Does this sound normal?

[BWMJ]

I recently noticed that if I have the heater fan cranking and give the truck a lot of gas, the fan drops down to low speed and then comes back to high when I let off. I've had some issues with this truck stumbling and wonder if this is an indicator of a poor charging system. '89 4.0 auto.

[Pete M]

Try cleaning up your battery cable connections and all grounds in the engine bay. :thumbsup:

[CWLONGSHOT]

I agree with Pete. This is not normal or correct. You have something electrical going on and you need to find it before you need another post here... How to open your hood after/during a fire!!!

 

Some time back, I upgraded my MJ's electrical with a big alt and new MUCH heavier wires. WHAT A DIFFERENCE!! It started, ran and produced better brighter lites!!! My cables where not allowing much electrical "flow". DC electric like whats in a vehicle is much like a water pipe, the bigger it is the better the flow. More power needs bigger wire. As wire ages its resistance raises. Causing the problems your experiencing. Start with new cables to and from the solenoid and starter and check the charging wire to the alternator.

 

CW

[garmj]

I recently noticed that if I have the heater fan cranking and give the truck a lot of gas, the fan drops down to low speed and then comes back to high when I let off. I've had some issues with this truck stumbling and wonder if this is an indicator of a poor charging system. '89 4.0 auto.

What you have is a vacuum leak its causing the air to come out the defrosters under hard acceleration and return to normal when not giving it the gas ,it only sounds like its changing speed. That also accounts for your stumble and poor fuel mileage. It can be any thing from a bad brake booster to cracked plastic vacuum lines to a bad A/C control panel.If it was a charging problem it would more likely have the opposite symptoms.

[shelbyluvv]

I recently noticed that if I have the heater fan cranking and give the truck a lot of gas, the fan drops down to low speed and then comes back to high when I let off. I've had some issues with this truck stumbling and wonder if this is an indicator of a poor charging system. '89 4.0 auto.

What you have is a vacuum leak its causing the air to come out the defrosters under hard acceleration and return to normal when not giving it the gas ,it only sounds like its changing speed. That also accounts for your stumble and poor fuel mileage. It can be any thing from a bad brake booster to cracked plastic vacuum lines to a bad A/C control panel.If it was a charging problem it would more likely have the opposite symptoms.

 

I would check and see if the line is hooked up to the "blimp" under the front bumper. They tend to rot off after awhile.