I have an 89 Formula 350 and every time I turn on my heat/ac blower on high my engine starts to run real bad, almost like it is skipping and the tach starts to bounce. Also if the car is moving the needle on the mph gauge starts to bounce as well. This happens only when I turn on the blower regardless if it is for heat or ac vent ect.
I have a brand new alternator and and I unpluged the ac compressor and still no change. I have no idea how the blower could cause this any ideas?

 

I know this problem is weird .....but any ideas?

 

This is a total shot in the dark but sense no one has posted…. The only thing I can think of is RFI getting in the computer through one of the sensor wires. How would you test for this? You could check the current pull from the motor at high speed, then replace the motor with a resistor (high wattage) that will pull the same current. If your car then runs normal, you know it is RFI. How would you fix this? Well, you would what to make shore that the ground to the motor is good. Make shore that the computer harness in the engine compartment is not to close the motor. You could try putting a ferrite core on the B+ line to the motor. But most of the time that a motor is putting out a high level of RFI, it is because the motor is bad (going bad). That should keep you busy, vary busy.

Or you could just go to the junkyard a get a cheep motor and try that.

 

You should scan it with a scan tool when this happens; check your voltages. The heater motor could be drawing enough power to cause a voltage drop down to where the ECM isn't happy; or it could also be emi as someone else suggested. Watching sensor input while it occurs should point you in the right direction.