Questions and Answers

GMC Topkick Taking On Water Q. Hello. I have a 1991 GMC Topkick with a 3116 Caterpillar Diesel engine, 5 speed transmission with a two speed rear axle, air brakes, and air conditioning with 240,000 miles showing on truck but only 50,000 miles on the engine. It has recently developed a water leak when it rains and a considerable amount accumulates on the passenger side floorboard.

On closer examination, the heater blower motor box contains water with a obvious drip and when the fan is turned on you can hear it picking-up water. I finally got that all dried out and thought maybe the air intake at the cowl was obstructed, so I removed the cowl cover and washed out the cowl. The heater blower motor box, once again, was filled-up with water. After looking at a service manual, it shows a seal between the heater blower motor box and the firewall.

One GM tech told me that this was a problem with GM trucks especially when they get a little age on them and are exposed to a lot of heat. Is this likely to be the problem and if it is can this seal be replaced without discharging the A/C and draining the cooling system? It is to the point now that if it rains any at all I have water in the floor and in the blower motor box.

Thanks,
Bill
Candler, N.C.

A. If it is that seal, you will need to remove the complete HVAC module to replace the seal. This also means the dash must come out and the A/C and cooling system drained.

Added 8/1/03

Hello. I e-mailed you a question a couple of weeks ago about my GMC Topkick getting water in the blower motor box. I think it is posted on page 68. After disassembling nearly all of the dashboard, I'll tell ya what I finally found.

Under the cowl cover in front of the windshield there is a small piece of sheet metal (about 10" long and 3-4" wide that sits in a recess stamped out in the cab assembly that is directly above the fresh air intake for heat and AC.

When these trucks are assembled, these plates are caulked in place. I imagine that after a few years this caulking has a tendency to crack and peel-out. When the truck was sitting level, the rain water went to the lowest spot in this recess that the plate was covering and found its way into the air intake leading to the blower motor box. I cleaned the recess and cover plate and re-installed it using silicone sealant and have not had a leak since.

All GM trucks 88-99 have this cover plate under the cowl directly above the blower motor air intake. I don't know of any reason it would be there other than to provide access to the air intake should it become clogged with leaves or other debris.