Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
Fans take huge electrical load bit this sounds like an alternator issue, is the idle speed normal ?
There are simple tests you can do but before testing it is important to check the battery has a good level of charge.
You can check the level of charge by measuring the voltage across the battery with the engine and everything else switched off this is called the open circuit voltage.
With engine running you would not expect to see below 13v. At 13.1v all the electrical loads are just being met but the battery is not getting any charge put back into it at all.
To get a true reading either measure this voltage after the vehicle has been parked for a few hours or if the engine has running turn the engine off and turn on the lights for two minutes or so, then switch the lights off.
Doing this strips the surface (false) charge off the battery and you can now check the voltage across the battery, if you read more than 12.4v you can test the alternator.
Normally no matter how many electrical loads are on the voltage at idle speed should never be below the battery's open circuit voltage.
At idle speed with no extra electrical loads on battery voltage should normally be in the range 13.4 to 14.5. increase the RPM to 2000 to 3000 and the voltage may increase but not above 14.9 volts. Normally at 2,000 RPM or above on average expect to see 14.0 to 14.4 volts.
Now repeat the test but with the head lights on and heater blower on at medium speed voltage should not drop below 13.1v at any point.
Finally with the engine at about 2,000 to 3,000 quickly switch off the lights heater and any other electrical electrical loads, at no point should the battery voltage go above 14.9 volts.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Strange thing is that ,I can turn on all other loads besides the blower that includes the rear defrost the voltage does not drop below . 13.4, the moment I turn on the blower with the lights on and that's all the voltage starts to drop below battery voltage.
jerry r wrote: Strange thing is that ,I car but i turn on all other loads besides the blower that includes the rear defrost the voltage does not drop below . 13.4, the moment I turn on the blower with the lights on and that's all the voltage starts to drop below battery voltage.
Alternators on modern cars are rated in the 80 to 160 amp minimum so it shouldn't be an issue. Fan motors do tend to draw more current as they get older but if it was way over spec I would expect it to blow a fuse. I would assume it has the smart charge system.
I'm not that expert on Ford Smart charging systems but to me that looks an issue in the lternator itself, there is a by-pass test for the system I will use the Google machine and post it,.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
The quick and dirty check on the alternator is simply to disconnect the 3 pin connector with this discconnected the alternator should produce a constat 13.75v as log as it is meeting the load.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Does it only dip into the 12V range with the blower on at idle? As in, you turn the blower on, and you start seeing 12.8/9 at the battery? Does the same reading improve if you raise the engine speed off idle?
yes at idle with the blower on,yes is rises with rpm increase .now the indicator light isn't coming. looks a like another bad alternator ,brought 3 remans all bad, thats why I thought I was over looking something.
Try another brand, if you have one available? I dunno how labor intensive these are to replace.
Really, though, the behavior you're describing sounds fairly typical of Ford/Mercury products of that generation. Even the Nissan crossovers. Almost none can stay above 13V with headlights and the blower on at idle. Raising the engine speed off idle allows the alternator to reach maximum output, which it usually can't do at idle.
at times it dropped to 12.3 I believe it would keep dropping if I didn't turn the blower off. No indicator light is not coming on even with initial key on bulb check
A quick test, put the loads on and check the voltage as you slowly increase the rpm from idle speed note the rpm at which the voltage crosses over 13 volts.
I suspect the engine computer is not controlling the alternator and the regulator built into the alternator is working in back up mode. From experience of British Ford models I would examine the wiring to the 3 pin connector on the alternator.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
Almost none can stay above 13V with headlights and the blower on at idle. Raising the engine speed off idle allows the alternator to reach maximum output, which it usually can't do at idle. 
:silly: