1996 Chevy full size PU- heater core plugged

I am helping a friend with a full sized 1996 Chevy truck. the heater core is plugged and there is no air getting through it. Whats the trick to getting the dam thing apart? got the fan out. there is four small
bolt on the heater fan housing top and bottom. removed those but the housing wont come off. Got the bottom plenum housing loose below the AC evap coil, but cannot get the back by the firewall to break loose. Whats the trick?
also, the fan does not have high speed. the resistor pack is ok. so is the HVAC control head (swapped parts). is there a relay somewhere to do this?
bob
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bob urz wrote:

Well bro in law managed to pry it out. the core was caked with garbage so much that air flow through it was nil. He did crack a chunk of the plenum while he was at it.
I think i got the high speed issue solved too. On some Gm trucks, the High speed blower setting goes through a separate relay above the plenum and blower motor. A different relay was no help. First, i determined that the relay was not engaging with the speed selector switch on high. The switch was ok, i had 12 volts there on high position, there was a small connector with 4 or six smaller wires that this circuit goes through that had a problem. I took the connector apart and work it and the relay clicks now. Its clicks, but still no blower. further testing with the fluke showed no power to the relay switched terminal. I did not have a shop manual, but a little sleuthing showed this feed went through a 3 pin connector at the bottom of the plenum and then through the firewall. Close inspection showed it melted some. I could not get it apart, but after working it i had the hot 12 volts back to the relay. a new connector or bypass with a butt splice should fix it. Seems a lot of people have this problem, but i had not seen this fix mentioned.....
bob
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Brother-in-law or not Bob, I'd make him buy me a new truck for breaking that piece.

Good sleuthing Bob. The AC relay is generally one of the first places to look for the no high speed problem, but I had not heard about any connector/wiring problems. That kind of tracing can get nasty and it's good to see info like this to keep for future reference.
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Mike Marlow wrote:

Well, it not my truck ;) Its for a friend of ours that runs a tow truck and salvages cars. Its on the tow truck. we got it back together, ALMOST done. first, the low speeds blew the fuse when it was put back together. bro in law pried off the screen from the evap coil when he replaced it and forgot and left it in the plenum. this screen then shorted out the blower motor resistor pack to ground. whoops.
So now, all the speeds work and the plenum is back together. got my vacuum pump out and sucked down the system for a couple of hours. Went to fill it and could not get the compressor to come on. hot-wired the low side switch, no go. tried a different AC relay, no go. I hot wired the relay to get the compressor to go and fill it with R134. seems to be cooling ok.
further testing showed that i do not have ground at the fuse box under the hood for the relay for the AC. the rest of the relays next ot it seem to be working ok. So my mission now, is the ground somehow separate, or where is it really connected to ground from the ac relay on the main fuse box?
I ran out of time, and taking the fuse box loose seems to be somewhat of a pain in the ass operation. If anyone has the factory shop manual on this and would care to enlighten me before i tear the fuse box off all help would be appreciated. What i need to know is where the small ground wire from the ac relay socket grounds at? all this wire does is complete the circuit to the AC relays coil. not a high current wire at all.
Bob
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bob urz wrote:

It's ground controlled. By the pcm.
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