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95 Bronco, 351W, E4OD, 4.56 gears, 35x12.50x15 Patagonia MTs.
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Discussion Starter #1
As stated, the airflow stopped in my vents when I was driving uphill under load for 30 seconds or so. After letting off the throttle, the vents promptly regained airflow. There is no obvious vacuum leak, with KOEO, the vacuum holds the dampers just fine for well over a minute, and tested after waiting, has enough reserve to cycle the damper from max ac all the way to floor and back again. I verified the vacuum check valve was operating and it was, although intermittent failure could be possible. The infamous white hose has been repaired, but cannot cause this issue, as my F350 has a broken white hose and does not have this airflow issue. Any other ideas for places to check?
 

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Yo two,
From 1996 Bronco Workshop Manual;
"
DIAGNOSIS AND TESTING
Pinpoint Tests
NOTE: Use Rotunda 73 Digital Multimeter 105-R0051 or equivalent when performing electrical pinpoint tests.






The fastest way to find a condition is to work with the tools that are available, which means working with proven diagnosis charts and the proper special tools for the system being worked on.

Refer to the following charts for performance testing procedures.

PINPOINT TEST A: INSUFFICIENT HEAT
A1 CHECK ENGINE COOLANT LEVEL
  • Allow engine cooling system to fully cool and check radiator coolant level.
Is coolant level low?
YesNo
FILL and BLEED engine cooling system. REFER to Section 03-03. RETEST system.GO to A2.
A2 CHECK HEATER HOSE ROUTING AND CONDITION
  • Check that heater water hoses are not crossed or routed incorrectly. Refer to Section 12-02 for heater hose routing.
  • Visually inspect condition of heater water hoses for pinched or collapsed conditions.
Are heater water hoses in good condition and routed properly?
YesNo
GO to A3.REPAIR or REPLACE heater water hoses as necessary. RETEST system.
A3 CHECK COOLANT FLOW THROUGH HEATER CORE
  • Start vehicle and allow to reach normal operating temperature.
  • Place temperature control knob in the full warm position.
  • Grasp inlet and outlet heater water hoses. Both heater water hoses should be warm to the touch due to coolant flowing through them.
Do heater water hoses indicate coolant flow?
YesNo
GO to A4.FLUSH heater core to remove obstructions. RETEST system. If flushing does not correct condition, REPLACE heater core. RETEST system.
A4 TEST THERMOSTAT
  • Start vehicle and allow to run for 10 minutes.
  • Feel the heater water hoses and the underside of the upper radiator hose.
Are the heater water hoses and the upper radiator hose cool?
YesNo
REPLACE thermostat. RETEST system.GO to A5.
A5 CHECK TEMPERATURE CONTROL DOOR OPERATION
  • Rotate temperature control knob from full warm to full cool and check temperature door for operation.
Does temperature door operate correctly?
YesNo
CHECK for head gasket leaks. REFER to Section 03-00 for information on cylinder heads. REPAIR as necessary. RETEST system.CHECK for binding, damaged or disconnected temperature door. REPAIR or REPLACE as necessary. RETEST system.

Vacuum Leak Diagnosis

The air handling system is designed to provide defrost when no vacuum is applied to any of the three air door vacuum motors. This is done to prevent a situation where defrost cannot be obtained due to a system vacuum leak. Instead, a leak in the vacuum control circuit will send all airflow to the defroster outlets. This condition may occur during acceleration (slow leak), may exist at all times (large leak) and may happen only when certain specific functions are selected, indicating a leak in that portion of the circuit.

The vacuum hoses used in the passenger compartment control circuit are constructed from PVC plastic material. The vacuum hoses used in the engine compartment are constructed of Hytrel. Because of the materials used, the vacuum hoses should never be pinched off during diagnosis to locate a leak. Use Rotunda Vacuum Tester 014-R1054 or equivalent to locate vacuum leaks. A wood golf tee can be used as a plug when it is necessary to plug one end of a vacuum hose for leak test purposes.

PINPOINT TEST B: DEFROST DOES NOT OPERATE
B1 CHECK SYSTEM AIRFLOW
  • With engine running and blower on high, check system airflow in each function selector position to determine which position(s) have incorrect airflow. Refer to vacuum application chart for correct system airflow.
Is airflow from the defroster outlets for all positions?
YesNo
GO to B2.GO to B13.
B2 CHECK VACUUM SUPPLY HOSE
  • Check vacuum supply hose to be sure it is connected to both the engine manifold and vacuum check valve.
Is hose disconnected?
YesNo
RECONNECT hose. GO to B1.GO to B3.
B3 CHECK VACUUM SUPPLY HOSE FOR LEAKS
  • Disconnect vacuum supply hose from engine manifold fitting and from check valve. Plug one end of hose and leak test with vacuum pump.
Does hose leak?
YesNo
REPLACE or REPAIR as necessary. GO to B1.CONNECT hose to manifold fitting. GO to B4.
B4 CHECK VACUUM CHECK VALVE
  • Check vacuum check valve for correct installation by removing the reservoir hose and the control assembly source hose (one at a time) from check valve and check for vacuum. Engine must be running for this test.


