Diode (?) in heater resistor pack

We're working on a friend's "new" 94 Cherokee and have replaced the busted
heater fan motor. We then tracked why the fan only worked on "High" to an
open
component in the heater resistor. It's shown as a switch in the wiring
diagram, and physically resembles a silicon diode. Shorting across it
restored all four fan speeds. Does anyone know what that diode-looking thing
is? And why it's there? I hate to buy a new resistor pack just to replace
the diode (?).

Diodes only conduct in one direction. So, I'm assuming that the cathode is
pointing towards the motor/fan....if this is the case, then it's there to
prevent the motor from shunting back power to other components back
upstream.

I don't know any other reason for it....not to say there isn't one.

When a motor spins, it has a magnetic/electrical field around it....when the
motor stops, this field collapses and fires current back up the line.
Diodes will help block this....but for different reasons, they stop working.

We just had a problem with a large UPS system at work. When we were
troubleshooting it, we had a large diode blow and send out a 2 foot spark.
Lucky for me, I had a spare pair of drawers in my locker!

Sometimes diodes open....other times they short. BUT, they also weaken
(don't block the way they used to)...but this leads up to one of the first 2
failures.

"Pat Moore" > wrote in message
...
> We're working on a friend's "new" 94 Cherokee and have replaced the busted
> heater fan motor. We then tracked why the fan only worked on "High" to an
> open
> component in the heater resistor. It's shown as a switch in the wiring
> diagram, and physically resembles a silicon diode. Shorting across it
> restored all four fan speeds. Does anyone know what that diode-looking
> thing
> is? And why it's there? I hate to buy a new resistor pack just to replace
> the diode (?).
>
>

Pat Moore did pass the time by typing:
> We're working on a friend's "new" 94 Cherokee and have replaced the busted
> heater fan motor. We then tracked why the fan only worked on "High" to an
> open
> component in the heater resistor. It's shown as a switch in the wiring
> diagram, and physically resembles a silicon diode. Shorting across it
> restored all four fan speeds. Does anyone know what that diode-looking thing
> is? And why it's there? I hate to buy a new resistor pack just to replace
> the diode (?).

Any numbers on it or bands?

Can you post a pic of it to alt.binaries.pictures.autos.4x4
or a link?

http://tinyurl.com/465fj
(radioshack)

If it's just a diode you can use any rectifier diode that
is rated high enough for the motor current. Overkill is fine.
276-1661 or 276-1653

However, it could be a different animial. You could be looking
at a thermal fuse designed to turn off the system should the
resistor pack overheat due to a seized motor.
http://www.thermodisc.com/productdet...p?ProductID=30
http://www.partsexpress.com/webpage.cfm?WebPage_ID=63
(lacks pictures)
http://www.circuitspecialists.com/prod.itml/icOid/5704


--
DougW

DougW proclaimed:

> Pat Moore did pass the time by typing:
>
>>We're working on a friend's "new" 94 Cherokee and have replaced the busted
>>heater fan motor. We then tracked why the fan only worked on "High" to an
>>open
>>component in the heater resistor. It's shown as a switch in the wiring
>>diagram, and physically resembles a silicon diode. Shorting across it
>>restored all four fan speeds. Does anyone know what that diode-looking thing
>>is? And why it's there? I hate to buy a new resistor pack just to replace
>>the diode (?).
>
>
> Any numbers on it or bands?
>
> Can you post a pic of it to alt.binaries.pictures.autos.4x4
> or a link?
>
> http://tinyurl.com/465fj
> (radioshack)
>
> If it's just a diode you can use any rectifier diode that
> is rated high enough for the motor current. Overkill is fine.
> 276-1661 or 276-1653
>
> However, it could be a different animial. You could be looking
> at a thermal fuse designed to turn off the system should the
> resistor pack overheat due to a seized motor.
> http://www.thermodisc.com/productdet...p?ProductID=30
> http://www.partsexpress.com/webpage.cfm?WebPage_ID=63
> (lacks pictures)
> http://www.circuitspecialists.com/prod.itml/icOid/5704
>

If it has three leads and looks like a diode otherwise, perhaps a
SCR switch?

