Heater Blower Motor not working. Fuses all OK. Blower Motor OK tested with resistors in circuit OK. Switch tested OK. I semm to be dopping volts between restors and switch connection. Diagram that I have is not correct. Is there any relay or anything else in circuit ?. Rgds.

Based on the detailed information you've provided, it seems that you've done a thorough job of testing the major components related to the blower motor system in your 1999 Chevy Tahoe.
Here are a few possibilities and additional steps to consider:

High Resistance in the Circuit: Even though the wiring and connectors appear to be in good condition, there might be high resistance somewhere in the circuit that only becomes significant under load. This could be due to a partially broken wire, a corroded connector, or a faulty switch. Carefully inspect the wiring harness for any signs of damage, wear, or heat exposure.

Issue with the Control Switch or HVAC Control Unit: The problem might be with the control switch or the HVAC control unit in the dashboard. These components can sometimes develop internal faults that are not immediately apparent. Consider testing the switch and control unit if possible.

Ground Connection Problem: A poor ground connection can cause voltage drop issues. Make sure the ground connections related to the blower motor circuit are clean, tight, and free of corrosion. This includes not just the ground for the blower motor, but any related grounds in the HVAC system.

Voltage Drop Test: Perform a voltage drop test on the blower motor circuit. This test is done by measuring the voltage across different parts of the circuit while the circuit is under load. A significant voltage drop across any specific part of the circuit would indicate where the resistance problem lies.

Check for Overloading or Short Circuits: There could be an issue elsewhere in the electrical system that's causing an overload or short circuit when the blower motor is connected. This might be difficult to diagnose without specialized equipment.

94 CHEVY Silverado ????,heat/ac fan not blowing. K1500?????? Hear clicking when selecting l'm, SET high but not blowing! WITH AIR OR NO A/C? FIRST IS FUSES, MAKE SURE THE BLOWER FUSE IS NOT BLOWN.
EASY NO? CHECK ALL FUSES FIRST. ON ALL ELECTRICS FAILING OF ANY KIND.
BAD FUSE, BAD WIRINIG , BAD CONNECTIONS. OR EVEN BAD RELAYS TO THE BLOWER OR BAD SWITCH BLOW.
THE OLD CARS , HAD A SPEED BLOW , RESISTOR BLOCK
THAT FAILS ON ALL SPEEDS BUT HIGH, SO ITS NOT THAT.
OWN A VOLT METER????
HERE IT IS, ANY OF THIS CAN FAIL THIS OLD
WE USES A VOLTMETER TO FIND THE POINT OF FAILURE.
A SHORTED BLOWER MOTOR CAN BLOW LEFT SIDE TOP
FUSABLE LINK.RIGHT FUSE BLOWN MAKES THE CONTROLLER DEAD.

Nowhere did you say you had checked the motor power supply - the typical heater motor circuit supplies the motor with power and provides a ground through the switch via the resistor for lower speeds and direct for high speed.

With late model fully computerised cars, switching duties are usually carried out by a black box at commands from the dashboard switch. Sometimes the circuits are unconventional which is why a circuit diagram is very desirable before troubleshooting.

Finding a wiring diagram an using a DVOM - digital volt ohm meter an testing the electrical circuit is the proper way to diagnose electrical problems . Looking at the wiring diagram for your vehicle i see it doesn't have a blower motor resistor , it is a blower motor contol processor . Electronically controlled .
HVAC Blower Motor Circuit
GMC Yukon HVAC Blower Motor Won Turn On

Automatic or manual climate control ?

• The blower motor does not operate correctly - manual climate control

• Circuitry short
• A/C blower motor resistor
• A/C blower motor switch
• Blower motor high speed relay

• Go To Pinpoint Test O .

• The blower motor does not operate correctly - EATC

• Circuitry short/open
• A/C blower motor speed control
• EATC module

• Go To Pinpoint Test Q .

Pinpoint Test Q: The Blower Motor Does Not Operate Correctly - EATC
Normal Operation Under normal operation, the blower motor control module receives a 1-2 volt signal from the EATC module through circuit CH218 (GN/VT) to determine the desired blower motor speed. The EATC module receives a feedback voltage from the blower motor control module through circuit VH219 (GY) to determine actual blower motor speed. When the highest blower motor setting is selected, the EATC module sends 12 volts to the blower control module through circuit CH217 (BN).

Possible Causes
• An open, short to voltage, ground or together in circuits CH218 (GN/VT), VH219 (GY) or circuit CH217 (BN)
• Blower motor control module
• EATC module
• If automatic HVAC system i would suspect Blower motor control module .

I would suspect blower resistor first. they are known for melting the wires at the connector. If problem isn't there its more than likely in the control head. Fuses ok?

Check blower motor resistor make sure none of the spring's are broken, resistor sit next to blower motor in housing, also check blower motor relay for voltage, also sit over by blower motor outside of housing and last check voltage at blower motor switch.Hope this was very helpful,good luck.

look in passenger foot well on blower case for 4 wire connector.



Behind right side of I/P, A/C-heater plenum


resister bad would still allow high speed. may be blower motor.

• The blower motor does not operate/does not operate correctly

• Open fuse.
• Tripped circuit breaker.
• Circuitry open/shorted.
• Heater blower motor switch (18578).
• Heater blower motor switch resistor (18591).
• Blower motor relay.
• Blower motor.

check the connections at switch and motor if good will need a volt meter or test light to check the switch and blower motor for power

The blower motor resistors are known for going bad. Usually its because the blower starts drawing too much current. What I would recommend doing if you can and have the tools, is to check the blower motor connector to make sure 12 volts is present. If so than I would recommend replacing the blower motor resistor. Then if the same issue ever happens again and the blower motor resistor is bad again, you will need to replace the blower motor and the blower resistor as the final repair.
Here is some info about testing the resistor using a multimeter it you have one:

DIAGNOSIS AND TESTING- BLOWER MOTOR RESISTOR WARNING: ON VEHICLES EQUIPPED WITH AIRBAGS, DISABLE THE AIRBAG SYSTEM BEFORE ATTEMPTING ANY STEERING WHEEL, STEERING COLUMN, OR INSTRUMENT PANEL COMPONENT DIAGNOSIS OR SERVICE. DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE, THEN WAIT TWO MINUTES FOR THE AIRBAG SYSTEM CAPACITOR TO DISCHARGE BEFORE PERFORMING FURTHER DIAGNOSIS OR SERVICE. FAILURE TO TAKE THE PROPER PRECAUTIONS COULD RESULT IN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY.
Both the front blower motor resistor and the optional rear heater-A/C unit blower motor resistor can be diagnosed using the procedure that follows. For circuit descriptions and diagrams, refer to Air Conditioning/Heater in Group 8W - Wiring Diagrams.

1. Disconnect and isolate the negative battery cable.
2. Disconnect the wire harness connector from the blower motor resistor block.
3. Check for continuity between each of the blower motor switch input terminals of the resistor and the resistor output terminal. In each case there should be continuity. If OK, repair the wire harness circuits between the blower motor switch and the blower motor resistor block or blower motor as required. If not OK, replace the faulty blower motor resistor block.

Here is the information on replacing the resistor:

REMOVAL

1. Disconnect and isolate the battery negative cable.
2. From the passenger foot well area, remove the two hex screws used to secure the blower motor resistor.
3. Unplug the front blower motor resistor from the wire harness connector.
4. Remove the front blower motor resistor from the HVAC unit.

Here is an exploded view of the HVAC box. The resistor is #63 in the picture and the actual part may appear slightly different:

View Full Image


If any issues such as no voltage at blower connector or damaged or melted connector at the resistor then let me know.