Symptoms
One axle's tires drop pressure overnight, or CTIS won't inflate or deflate at all. The fronts and rears are paired, so a leak can be in either tire, either hub seal, the lines, the manifold solenoids, the over-pressure valve or the pump.
The fix, step by step
- Spray soapy water on the tires, rims, lines and fittings.
- Inflate to normal, pull both quick-disconnects at the wheels and leave it overnight.
- If the cab gauge drops, the leak is in the lines, manifold or pump; if not, reconnect one wheel at a time to find the leaking tire or hub.
- Bubble-test each hub vent for a bad spindle seal.
- Check the braided hose swivel at the hub with the wheels turned both ways.
- Clean sand out of the manifold solenoid valves.
- If the pump won't run, jump it from the battery, then check the 5-amp (1G) and 40-amp fuses and the relay, per Gil Herman's checklist on Flash Off-Road.
Tools & part numbers
- Spray bottle
- 7/8-in and 1-3/16-in deep sockets (the latter sold as a GM oil-sender socket)
- 9/16-in wrench
- multimeter
- jumper lead
Where to buy the parts
Stainless Steel Inboard CTIS Lines – full kitPredator Inc ↗Amazon ↗
CTIS kit (exposed / hidden)Predator Inc ↗Amazon ↗
Wheel seal, 1998 H1 6.5 TD (catalog)RockAuto ↗
Hunting a used or discontinued part? Search eBay Motors for AM General Hummer H1 parts →
Rated for a competent home mechanic with a decent tool kit. Confirm torque figures, fluids and procedures against the factory service manual for your VIN, support the truck on stands rather than a jack, and work at your own risk (disclaimer). Some parts links are affiliate links, which pay us a small commission at no cost to you.