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Digging into the Dash





1996 GMC Sonoma 4x4



Most of the HVAC parts we're interested in are located behind the dash, which makes this job somewhat unpleasant. Later S-series models (like my 2004 Blazer) were redesigned so the actuators could be accessed without removing the dash. Unfortunately for me, I had to do some dash work to get at the Sonoma actuators.


The process begins with removing the many plastic trim pieces which conceal the underside of the dash. The best way to do this is to get yourself a 7mm socket driver (or better yet, a cordless drill with a socket adapter) and start unscrewing. If you're smarter than me, you'll carefully segregate all of the screws and bolts, label them in Ziplock baggies, and take a bunch of digital photos. When the job is done, you won't remember where they all go...trust me.


All the under-the-dash plastic came out, as did the trim around the instruments. I removed the radio, the heat and A/C controller, the ash tray, and the glove box door. The last two components might not have needed to be removed, but I was doing this for the first time and wasn't sure.


It's important to disconnect any electrical harnesses that may interfere when the dash is moved. The GM designers were kind enough to allow the dash to pivot forward for access to the components behind it. This is nice, because the whole dash can remain in the vehicle for this job. However, if there's not enough slack in the wires, some of the harnesses may be stretched when the dash pivots forward. Disconnecting some of the harness clips is tough to figure out, but a small flat-blade screwdriver and a set of metal picks helped out greatly. A small retractable mechanic's mirror is also helpful, as it's difficult to see behind some of the harness connectors that must be disconnected before certain trim pieces can be removed.


Once all the trim pieces on the front and underside of the dash are removed, the defroster grille and the dash speaker covers must be removed. My Sonoma had 4 metal screws under these grilles which secure the dash to the firewall. The defroster grille is tricky - this is one of the most commonly broken pieces of dash removal. The grille is held in by several metal clips and must be carefully pried out.


Most of the various S-series owner websites have "Stickies" in their forums that cover dash removal procedures. These are often contained in heater core replacement discussions, where the entire dash has to be removed from the vehicle. The website I gleaned the most info from was the now defunct ZR2USA.com, which was focused on the ZR2 version of S-series vehicles (like my Blazer).





Above: For pretty much any inside cab work, removing the seats frees up an incredible amount of space. The steering column cannot be dropped without removing the driver's seat, which is attached by 4 studs in the floor.

As a bonus, I found 42 cents!

I definitely should have taken more digital pictures during this project. It required removing at least 25 screws, nuts and bolts. Plus, I did this project over two weekends. What you think you'll remember is often very different from what you will actually remember.





My next task was dropping the steering wheel. This is necessary to allow the dash to pivot forward on its hinges. I removed the front seat to allow more room to move around plus, the steering wheel needs to be dropped to the floor to allow the dash to pivot forward far enough. It's pretty simple - four bolts and one small trim piece around the bottom of the seat. Looking back on it, I would have dropped the steering column before removing the trim around the instrument gauges. The trim fits pretty tightly around the column.

Dropping the steering wheel, for me, was a better alternative than removing it altogether. The dash could pivot forward and rest against the steering column, leaving me free to work behind the dash on my own. Also, the 1996 Sonoma has its shifter lever on the steering column, which meant I didn't have to remove it or detach anything from it. The various wiring harnesses do need to be disconnected, however.

The column is attached with four 15mm nuts: two under the dash and two where the steering shaft attaches to the frame. There's also a nut and bolt through the shaft coupler, which I removed, although I'm not sure if this was necessary.





Above: Here's the main wiring harness for the steering column. The GM engineers really did not want this harness to accidentally disconnect. While most harnesses use various types of clips which snap together when the harness ends are joined, this one has a 7mm bolt to keep it firmly in place. Makes sense, since there's a lot going on within these wires. The yellow harness in the next photo below even had an extra "safety" clip (orange). These wires control the airbag. When airbags fail to deploy, car makers tend to lose lawsuits...very expensive lawsuits.





Above: Steering column nuts removed column pulled out about an inch to clear the studs in the red part of the frame in the photo at left.

As mentioned previously, I also removed the bolt from the shaft coupling near the firewall, which may have been what allowed me to pull on the column just enough so that it cleared the studs and could drop to the floor (which is possible because of the universal joint in the shaft).

Note to the wise: get a cordless impact driver. Priceless.

Below: With the steering column dropped, you can begin detaching any wiring harnesses that may not have enough slack when the dash is pivoted forward. After the screws are removed from under the defroster and dash speaker grilles, there are two 10mm bolts under each corner of the dash, which upon removal, will allow the entire dash to pivot forward. This is possible because of metal pegs under each corner of the dash, which act as a hinge when all the screws and bolts are removed. In this photo, the dash has pivoted forward and rests on the steering column.







