It is hot in Florida and as I drive my '99 XK8 with 172k on it the AC works fine on auto. About three times a day while stopped at a light the fans will slow and the voltage meter drops to about 10v and works hard. I have had the battery and alternator checked and all passed with "good" at auto parts store. The alternator is less than a year old and the battery about a year as is the serpentine belt.
Any ideas?
Thanks!

 

Sounds more like an electrical gremlin than an AC problem. When the problem presents itself, does it then recover or does the voltage stay low? What happens to the engine speed - does that drop as well?

 

Thank you for the reply.
At speed there is no issue, only when stopped and only when in the 90'sF
It drops and I panic and turn off the ac which removes the problem. I wait a minute and turn it back on and it is fine.

 

BELT slipping??????

Crank Damper delaminated???

bob

 

Reportedly, the earlier 4.0 cars have a 2-speed radiator fan system (low and high I suppose). When the AC is on, the fan is on Low, no matter the temperature. When you come to a light, the air flow diminishes, and the coolant temperature goes up.

From what you are describing, there could be some form of electrical issue with the High circuit. I would check the fuses as well to make sure they are per spec. I would think that if the current is so high as to drop the voltage to 10V, the fuses would protect the wiring. The system has relays, too, and they need to be checked. The electrical wiring diagrams are available online for complete analysis.

Separately, folks have reported many times on their car's cooling system needing to go to high fan at low vehicle speed. These situations usually call for some form of service to the cooling system. Starting with water pump, thermostat, and sensors. Have you seen any evidence of leaks or a need to refill? Have you had any cooling service recently? Is there an off chance cooling fluid of different nature were mixed and are gelling, causing the overheat in the first place?

Just my thoughts. Best of luck, keep us posted.

 

Larue
I once had a similar symptom when a fused cable I ran from the battery to the amps in the trunk (different car) touched an exhaust pipe on certain turns and bumps. The exhaust melted the insulation and I'd get these real short short circuits. The symptoms were awfully similar and it would explain the voltage drop. Hope it helps in some way. John

 

I do not know about Jaguar, but my Mercedes has a higher idle setting it would assume, when the A/C was switched on, increasing the idle speed to compensate for the added draw when running the compressor.

 

Hi, My guess is that the alternator you replaced last year doesn't have a high enough current rating, especially at operating temperature. You had it checked as "good", but that doesn't mean it will put out the current the circuit demands. The fact that you see the output drop to 10V indicates there is more current demand than the alternator can supply.
I would check:
1) the rating of the alternator (I believe this should be 110A),
2) the pulley diameter, since a larger alternator pulley will provide lower ratio, and especially at idle this will have a dramatic effect on output current.


All the best,


Jerry Lippmann