How to Check Fluid Level on an Allison Transmission Step-by-Step

Checking the Fluid Level on an Allison Transmission

The Allison transmission is a heavy duty automatic transmission often found in trucks, RVs, buses and other large vehicles. Keeping the fluid level correct in an Allison transmission is important to ensure proper performance and durability.

Locating the Dipstick

Unlike most automatic transmissions, Allison transmissions do not have a regular dipstick tube to check the fluid level. Instead, Allison transmissions have a check plug on the side or bottom of the transmission pan. To check the fluid level, the vehicle must be on level ground with the engine running.

The check plug will be a hex head plug, usually with a locking mechanism to prevent it from vibrating loose. The size of the check plug can vary between models, but is often in the 9/16 to 5/8 inch range. Ensure you have the properly sized hex tool before attempting to remove the check plug.

Checking the Fluid Level

With the vehicle running and properly warmed up, locate the check plug on the transmission pan. Using your hex tool, carefully break loose the check plug; fluid will begin to drip from the hole once the plug is loosened. Allow any built up pressure behind the plug to dissipate before completely removing.

The correct fluid level is reached when fluid starts to drip from the check port, also often described as fluid "weeping" or "seeping" from the hole. If no fluid comes out, the level is low. If fluid streams or gushes from the port rather than dripping, the fluid level is too high.

Adjusting the Fluid Level

If the fluid level is too low, more fluid will need to be added through the transmission dipstick tube. Add a small amount at a time, rechecking the fluid level at the check plug periodically. Most Allison transmissions hold between 12-25 quarts of transmission fluid. Refer to your vehicle's service manual for the factory fill capacity and recommended fluid type.

If the fluid level is found to be too high, excess fluid will need to be drained from the transmission. This will require loosening the pan drain plug to allow some fluid to drain out. Take all appropriate safety precautions whenever underneath a vehicle. Check and recheck the fluid level at the check plug until the proper seepage/weep point level is reached.

Related Service Items

Anytime the fluid level is checked, also inspect the condition of the fluid coming out of the check port. Fluid that is very dark, smells burnt or has visible metal shavings likely indicates a bigger problem within the transmission. If the fluid condition is poor, a complete fluid exchange service should strongly be considered.

The transmission filter helps remove debris from the fluid and should periodically be changed. Most manufacturers recommend changing the filter every other fluid and screen change. Consult your owner's manual for the service intervals recommended for your Allison transmission.

Conclusion

Checking and maintaining the correct fluid level is vital to maximizing the performance and longevity of your Allison transmission. Simply remove the check plug, verify proper fluid seepage/weep, and top off the level from the dipstick tube as needed. Refer to your vehicle owners manual for specific details and procedures related to your transmission model.

FAQs

Where is the dipstick located on an Allison transmission?

Allison transmissions do not have a traditional dipstick tube. Instead there is a check plug on the transmission pan that is used to verify the fluid level.

What type of fluid is used in an Allison transmission?

Allison transmissions use a specialized synthetic transmission fluid. Consult your owner's manual to determine the exact type of fluid recommended by the manufacturer.

How often should I check the fluid level?

As part of a regular maintenance schedule, the fluid level should be checked about every 10,000-15,000 miles. Check the fluid more frequently if you notice any changes in transmission operation.

What happens if the fluid level gets too low?

Low transmission fluid can cause internal components to wear out faster from lack of lubrication. Running the transmission with very low fluid can eventually lead to internal failure.