Spot the actuators: blue line is attached to the defroster vent actuator orange line goes to the recirculation actuator (opens a vent that pulls in air from inside the cab). The gray line is the vacuum supply line. The heater core is in the black plastic container under the colored vacuum lines.



Here's a sampling of what you'll see with the dash tilted forward:







Above: This is the mode actuator, on the left side of the defroster vent door. It is attached to a door that directs air either to the defroster vent, the floor vents, or both. The mode actuator has two vacuum lines attached to it: one to pull the lever, and one to push it. The actuator arm was a real pain to unhook from the mode door below. There's very little clearance to detach the arm.





Above left: Mode actuator removed for testing. With vacuum applied to the top inlet, the lever pulls up on the mode door, which sends air to the floor. Vacuum applied to the lower inlet forces the lever down and sends air up to the dash vents. Like all the actuators, this one tested ok for holding vacuum, which leads me to believe I'm either lucky, or the rubber diaphragms are more
durable than most people think. This actuator held very little fluid when I suck-tested it with my Mity-Vac.

Above right: The actuators all seemed to hold decent vacuum. The MityVac was a great help in diagnosing vacuum leaks. My problem seemed to be more about dry rot in a couple of the vacuum lines, than failing actuators. I replaced all HVAC actuators anyway, since they don't cost too much.





Above: Here's the actuator that controls the defroster vent. When vacuum enters the blue line, the actuator lever pulls down and the defroster flap (brown door) opens. When open, the defroster vent is actually blocked, which causes the air to be directed to the dash vents.





Behind the HVAC dash controls is the plug shown in the photo below, which distributes vacuum to the various actuators. This is a common source of transmission fluid. Leaking fluid typically drips down into the ash tray. The photo shows evidence of fluid in the lines, particularly on the top left holes. The orange line had little evidence of oil contamination, probably due to the fact that it controls the recirculation vent.





I almost never use the "MAX" air conditioning setting, so that line didn't get much chance to be contaminated with fluid.


With my MityVac, I was able to test the actuators by applying vacuum to each line, before digging into the dash. All actuators seemed to function ok, and this was further proven when I pulled out the actuators and bench tested each of them. Various online forums have much discussion on this topic, with many people finding that the actuators no longer worked after fluid entered the system. This wasn't the case with me, but I still wanted to get behind the dash and properly clean out all of the vacuum lines.


The photo below shows a vacuum line connector located in the dash. I disconnected the lines and blew compressed air through each one. It takes some time, but that's about the only way to clean them out. The colored vacuum lines were all in good condition, with no evidence of leaking. Not bad for an old truck....







New parts: 3 actuators, vacuum cannister, HVAC
controller, and some new vacuum line.





It wouldn't be a truck project without a few extra parts leftover after everything was reassembled. The trim piece came from the driver's seat, and was too much of a pain to reassemble, for what it was worth. The rest of the parts? Who knows. Everything fit together fine.





Update January 2012





Lessons in aftermarket parts



Snow took quite awhile to arrive in the Midwest in 2011-12, but when it did, the Blazer's 4WD wouldn't work. After much use of my Mity Vac to trace leaks, it became clear that the hoses weren't connecting tightly to the transfer case vacuum switch. The transfer case would engage the switch, but it couldn't maintain enough vacuum to power the 4WD actuator under the battery tray.​


I used a Dorman aftermarket vacuum switch with the Blazer, and apparently the 3 tubes sticking out of it weren't quite large enough in diameter to fit snugly inside the hose connector. It was an easy fix, though. I simply wrapped pieces of duct tape (what else?) around the tubes, and they then fit nice and tight into the hose connector. Problem solved...or so I thought





This was my original duct tape solution. It didn't work. Avoid the Dorman switch.



Here's a word to the wise: Stay away from the Dorman vacuum switch. The one I installed in the Blazer failed a year later and let huge amounts of transfer case fluid contaminate the vacuum system. The vacuum reservoir was so full of fluid that I threw it out and put in a new one. Two of the actuators in the dash, as well as some of the vacuum lines under the hood, were leaking around the line connectors. I was not happy. The Blazer now has a GM vacuum switch.


The Blazer had its own set of transfer case switch problems. Click on the link below to ready all about it.


Blazer Transfer Case Switch

Vacuum System Diagrams



To properly diagnose and repair issues related to the vacuum system, it helps to have diagrams. Please click on the link below to view some of the diagrams I've collected over the years.


Vacuum System Diagrams

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