I have a 85 XJS HE v-12. The heat inside the car comes and goes. Two days ago coming home from work, heater on defrost plenty of heat. Car got warm so I turned it down to low. Air came out of the vent cool/cold. Tried
to put it back to defrost, nothing just stuck on vent and cold.
Yesterday on my way to a meeting, out of the blue the air started
changing were it came out of. I had it on defrost and had heat but
it would go to the vent,defrost,vent,defrost. I had heat so I
didn't mess with it. Today came home from work had heat on defrost,
car got warm turned it down to low, came out of the vent cool/cold.
I can change the defrost and high but have no heat. Checked the
heater control valve, I have heat on both hoses, in and out, but no
heat in the car. What gives?? With the weather turning cool I
need to fix this ASAP.

Any ideas as to why I have no heat in the car most of the time.
I'm not sure how the heat selector works, so any help here would be
very appreciated.

Thanks,
Bill

Submitted by keith.loretta@… on Sun, 09/28/2008 - 21:12

Bill, I'd bet you a dollar to a donut its your AC amplifier. When defective it does exactly like your explaining. Actually thats what it controls when its working properly. The noises your hearing is exactly what you suspected, servos. The fact thats its all working, more or less, at different times, shows that these things are actually working. The defrost will only work on high speed. The AC Amp is located under, and in front of your console, drivers side. A small, cigarette pack size black box, with several wires attached to it. You'll get a real case of heartburn when you check prices at various suppliers. Anywhere from $250.00 and up. BUT, have no fear, I located one for my car at Caulfield Jags website, think it was $180, or there bouts. Shipping was 3 days. Their aftermarket amp is shaped and sized like a 12 gauge shotgun shell, and is better than the original. Just attach the wires, don't even have to remove the old amp. I've had mine for 3 summers, and working great. My symptons were identical to yours BTW.

Submitted by buck_1one@yahoo.com on Wed, 09/17/2008 - 04:41

Here is some more info I have. I'm loosing my heat because for some
reason the system is going into A/C mode even though I have made no
changes to the controls. I can hear something make a whinning type
of noise, I'm guessing that is the servo. When this happens the
system changes.

Bill