I have a 2001 ZR2 with what seems to be a "factory" front 4WD unit. I was given a locking diff. for it should I want to convert. It looks completely different from my front axle. I noticed in a wrecking yard where there were 2 ZR2 hulks there that they both had the "locker" that looks like the one that is sitting on the floor of my garage. I traced the wires and hoses from the top of the axle housing up to a vacuum actuator under the battery tray so it appears this is a factory set-up also. Was this an option? Is there a code for it? One of them must have ceased working through the vacuum as it had an aftermarket throttle cable hooked to it mounted under the dash. I am assuming this was an air-actuated locker (via the vacuum diaphragm) that has the capability of being manually operated so it would be possible to install this in my vehicle w/o too much trouble. Of course it would need to be gone through first...
Any thoughts of inspiration on this?
Sorry if I am repeating the same old questions but I wasn't able to find much intel on this on this forum
Thanks.

 

There were no S-Series trucks with front locking differentials. The vacuum actuator under your battery tray is the actuator that engages the front differential. When it is in resting position (2wd) the front wheels spin independently. When the 4x4 system is engaged, that vacuum actuator pulls a gear collar inside the front differential and engages the front axles with the power being sent from the transfer case. The front differential is still an open diff, meaning that the front two wheels can spin independently from one another if one has more traction than the other.

Because the vacuum hoses can degrade and sometimes result in the 4x4 system not engaging due to poor vacuum, some people have replaced the vacuum actuator with a manual 'posi-lock' system to engage the 4x4. I suspect that is what you have found.

It is generally considered to be a bad idea to add an aftermarket locker to the aluminum front differentials on these S-Series trucks as they cannot stand up to the additional stress and load created by a locking differential.

 

I agree with everything else you said, but, correct me if I'm wrong, but didn't the ZR2 models have an iron front diff housing? I seem to remember reading that somewhere

 

The front diff I have on the floor is aluminum. The wrecking yard ones I am not sure but they looked cleaner (as in aluminum) than an iron diff. would be after many years. Going to pull my front skid plate and compare the 2...

 

Found this quote reading El Beautor's build thread...

"The differential that I bought is from a '98 ZR2, so I had to use '98+ CV axles. If my diff was from a '95-'97 S-series truck I could have reused my original CV axles. There's no difference between the front differentials from the regular S-series to the ZR2's, other than the manual transmission ZR2's came with an iron housing differential as opposed to the aluminum one used in the automatics and non-ZR2's."

 

Ah, OK. That's what it was. The manuals had the iron diff, not just the ZR2. Thanks for the correction

 

So are the iron front differential assemblies compatible with non-ZR2 Blazers? Is the difference the length of the CV-axles?

 

I believe it was only 5 speed ZR2s that came with an iron diff, and it was not all 5-speed ZR2s. They only way to tell is to crawl under the truck with a magnet. I think it depended on what plant they were made in and certain build-years, but I don't remember for sure off the top of my head. If you can find one, they are a direct swap to any S-series truck as long as the gearing matches (3.73 was the factory gear ratio for ZR2s). They are really hard to find, but supposedly they would be able to handle a locking diff due to having a stronger housing that can handle the stress.

There's a lot of info about them on ZR2USA if you can find it in the archive. Apparently there was never a different part number for the iron diff, and most GM parts folks didn't know that they existed. The only way to get one is to find it in a parts-truck.