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This information is free from me to you. Hopefully it will save you both time and money
GMC 1993 Suburban
4 Wheel drive not engaging.
Below is what I discovered was the problem.
Replace the Thermal Actuator on the front 4 wheel drive differential.
The Thermal Actuator in the front differential housing uses heat to heat up the oil inside the actuator itself. Causing the shifter to engage the front wheel drive and a light switch on the front differential.. The light switch activates the light on the floor shifter inside the vehicle that it has engaged. I was told by my dealer the only fix was through GM in which they change you over to an electric motor so you don't have to wait for the 4wd to engage the front differential. Engagement is slower in colder temperatures but for the price difference in cost I can wait a second or two.
Well if you believe everything you read on the internet, it shows the only fix is a new electric motor kit from GM for around $250.00 and a little wiring. Not so. You can still buy the thermal engagement switch new for these trucks.
Just check with NAPA Auto Parts. Their part number for my 1993 Suburban is FWD12 and has the round 2 prong connector. Be sure to see if you have round or flat 2 prong connectors inside the end of the connector before ordering. I think part number FWD19 has the flat spade connectors. Anyway NAPA had a new Thermal Actuator in stock for $70.00.
NAPA Part # FWD12 Round Wire Prongs, FWD19 for Flat Wire Prongs
Another part number 7 SKU's 1430000, don't know who's part number this is but a car salvage yard in Lansing sells them.
Fix it yourself:
You don't need to take your truck to a repair shop. It took me less than 10 minutes to replace the unit. Before I changed the unit I did make sure there was power to the unit at the connector. I shut the truck off then turned the key forward but did not start it up. You will need to put the main shifter in park. Then you will need to put the floor shifter in 4wd all the way back. Unhook the connector on your front axle and test it for power from the truck with a simple light tester. The light worked so I figured it was in the Thermal Actuator. Used a crescent wrench to unscrew the old actuator, put a little orange Permatex on the seating surface of the new one to avoid any leaks, tightened it up and re-hook the connector. Got in the truck, started it up and backed it up and low and behold, the 4wd light on the floor shifter lit up and it works. It's that easy.
Differential oil did not come out the actuator hole so you don't need to drain it first before removing the actuator.
OEM Actuator 1993 Suburban Front 4wd Actuator
NAPA Auto Parts, Part # FWD12
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