or worsens, more and more variations in antifreeze formulas will be marketed. It is, therefore, recommended that you purchase a "refractometer" and encourage your technicians to use it. Refractometers test for the amount of glycol in a coolant mixture by measuring the speed of light as it passes through the fluid and are not affected by the specific gravity of the glycol. The freeze protection of solutions of ethylene glycol in water, solutions of propylene glycol in water, and solutions of mixtures of propylene glycol and ethylene glycol in water may all be tested with sufficient accuracy using a refractometer.

Refractometers are available from several sources including:

Kent Moore* J23688 Fahrenheit Scale J26568 Celsius Scale

Misco Products*

IMPORTANT: Refractometers and hydrometers test only the FREEZE PROTECTION provided by a coolant.

*"We believe these sources to be reliable. There may be additional manufacturers of such equipment. General Motors does not endorse, indicate any preference for, or assume any responsibility for the products or equipment from these firms or any such items which may be available from other sources."

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.