Only hi speed on ac blower motor - 1981 Chevrolet Corvette

Probably something going on with the blower motor control Processor . Hooking a scan tool an checking for DTC'S - diagnostic trouble codes would be the first step in a diagnostic procedure for a problem like this .

Auxiliary Blower Motor Control Processor
The auxiliary blower motor control processor is an interface between the auxiliary HVAC control module and the rear blower motor. The auxiliary blower motor control processor monitors supply voltage and ground circuits to the rear blower motor. The auxiliary HVAC control module provides a pulse width modulation (PWM) signal to the control processor in order to command the blower motor speed. The auxiliary blower motor control processor uses the blower motor ground as a low side control to adjust the blower motor speed.
The blower motor forces air to circulate within the vehicles interior. The vehicle operator determines the blower motors speed by placing the blower motor switch in a desired speed position or by selecting automatic operation. In manual operation, once a blower speed is selected, the blower speed remains constant, until a new speed is selected. In automatic operation, the HVAC control module will determine what blower speed is necessary in order to achieve or maintain a desired temperature.
As the requested blower speed increases, the following conditions occur:
• The HVAC control module increases the amount of time that the blower motor speed control circuit is modulated to ground.
• The voltage and duty cycle, measured between the blower motor speed control circuit and ground, decrease.
As the requested blower speed decreases, the following conditions occur:
• The HVAC control module decreases the amount of time that the blower motor speed control circuit is modulated to ground.
• The voltage and duty cycle, measured between the blower motor speed control circuit and ground, increase.

Check the AC fan relay
Symptoms Of A Defective Blower Motor Relay
The blower motor relay's job is to basically bypass the current that passes thru' blower motor resistor and deliver it directly to the blower motor. With the blower motor getting full battery current, it'll run in HI speed.

The most common symptom you'll see, when the blower motor relay fails in your Chevy S10 (GMC S15), is that the blower motor will not run in HI speed (but will run in LO, M1, and M2 speeds).

In some cases, the blower motor will fail and the blower motor will not run in HI but you'll have all of the other speeds working.

https://easyautodiagnostics.com/gm/2.8L/blower-motor-relay-multimeter-test-1

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Could be a number of thing's ! Testing the electrical circuit is the way to find out what's wrong . Using a wiring diagram and a DVOM - digital volt ohm meter . Videos on youtube showing how to test it ! Without testing i would only be guessing . Does it work in high speed ? In high speed it by-passes the resistor . If it doesn't work in any speed more then likely the blower motor is the problem !
Air Speed
The HVAC control assembly applies ground to the blower motor control circuit that corresponds to the selected blower speed. The resistors and the blower motor are in a series circuit. The following list represents the number of resistors in series with the blower motor per particular speed request:
• Low speed--3 resistors
• Medium 1 speed--2 resistors
• Medium 2 speed--1 resistor
When the operator requests High speed, the HVAC control assembly applies ground directly to the blower motor through the high blower motor control circuit. The resistor card has a fusible link which, if blown, will require that the card be replaced.

Blower Motor and Resistor Diagnostics by Wells

Hi Elaine , What make vehicle are you working on ? Most manufactures have a relay for hi speed an have a blower motor resistor for all other speeds . Depending on make vehicle control modules need programming in some cases . Blower Motor Problems Auto Repair Help

SOUND LIKE BAD BLOWER MOTOR RESISTOR.CHECK BLOWER MOTOR HI BLOW FUSE AND RELAY.YOU WILL NEED OWNERS MANUAL IN GLOVE COMPARTMENT TO LOCATED BLOWER MOTOR FUSES AND RELAYS.

blower motor resistor is burnt up
located upsteam in air plenum from blower motor
passenger side compartment at foot
locate motor then you should spot resistor close by

Hi!!
When the Blower Motor Resistor (controls blower speeds) goes bad, the blower motor works only on HIGH SPEED. REPLACE IT.
BLOWER MOTOR RESISTOR Removal & Installation

• Disconnect negative battery cable.
• Remove right lower instrument panel insulator panel.
• Disconnect blower resistor connector located behind right side of dash.
• Remove blower resistor screws and blower resistor.

To install, reverse removal procedure
Good Luck!! A HELPFUL - 4 THUMBS - rating for this solution would be greatly appreciated. Thank you for using FixYa!!

Replace the blower motor resister,this is the speeds for the blower motor,it is located very near the blower motor,and is mounted on the blower box,and it actually goes inside the box,to keep it cool.The reason for it`s failure is the blower motor,even though the blower motor sounds,and runs good,it is drawing too many amps.If this was at all helpful,please rate,thank you.

Usually when the blower fan only operates on hi speed, its because of a failed blower motor resistor. You still have hi speed because on hi, the power going to the blower motor goes directly to it, as opposed to the lower speeds, when the power is routed through the resistor, thus lowering the voltage, which in turn lowers the speed of the fan. The resistor is usually located near the blower motor itself, under the hood back near the firewall.

here is what my book says:
Blower Motor Speed Control To Remove:

1. Remove the right lower instrument panel insulator.
2. Disconnect the electrical connector from the blower motor speed control.
3. Remove the blower motor speed control mounting screws.
4. Remove the blower motor speed control.