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1973 Pontiac Grand Prix Heater Blower circuit heat
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one2da1 Offline
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Post: #1
1973 Pontiac Grand Prix Heater Blower circuit heat
My problem relates to HOT heater circuit cables when key is turned to RUN, AC/Heater control in ON or OFF position, and fan speed switch in position LOW, M1, M2, and heater fuse blow in HI position. Blower runs at selected speed but cables from the fuse block to the resistor is HOT at every connection point, ie, fuse block fuse - output side, AC/Heater selector control - brown cable, and blower resistor. If selector control is moved to MAX AC (by pass resistor) the cables COOL down while blower is at top speed. The resistor has 2 wire-wound resistors of less than 1.5 ohms each and one silicon power rectifier diode of about 19 ohms; can anyone explain why this diode is part of the blower resistor when the AC power at the alternator is already rectified to DC? Or is there any way to explain COOL cables at MAX AC and HOT cables when fan speed switch is in use. Also is there any sure test to determine if the diode or complete resistor pack is BAD? There is also heat damage to the fan speed switch. Can the blower itself be bad and how to check?
03-22-2008 06:35 AM
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re-tired Offline
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Post: #2
 
Lots of questions Ill try to answer some .The diode is a blocking diode .Its there to keep voltage from feeding back into a circuit . The rectifier assy in the alternator, (actually there are 6 diodes in it) rectify the ac to dc. A zenier diode only allows a certain voltage to pass. You say the switch has heat damage . Thats due to something drawing to many amps . The 1.5 ohm resistors can't, a diode can't. That leaves the wiring (short) and/or blower motor. You would need to put an amp meter in series with the motor to measure amperage. I would replace switch ,motor and repair any loose connections/connectors ( they create heat also). <!-- m --><a class="postlink" href="http://www.autozone.com/az/cds/en_us/0900823d/80/21/73/09/0900823d80217309/repairInfoPages.htm">http://www.autozone.com/az/cds/en_us/09 ... oPages.htm</a><!-- m -->

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03-22-2008 10:25 AM
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one2da1 Offline
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Post: #3
 
Thanks for the info re-tired!! Today replaced blower motor and with no change. Only have multimeter, are amp meters costly? Cable form resistor to motor is cool and has always been. When you say SHORT is that insulation on cable is broken and wire ground to metal. I have had this car since 1990 and this condition has always been a problem, 1000s of fuses blown (underhood fuse at times also), doing a resto and would like to correct before dash is installed, any other ideas would very helpful. Can you give more info on the rectifier (blocking diode), not clear about "voltage feeding back", back from what or where?
03-22-2008 11:36 PM
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re-tired Offline
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By short I mean a hot(POS) wire making direct contact with a ground (neg) such as the metal body or making contact with another wire in a bundle . look for melted , bare or pinched wires. Another cause of high amps is high resistance due to loose or damaged connectors or switches .You say switch was damaged , unplug the switch, motor and reistor assy, read each to ground with meter on ohms . read wiring from switch to motor and resistor to ground all should not read anything . Read thru swiitch ,should not be any resistance.

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03-23-2008 01:58 AM
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Harvester Offline
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73 Pontiac
How did you get the 19 ohms through the diode? In the conducting direction, the number you found is far too high; in the blocking direction, that number is far too low. In the conducting direction, you should find only a few tenths of an ohm, but the level depends on the ohmmeter you are using and the scale on which you are measuring the resistance. Generally, ohmmeters apply a higher voltage to the component being measured when you select the higher resistance scales. You will get a better indication of diode function if you use the highest resistance scale you have on the instrument. If the diode turns on fully in the forward (conducting) direction, you will see almost zero ohms on the meter if you use the highest scale available. Using the lowest scale, you will find several tenths of an ohm resistance. For the reverse (blocking) direction, you get the best results again using the highest resistance scale, because you are stressing the diode with the highest voltage you have available. If it doesn't show infinite on the highest scale, the diode is leaking in the reverse direction. But, it may show infinite in the reverse direction if you use a lower resistance scale.

Another reason for your high number of blown fuses is resistance in the fuse holder itself. On older cars as yours, the fuse holder contacts weren't large enough to carry the sustained high current that came with air conditioning systems, so the contacts over-heated in normal use and melted the fuse. The holder and fuse contacts themselves would also show evidence of overheating. That condition led to a lot of short-chasing and parts changing that didn't correct anything. If the fuse holder is your trouble, the only remedy for that is to replace the heat/AC fuse with an in-line ATO-type fuse holder that can handle 25 amps or so continuously.

It sounds like you have more than one trouble with your car, but don't overlook the fuse holder as a cause of the blown fuses.

I didn't mean to turn this reply into an electronics lesson on ohmmeters and diodes, but I hope this helps. Keep in touch.

Harvester
03-24-2008 01:44 PM
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re-tired Offline
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Post: #6
 
Thanks for the addtional indepth answer harve :wink: . I am typing challenged .(lazy) Maybe next week we will get into transistor therory then logic circuits .Cant wait for nano devices to hit market. Then we can have a ball teaching todays whiz kids points/condenser operation and dwell angle . :shock: :roll:

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03-24-2008 02:15 PM
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joesmokabo Offline
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Post: #7
 
I just had a flashback of flunking electronics in the 7th grade.

I wish I would've paid attention.

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03-26-2008 05:49 PM
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one2da1 Offline
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Just back from a serious computer problem. Thanks Harvester, your input give me hope, but I bench tested a new diode, with my same results. Will use your method to see if I can get the readings you indicated. Current testing includes removal of fan switch (loose male connectors) and using jumpers at the female connector for fan activation, and cables are still HOT. Re-tired your testing procedure will be checked also, back with results in a few days.
04-02-2008 02:35 AM
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jcdkool Offline
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Post: #9
RE:
(04-02-2008 02:35 AM)one2da1 Wrote:  Just back from a serious computer problem. Thanks Harvester, your input give me hope, but I bench tested a new diode, with my same results. Will use your method to see if I can get the readings you indicated. Current testing includes removal of fan switch (loose male connectors) and using jumpers at the female connector for fan activation, and cables are still HOT. Re-tired your testing procedure will be checked also, back with results in a few days.

Well, this is my thread from another life!!!! Just found my own answer today. With 1000's of heat changes at the fuse holder output side had turned the metal connector to something that was very much nonconductive, thus the heat build-up(resistance). Replaced connector with better similar one and 93% heat decrease, can feel some heat by hand touching at high speed, but not enough to cause fear of fire.
12-30-2015 01:40 AM
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