Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation


We have an automatic 1999 sierra 1500 GMC pickup. Just the other day a light came on that told me to “service the 4WD”. To the left of the steering column there are buttons to press which allow me to engage whichever drive I need. Ever since the service light came on I no longer can switch drives. Usually there is a light on the two wheel drive button but now there is no light. While driving down I-90 the other day it was raining VERY heavily! (the 4WD was NOT engaged!)I suspect the water from the road slamming under the truck caused something to come ajar! Like a wier or a switch? Do you have any suggestions as to where my husband should look for the problem? We live in a area where four wheel drive is a must (Summit at Snoqualmie). In order to increase our odds of survival my husband has decided to load up the pickup bed with snow! Beacuse our limited budget this is the best we can do for now! We are a four wheel drive family that has now been forced to become a two wheel drive family!It seems like it could be an easy fix any suggestions?

The light comes on when the 4WD system detects a fault. Until the fault is corrected, the 4WD will be off. You’ll need to find a shop that fixes the 4WD system. They need to get the fault code to know what is wrong. The dealer will do it, but does charge a lot. An independent mechanic at a 4X4 shop is a better bet. Anything else is a waste of time and money.

From my readings about mine, one of the more common problems is a bad electrical connection on the transmission.

Do you have the Autotrac System (2wd, Auto 4wd, 4wd Hi, 4wd Lo) or the Instatrac (2wd, 4wd Hi, 4wd Lo) sytem?

When the Autotrac system acts up on my 2000 Blazer resetting the TCCM (Transfer Case Control Module) by disconnecting the battery usually helps. This trick worked until 4wd gave up completely in my Blazer last week.

Check the vacuum lines in the engine compartment. GM uses a vacuum actuator to engage the front axle. While I was troubleshooting I found most of the vacuum lines under the hood were in bad shape. They’re not leaking yet, but I plan on replacing them in the spring.

The problem with my Blazer was a 2 port vacuum valve (Part #15165891, ~$65). The valve controls the vacuum to the axle actuator. When I removed the lines from the valve and connected them the 4wd started working. The valve is mounted on the firewall behind the distributor on my Blazer. I think it’s mounted on the transfer case in the Instatrac system.

There is also a switch mounted on the axle that signals the TCCM that the axle is engaged. Might be worth checking the connection.

Here are links to discussions that helped me.

In short, reset the TCCM, check the vacuum hoses and the electrical connections.

Good luck,

Ed B.