Defrost Help!!!

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74.Dodge.Dart.S

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Hey quick question again. On my 74 Dodge Dart Sport, I cant seem to figure out what is wrong with the defrost. All the other switches work (hot, cold, fan speeds ect.) But I cant get it to switch switch at all from defrost to vents. Any one know what I can do to get it to work again?
 
So if I understand this correctly, you are saying that it blows through the defrost vents all the time, correct? If that is the case, it has lost vacuum to the HVAC box. The default position for the system without vacuum on it is through the defrost vents. I would look for a disconnected, broken or rotted vacuum connection.
 
It has ac. But yes its mainly on defrost with only a little air coming out of the vents because of the flap isn't all the way closed. (the flap is free moving too) This may sound dumb but where would I start looking for a leak? Does the vacuum come from the brake booster or where does it get its pull from? I know that its connected to the controls unit because i have taken the dash out and installed a used but newer unit into it... thanks for the response guys!!
 
Just follow the vacuum lines. They run through the firewall and under the dash.
 
also make sure that the doors for the vacuum actuators are not stuck in position...
 
Under the hood... I would first look for a vacuum reservoir. It might look like a ball. 2 vacuum lines attached. One goes inside the cab. Other goes back to engines vacuum supply ( fitting in intake that looks like a tree ). Once you prove the vacuum supply going into the cab is good, you know where the fault isn't at.
 
TrailBeast said:
And split right where they go on the switch, right? :D
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IIRC, the distribution function is handled by a lever from 1973-76 models. The vacuum line from the engine goes through the firewall into the function lever switch. On the pushbutton controlled A/C, it was slangly called the harmonica. If everything else works well, suspect the hose from the harmonica to the dashpot that controls the defroster door. Check to make sure the harmonica isn't cracked, too. You can test the dashpot, too, by running a suction hose directly from the engine to the dashpot. If it moves and does not hiss when you connect the hose, it is probably OK.
 
So i was able to go take a look at my car. I don't think that it is getting vacuum. I found the two vacuum hoses underneath the dash on the passenger side, and when I pulled off both neither had suction. I also found one on the drivers side where there's a vertical rod coming off of it. it didn't have suction either. So I think I need to back up and take a look at the intake manifold now...
 
74.Dodge.Dart.S said:
How much suction is needed for it to work?
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Not a whole lot. I can't recall if I can move a dashpot by sucking on a hose or not. Why worry about about oxygen deprivation when the engine is a great vacuum source and needs to be tested anyway.

The best way to test is to buy about 8 feet of appropriately sized hose. Run hose from vacuum tap directly to each dashpot. Once dashpots have been tested, it's time to check individual lines.

IIRC, the vacuum hoses through the firewall go from the engine to the harmonica, the other from the harmonica to the hot water supply valve under the hood. The line from the engine to the harmonica, I think, has a Tee that connects to the vacuum accumulator/amplifier.

Since the hose from the engine to the harmonica is the most critical, test it first. No vacuum to harmonica, nothing else works. Start engine, disconnect hose at harmonica. There should be suction at all times on this hose at all times. The line to the accumulator/amplifier should also have full-time vacuum. Get these working properly first. The hot water valve line should only have suction when the A/C is selected.

I don't recall much about where the lines off the harmonica are supposed to go, so check them one at a time assuming each is connected to the correct door dashpot. After all have been tested, cut up the test hose to replace bad lines, if any.
 
Ok So I have another question. I figured out that there was no vacuum on the system. (The owner before me didn't have it hooked up at all) So my question is, Can I just hook it up to a "T" off of my brake booster? Or since my 4bbl Carb has a port off of it (the one that typically could run to the auto trans) can I put a line off of that or is that going to affect performance in any way?
 
74.Dodge.Dart.S said:
Ok So I have another question. I figured out that there was no vacuum on the system. (The owner before me didn't have it hooked up at all) So my question is, Can I just hook it up to a "T" off of my brake booster? Or since my 4bbl Carb has a port off of it (the one that typically could run to the auto trans) can I put a line off of that or is that going to affect performance in any way?
Click to expand...

Use any intake manifold source. The brake booster is 3/8", I would try to find a smaller port. Sometimes there is a vacuum tree with more than one outlet in the intake and it also has the big port for the brake booster, use one of the smaller "legs" of the "octopus", or use one from the base plate of the carb. Or use a tee in one of the smaller diameter lines, as long as it's intake vacuum, not ported vacuum which is for the vacuum advance on the distributor...
 
Sweet Ill try the other one on the base of the carb first! I know its not the ported vacuum because that is already hooked up to the distributor. But Thanks for the help!!
 
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