Stalling when coming to a stop with A/C on

fstcoup

SCCoA Member
Has anyone run into this problem. I have two 90"s one auto the other a 5 speed and it affects both the same way. Both A/C systems are all new other than the evap and I have changed the IAC twice in the 5 speed and it made no difference. This only happens with the A/C on and when slowing to a stop, when I switch the A/C off no problems in either.
Any help would be appreciated.
 

DOUG H

SCCoA Member
I would agree that it's a IAC issue, but you've eliminated that possibility.
Base throttle plate setting/gap? I dont think you should ever have to touch it
though? I think its even locktited to discourage people from messing with it.
A/C overcharged & head pressure is too high?
 

firstandtenth

SCCoA Member
I've experienced this several times over the 32 years I've had the car, but it is pretty rare that I use the AC. It is worse with the AC on, but happens when it is off - more often with quick deceleration. Most recently it happened as I stopped at the entrance gate for a Ford show - just a little embarrassing. I reached out to the best Ford techs I know and there are a few possibilities. One thought is the MAF malfunctioning and sending an error code. Other thoughts are a vacuum leak (I haven't found one yet) and TPS malfunction. One of those technicians just installed a new compressor for my AC last year and believes the pressure was correct after install (can that change?). I have always noticed a drop in rpms when the AC cycles on, but comes back up quickly. Sorry I don't have a solution, I just have a similar problem. Like fstcoup, I would appreciate any thoughts.
 

fstcoup

SCCoA Member
The auto's engine was removed from the car and all mating surfaces resealed, did a smoke test no leaks. The 5 speed was also rebuilt and had no issues until about 200 miles. pressures are correct for both. The 5 speed has a MP 95mm TB the auto a 70mm Edelbrock TB both MAF's have been cleaned and inspected and they have no other issues.
 

DrFishbone

SCCoA Member
The auto's engine was removed from the car and all mating surfaces resealed, did a smoke test no leaks. The 5 speed was also rebuilt and had no issues until about 200 miles. pressures are correct for both. The 5 speed has a MP 95mm TB the auto a 70mm Edelbrock TB both MAF's have been cleaned and inspected and they have no other issues.
Are you sure that the TB's blade stops are set up properly? Basically, the IAC should be operating at the lower end of its range at idle. The simplest way to check is see if the car will hold an idle (A/C off, in park) with the IAC unplugged. It will probably get close to dying when unplugging the IAC.
 

J dot Miller

SCCoA Member
I had that problem before and the two speed fan motor had a short on one of the speeds.

If you have a scanner you can test both of the fan speeds (yes your fan has a low and a high speed). If not put your AC on. Your low speed fan should turn on. If it does not then you are getting close to the problem.

Turn off the car and disconnect the fan motor. You can check the resistance between the motor connectors. Resistance should peg the scale between two the ends and the center wires. If not you have a suspected bad fan motor.

If you do not have a ohm meter start the car and check the voltage at the connectors. The middle to one of the ends should read more than 10.5 volts. If it does then you motor is bad. Do not run the car long with the fan disconnected.
 

DOUG H

SCCoA Member
So, what do you suspect was causing the engine to stall with the shorted fan speed/motor?
Drawing the system voltage down too low & causing ECM to faulter or there was so much additional
load placed on the alternator that with the load from the compressor as well,that the IAC couldn't compensate enough
or quick enough? Possibly without the fan running, the high side(head pressure) of the A/C system got too high without airflow through the condenser & put too much workload on the compressor ? May be a combo of all. I would've thought a short would have blown the fuse or popped the breaker(s) for fan though. I've never experienced that issue though(yet).
 

J dot Miller

SCCoA Member
It was many years ago. I can only guess that it was a combination of low RPM, low speed fan short and compressor load. I did not solve the problem until after it was stalling every time I stopped at a light. It could have damaged my alternator as it was the same year I replaced my alternator. I had no issues with my IAC and ECM. No blown fuses. Maybe there should be some fuses added to the fan motor wiring...
 
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