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FlatTwin

Heating and A/C system problems

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Guys, Am still having problems with the heating and cooling in the cab of the truck. Driving home tonight it was getting a bit chilly here, in the mid 50's. Turned the heat control from full cold to mid-way up the scale. Nothing happened at all. Had the blower on fresh air. Turned it 3/4rd hot and still no change. Finally went full tilt hot and about 3-5 seconds later heard a vent door shut and immediately it got full hot.Ok, so we know that there is hot water in the heater core. Turned the temp control back to 1/2 scale and no change. Turned it back to full cold and no change, its still full tilt hot. After about 15 seconds it went from full hot to full cold outside air. Stayed that way for 30 seconds or so then "clunk" the flapper door somewhere in the dash closed and full hot again.It then cycled back and forth like that all the way home. Its got a mind of its own. I need to make a trip back to the mid-west next week and would really like to have heat on the way there. Anyone have something like this happen or ideas or suggestions where to look? Thx,Joe

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right out of the FSMVacuum control is used to operate the mode doorsin the heater-only and HVAC housings. Testing of theheater-only and a/c heater mode control switch operationwill determine if the vacuum, electrical, andmechanical controls are functioning. However, it ispossible that a vacuum control system that operatesperfectly at engine idle (high engine vacuum) maynot function properly at high engine speeds or loads(low engine vacuum). This can be caused by leaks inthe vacuum system, or by a faulty or improperlyinstalled vacuum check valve.A vacuum system test will help to identify thesource of poor vacuum system performance or vacuumsystem leaks.

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On the passenger side of the cab near the trans hump there is a electric stepper motor that controls the blend door for heat and cold. Rather common to see failures with the step motor or the blean door itsself. There is a rebuild kit for the blend door but I don't have the information...

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Guess, Thanks for the info but how does one do a "vacuum system test?" I do not have the manual you do so kinda in the dark here.Michael, That could be it. I can hear the flapper door opening and closing back and forth. I had thought that something controlling it was bad but could be the door itself sticking or not moving. Will look for it tomorrow.Joe

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Guess, Thanks for the info but how does one do a "vacuum system test?" I do not have the manual you do so kinda in the dark here. Michael, That could be it. I can hear the flapper door opening and closing back and forth. I had thought that something controlling it was bad but could be the door itself sticking or not moving. Will look for it tomorrow. Joe

Here is the books... http://forum.mopar1973man.com/downloads.php?do=cat&id=2 The blend door is electric controlled not vacuum...

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I believe that diesels use a vacume pump, not engine vacume to operate vacume accessories. Check me if I'm wrong...Old school gas engines create vacume beneath the throttle plate at less than WOT. At WOT the engine vacume goes to "zero". Engine speed is controled by the throttle plates which regulate the amount of flow of fuel air mix. Fuel air mix is created by airflow through a venturi. The low vacume at WOT is why the earlier text refers to a vacume check valve which holds some vacume for use when vacume is low & some vacume resevoir.A diesel on the other hand has no throttle plate, the intake is clear & the engine draws in the same amount of air on each stroke.. Engine speed is controled by fuel metering. My old gas engine kit from my mechanic days has a dual fuel pressure / vacume guage which would be tapped into the appropriate port for testing.Russ

Edited by flagmanruss

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I believe that diesels use a vacume pump, not engine vacume to operate vacume accessories. Check me if I'm wrong... Russ

Yeap your right... The vacuum pump is just forward of the power steering pump. There should be a vacuum line from there to all accessories. Mostly back inside the cab for HVAC controls. If there was a lack of vacuum the vents automaticaly set to DEF mode for safety. so if you can control placement of air then vacuum isn't a issues.

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Here is the books... http://forum.mopar1973man.com/downloads.php?do=cat&id=2 The blend door is electric controlled not vacuum...

File above at PDFTown supposedly used to be available for free, but now says that its only available for "premium" pay users. Am unemployed so money is short. Other file for 2nd generation trucks requires a password and it does not like mine, even though its the same one I use to log into the forum. Next idea?

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