Is vacuum available at both check valve ports?
YesNo
RECONNECT both hoses. GO to B6.GO to B5.
B5 INSPECT VACUUM CHECK VALVE
  • Inspect vacuum check valve for an obstruction and/or proper operation. Airflow through the check valve should be in the direction toward engine (engine connection side is marked VAC).
Is check valve plugged or obstructed?
YesNo
REPLACE check valve. GO to B1.GO to B6.
B6 LEAK TEST VACUUM CHECK VALVE
  • Remove vacuum check valve from vehicle. Connect 2 hoses and a Tee fitting to outlet ports of check valve. Connect Rotunda Vacuum Tester 014-R1054 or equivalent to Tee fitting.



  • Pump 15 inches vacuum on check valve and observe gauge reading. If vacuum loss exceeds 1 inch per minute, remove check valve from tester and plug vacuum hoses. Pull a vacuum with the tester to be certain hoses and tester are not cause of leak.



  • If tester and hoses do not leak, check valve is cause of leak.
Does check valve lose more than 1 in. vacuum in one minute?
YesNo
REPLACE check valve. GO to B1.REINSTALL check valve and GO to B7.
B7 CHECK VACUUM RESERVOIR
  • Connect vacuum tester to vacuum reservoir and leak test reservoir. Reservoir must hold vacuum.
Does reservoir leak?
YesNo
REPLACE reservoir. GO to B1.CONNECT hose to reservoir. GO to B8.
B8 CHECK SUPPLY HOSE FOR LEAK
  • Disconnect and plug BLACK supply hose at control assembly. Disconnect other end of supply hose from check valve and leak test hose with vacuum tester.
Does supply hose leak?
YesNo
REPAIR or REPLACE hose. GO to B1.REMOVE plug and RECONNECT vacuum hose. GO to B9.
B9 CHECK CONTROL ASSEMBLY
  • Disconnect vacuum harness at in-line multiple connector near control. Plug all vacuum hoses except BLACK. Connect vacuum tester to BLACK hose. Select each function position, apply 15 in. vacuum and check for vacuum drop. Drop should not exceed 1 in. vacuum per minute for any function position.
Does drop exceed 1 in. vacuum per minute?
YesNo
NOTE function positions where vacuum drops. GO to B10.GO to B11.
B10 LEAK TEST CONTROL ASSEMBLY
  • Remove vacuum harness from control assembly. Connect vacuum tester to control supply port and plug control port that indicated a leak in Step B9. Select function positions noted in Step B9 and apply 15 in. vacuum after selecting each position. Vacuum drop should not exceed 1/2 inch per minute.
Does vacuum drop exceed 1/2 inch in one minute?
YesNo
REPLACE control assembly. GO to B1.GO to B12.
B11 CHECK SUPPLY HOSE
  • Disconnect BLACK supply hose from check valve. Connect vacuum tester to supply hose and operate tester as if pulling a vacuum. If tester can pull a vacuum, hose is plugged. If tester pulls a partial vacuum, hose is restricted.
Is hose plugged or restricted?
YesNo
REPLACE supply hose. GO to B1.GO to B15.
B12 LEAK TEST JUMPER VACUUM HARNESS
  • Plug one end of vacuum hose that indicated a leak in Step B9. Apply 15 in. vacuum to hose and observe vacuum gauge. Vacuum should not drop.
Does vacuum drop?
YesNo
REPAIR and/or REPLACE vacuum jumper harness. GO to B1.CHECK connection of vacuum harness to control assembly for leak. REPAIR as necessary. GO to B1.
B13 EVALUATE SYSTEM AIRFLOW
Is the airflow in Step B1 correct for each function position?
YesNo
GO to B14.GO to B15.
B14 ISOLATE LEAKING VACUUM CIRCUIT
  • Repeat step B1 and accelerate engine speed for each function position.
Does airflow go to defrost during acceleration?
YesNo
GO to B19.GO to B15.
B15 REVIEW VEHICLE HISTORY
Did system function properly prior to this complaint?
YesNo
GO to B18.GO to B16.
B16 CHECK VACUUM HOSES
  • Check each vacuum hose to be sure it is not plugged.
Is a hose plugged?
YesNo
REPAIR or REPLACE as necessary. GO to B1.GO to B17.
B17 CHECK VACUUM HARNESS
  • Compare vacuum hose colors in each vacuum harness to the vacuum schematic.
Do the hose colors agree with the schematic?
YesNo
GO to B18.REPLACE vacuum harness. GO to B1.
B18 CHECK VACUUM CIRCUIT
  • Check vacuum circuit for pinched or kinked vacuum hose.
Is hose pinched or kinked?
YesNo
REPAIR as necessary. GO to B1.GO to B19.
B19 CHECK VACUUM CIRCUIT CONNECTION
  • Check each vacuum hose connection to determine if it is partially connected or disconnected.
Is a vacuum hose connection disconnected or partially connected?
YesNo
RECONNECT hose. GO to B1.GO to B20.
B20 CHECK VACUUM HOSE
  • Disconnect both ends of vacuum hose in suspected circuit. Plug one end and leak test with vacuum tester.
Does vacuum hose leak?
YesNo
REPAIR or REPLACE hose. GO to B1.GO to B21.
B21 CHECK VACUUM MOTOR
  • Check vacuum motor for leaks with vacuum tester.
Does motor hold vacuum?
YesNo
GO to B22.REPLACE vacuum motor. GO to B1.
B22 CHECK VACUUM MOTOR INSTALLATION
  • Check attachment of vacuum motor arm to damper door.
Is motor arm attached to door or door crank arm?
YesNo
CHECK for binding or damaged damper door. REPAIR as necessary. GO to B1.CONNECT motor arm to door and/or crank arm and CHECK operation. GO to B1.
PINPOINT TEST C: INSUFFICIENT A/C COOLING
 

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1984, 300 L6, smogless, manual 3speed with overdrive.
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Did your air divert to a different location, defrost or floor..... or did the fan cut out? There’s a “squirrel box” fan in your heater box with the core, the vacuums just switch the doors for selected output.
 

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take the vacuum line off the air box in the engine bay and connect it to a vacuum ball. i had the same issue and that fixed it.
Yup. The blend doors are controlled by vacuum, and when the engine is under load, there's very little to no vacuum. This will mess with the blend doors.

In the engine bay, there's either a ball, or a thing that looks like a coffee can. This is a vacuum canister that stores a large amount of vacuum (and will keep it for a while with the engine under load) for things like your heater controls. If it's not connected to that, it should be.
 

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On a 1995 Bronco/F150 the HVAC vacuum reservoir is mounted to the blower/condenser box:
158860
 

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1990 Bronco Custom 5.0
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take the vacuum line off the air box in the engine bay and connect it to a vacuum ball. i had the same issue and that fixed it.
Where did you find one? I'm in a 1990 5.0.

It's just the one vacuum line connecting to the ball instead of the air box, correct?

Sent from my SM-G986U using Tapatalk
 

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September 15 FOTM
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Where did you find one? I'm in a 1990 5.0.

It's just the one vacuum line connecting to the ball instead of the air box, correct?

Sent from my SM-G986U using Tapatalk
junkyard or parts store. its not brand or model specific. just any vacuum ball with one vac line port.
yes take the line off the air box and put it on the vacuum ball
 

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1990 Bronco Custom 5.0
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Does someone have a picture of this installed? It seems simple but for some reason it's not making sense to me.

Below is the one I got. To use the vacuum line from the airbox, I'll need to modify it to fit and which side do I plug it into?

I feel like I'm overcomplicating this haha
 

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1989 Bronco, Eddie Bauer, Raven Black
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I'm also having this issue but airflow like OP stated just stops. It's not coming out the defrost. Under load. I'll dig into it with the info provided.
 

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September 15 FOTM
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Does someone have a picture of this installed? It seems simple but for some reason it's not making sense to me.

Below is the one I got. To use the vacuum line from the airbox, I'll need to modify it to fit and which side do I plug it into?

I feel like I'm overcomplicating this haha
plug the line into the side that fits and cap off the other one. you shouldnt have to modify the line, it should fit one of the sides fine
 

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95 Bronco, 351W, E4OD, 4.56 gears, 35x12.50x15 Patagonia MTs.
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242 Posts
Discussion Starter #20
I'm glad I stirred up some relevent info for everyone! The Bronco did it again today, after trying to make it act up for a week, I'm leaning towards an intermittent issue with the check valve. I'll dig in tomorrow and vacuum test some things as well.

Last weekend I was able to verify the blower motor is happy, and the evaporator has been cleaned. It was nasty, I have better airflow now as well. However, this did not correct my issue since I didn't find any problems.

Upon further inspection, it may be partially transitioning to defrost and then back to AC, and not allowing fully flow through defrost while I check. My ill fitting dash mat makes this diagnosis interesting to confirm while driving down the freeway.

Thanks again for all the tips.
 
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