Heater is speed dependant

[Dan White]

The heater/defroster on my 2001 Pontiac Montana blows warm air only at faster speeds. Engine temp. holds fine. I can be driving for hours at highway speed and heater blows warm air but as soon as I slow to city driving it starts blowing cold air. Any ideas???

[tmorris1]

There is a TSB from GM on this. There isn't enough coolant flow through the heater core a low engine speed, so they have an electric pump that keeps the coolant flowinging at low RPMs.

[1Bradymichael]

That does make sense...I have a brother with a 2000 Montana without the pump,and mine has the pump.I just assumed it was for the rear heat.I am guessing you are talking about the pump that sits above the air filter? Mine is a 2000 year as well.

[tmorris1]

Here is the bulletin. Looks like it is above the air filter.



A/C - Poor Heat at Low RPM When Cold

File In Section: 01 - HVAC

Bulletin No.: 00-01-38-011B

Date: April, 2002

TECHNICAL

Subject:
Poor Heat with Cold Outside Temperatures at Low Engine RPM
(Install Auxiliary Coolant Pump)

Models:
1997-1999 Chevrolet Venture
1997-1999 Oldsmobile Silhouette
1997-1999 Pontiac Trans Sport
1999 Pontiac Montana

Attention: Implementation of this Service Bulletin by "GM of Canada" dealers requires prior District Service Manager approval.

This bulletin is being revised to correct the diagnostic information under the Correction section. Please discard Corporate Bulletin Number 00-01-38-011A (Section 01 - HVAC).

Condition

Some customers may comment on poor heat in the vehicle passenger compartment at low engine RPM. This condition is most prevalent at outside temperatures below 0°C (32°F).

Cause

The vehicle's heater core may not have sufficient engine coolant flow while the vehicle is idling. Anytime the engine RPM drops to idle speed, the temperature of the heated air may decrease to an unacceptable level even if the engine is already at full operating temperature.

Correction

A service only kit has been released which contains an electric auxiliary coolant pump. The components of this kit will boost coolant flow through the heater core whenever the outside air temperature falls below 0°C (32°F). The service kit is similar in design to the factory installed models on the 2000 model year vehicles. Any 1997-1999 vehicle that comes into the dealership that has not had the auxiliary coolant pump service kit installed and has an insufficient heating condition may need to have a pump kit installed.

Any 1997-1999 vehicle that comes into the dealership that has had an auxiliary coolant pump service kit installed and has an insufficient heating condition should be diagnosed using Heating Insufficient-Auxiliary (SI Document ID 793821) in the 2000 model year Service Information. This diagnostic and the HVAC schematics are compatible for both the installed 1997-1999 service kit and the 2000 OEM auxiliary coolant pumps. The following procedure contains specific instructions for retrofitting vehicles equipped with the following RPO codes: