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Ask Juan Crespo Your Own Question
Juan Crespo
Juan Crespo, Tech Trainer
Category: Mazda
Satisfied Customers: 1523
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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I have a 99 Mazda protege dx 1.6 automatic transmission with

Customer Question

I have a 99 Mazda protege dx 1.6 automatic transmission with 154000 miles which runs great as long as you do not turn on heater or ac. When it is turned on, the engine stumbles/misses every time the ac compressor engages, more so when transmission is in gear and at a full stop. I have checked the battery and charging system and both tested good! Checked all vacuum hoses, gaskets and found no vacuum leaks. I do have code p0420 ( catalyst efficiency below threshold ). Throttle response is excellent and have no other engine management issues. Will the heater still work if I disconnect the ac compressor clutch? Any help would be greatly appreciated, thank you and have a very happy new year. Robert (Tito) Toro, Clarksville Tn ***@******.***
Submitted: 1 year ago.
Category: Mazda
Expert:  K. Wiggins replied 1 year ago.

Good morning Robert and happy new year! The heat will work fine without the compressor clutch being plugged in. I would unplug the clutch and see how it does then.

It does sound like the compressor may be going bad and putting a heavy load on the engine causing the problem here.

Let me know how it goes and we can work further if needed.

Customer: replied 1 year ago.
Just disconnected the compressor clutch, it does not engage when heater is turned on. There is no change in engine performance, it still stumbles/misses as before. What else can l check/test to correct this problem?
Expert:  Juan Crespo replied 1 year ago.

Hi there.

Help me understand what's going on so I can help you diagnose the issue; the way I first understood, it seemed that you said the engine missed/stumbled only when the A/C compressor was engaged. Can you remove the blower fuse and tell me if the stumblking goes away?

Customer: replied 1 year ago.
The engine stumbles/misses when the heater blower switch is turned to position 1,2,3 or 4. The battery and alternator checked very good at local Autozone (I am a 14 yr. Autozone employee). Juan Crespo (Mazda mechanic) suggested I remove the blower fuse, it disables the blower motor and would not be able to troubleshoot the problem. I cannot find troubleshooting information for this problem. K. Wiggins (ASC Tech) suggested the ac compressor could be putting a heavy load on the engine. I disconnected the compressor clutch and the problem is still there. I question how an accessory circuit (blower motor) will have an effect on engine management. I couldn't find any TSB that addresses this issue. I need troubleshooting guidance.
Expert:  Juan Crespo replied 1 year ago.

Since you state "The engine stumbles/misses when the heater blower switch is turned to position 1,2,3 or 4." The logical thing to do would be to remove the fuse to confirm the blower circuit is where the problem lies that is causing the stumble. Once we knew that for a fact, we could then reinstall the fuse and proceed to check each of the components connected to that circuit (blower motor, resistor, switch) until the culprit was found - which would best be accomplished by mesuring voltage drop on each.

I'm a retired Master Tech and Technician Instructor, but if you feel the advice I'm giving you is wrong just let me know and I'll opt out of the question. Otherwise, we can work togrether and get your problem solved. Your choice.

Customer: replied 1 year ago.
Juan, I do not doubt your experience or technical expertise and would like to continue with you. I removed the 40amp "heater" fuse like you said, the car ran ok, no stumble, then I put the fuse back in and left the blower switch in "0" position, drove the car again, no stumble. I got home and while the car was still in drive, I turned the switch to position "1" and "2", the car started stumbling again. Can you send me the procedures for checking/testing the other components in that circuit? My work schedule will be tight starting tomorrow, but will somehow find the time to continue. I thank you very much for sticking with me. Prospero ano nuevo! (Just in case, the name and face are undeniable, you are Latino, so am I) respectfully Tito.
Expert:  Juan Crespo replied 1 year ago.

Thank you, ***** ***** me a few minutes to l0ok up the diagram and formulate the diagnosis. Be back in a few...

Expert:  Juan Crespo replied 1 year ago.

Hey Tito. After studying the diagram, I've come to the conclusion that the only thing that could be pulling that much current and still not cause the 40 amp fuse to blow, would be the blower resistor. So, to confirm, with the ignition off, turn the fan switch to position 4 (MAX) so ground is applied directly to the motor without going through the resistor; then start the engine. If there is no stumble, the resistor is bad and needs to be replaced. To do so, you'll need to remove the glove box to gain access to the blower motor housing. Once the glove box is out of the way, you'll see the resistor fastened to the housing by two Phillips screws. Unplug the resistor and remove the two screws, then pull the resistor out of the housing.

Autozone lists the resistor as Duralast part number JA1169.

Best Regards.

Customer: replied 1 year ago.
Juan, I just got home from work, read your message and did what you said, the engine did not stumble! Earlier today I ordered the blower resistor and will get it tomorrow. I won't be able to replace it until Saturday or Sunday. Will I still be able to communicate with you and let you know the outcome? I feel confident, you are right and would like to let you know. Again thank you so very much for hanging with me and know how grateful I am for your help! Your friend, Tito.
Expert:  Juan Crespo replied 1 year ago.

Thank you for sharing the good news. Looking forward to hearing from you soon.