I agree with DougW,
I had also thought it was a thermal fuse like on some of the VWs that I work
on.
You should be able to just replace it and save some $$$. ;-)

later,
dave AKA vwdoc1
88 GW
88 XJ

"DougW" > wrote in message
news:9%VBd.4478$2_4.3947@okepread06...
> Pat Moore did pass the time by typing:
>> We're working on a friend's "new" 94 Cherokee and have replaced the
>> busted
>> heater fan motor. We then tracked why the fan only worked on "High" to
>> an
>> open
>> component in the heater resistor. It's shown as a switch in the wiring
>> diagram, and physically resembles a silicon diode. Shorting across it
>> restored all four fan speeds. Does anyone know what that diode-looking
>> thing
>> is? And why it's there? I hate to buy a new resistor pack just to replace
>> the diode (?).
>
> Any numbers on it or bands?
>
> Can you post a pic of it to alt.binaries.pictures.autos.4x4
> or a link?
>
> http://tinyurl.com/465fj
> (radioshack)
>
> If it's just a diode you can use any rectifier diode that
> is rated high enough for the motor current. Overkill is fine.
> 276-1661 or 276-1653
>
> However, it could be a different animial. You could be looking
> at a thermal fuse designed to turn off the system should the
> resistor pack overheat due to a seized motor.
> http://www.thermodisc.com/productdet...p?ProductID=30
> http://www.partsexpress.com/webpage.cfm?WebPage_ID=63
> (lacks pictures)
> http://www.circuitspecialists.com/prod.itml/icOid/5704
>
>
> --
> DougW
>

I ran thru my service manual and couldn't find a description of what the
diode is for. Well, my patiend ran out (thick book)....but it sure does
list how to replace it more than once.

I'd hazard a guess that the motors have thermal shutdown themselves (most of
them do for the last bunch of years).

Anyhow, replace it....I couldn't find a part number in my book, but
hopefully there's something left of your diode that you can read the numbers
off of.

cheers

"dave AKA vwdoc1" > wrote in message
m...
>I agree with DougW,
> I had also thought it was a thermal fuse like on some of the VWs that I
> work on.
> You should be able to just replace it and save some $$$. ;-)
>
> later,
> dave AKA vwdoc1
> 88 GW
> 88 XJ
>
> "DougW" > wrote in message
> news:9%VBd.4478$2_4.3947@okepread06...
>> Pat Moore did pass the time by typing:
>>> We're working on a friend's "new" 94 Cherokee and have replaced the
>>> busted
>>> heater fan motor. We then tracked why the fan only worked on "High" to
>>> an
>>> open
>>> component in the heater resistor. It's shown as a switch in the wiring
>>> diagram, and physically resembles a silicon diode. Shorting across it
>>> restored all four fan speeds. Does anyone know what that diode-looking
>>> thing
>>> is? And why it's there? I hate to buy a new resistor pack just to
>>> replace
>>> the diode (?).
>>
>> Any numbers on it or bands?
>>
>> Can you post a pic of it to alt.binaries.pictures.autos.4x4
>> or a link?
>>
>> http://tinyurl.com/465fj
>> (radioshack)
>>
>> If it's just a diode you can use any rectifier diode that
>> is rated high enough for the motor current. Overkill is fine.
>> 276-1661 or 276-1653
>>
>> However, it could be a different animial. You could be looking
>> at a thermal fuse designed to turn off the system should the
>> resistor pack overheat due to a seized motor.
>> http://www.thermodisc.com/productdet...p?ProductID=30
>> http://www.partsexpress.com/webpage.cfm?WebPage_ID=63
>> (lacks pictures)
>> http://www.circuitspecialists.com/prod.itml/icOid/5704
>>
>>
>> --
>> DougW
>>
>
>

It's a thermal fuse. You can get a replacement at Radio shack or just solder
a wire around it. Mine blew because the motor needed lubed. Yes, it's just
like the one used on VW's.

JoBo

"Pat Moore" > wrote in message
...
> We're working on a friend's "new" 94 Cherokee and have replaced the busted
> heater fan motor. We then tracked why the fan only worked on "High" to an
> open
> component in the heater resistor. It's shown as a switch in the wiring
> diagram, and physically resembles a silicon diode. Shorting across it
> restored all four fan speeds. Does anyone know what that diode-looking
> thing
> is? And why it's there? I hate to buy a new resistor pack just to replace
> the diode (?).
>
>

I've heard Bill and Mike both talk about a separate diode that's been
replaced at times....what is this diode for?
Again, I can find the replacement instructions for it in my service manual,
but haven't been able to find a description of what it's for!


"Jo Bo" > wrote in message
...
> It's a thermal fuse. You can get a replacement at Radio shack or just
> solder a wire around it. Mine blew because the motor needed lubed. Yes,
> it's just like the one used on VW's.
>
> JoBo
>
> "Pat Moore" > wrote in message
> ...
>> We're working on a friend's "new" 94 Cherokee and have replaced the
>> busted
>> heater fan motor. We then tracked why the fan only worked on "High" to
>> an
>> open
>> component in the heater resistor. It's shown as a switch in the wiring
>> diagram, and physically resembles a silicon diode. Shorting across it
>> restored all four fan speeds. Does anyone know what that diode-looking
>> thing
>> is? And why it's there? I hate to buy a new resistor pack just to replace
>> the diode (?).
>>
>>
>
>

Hi Steve,
Not I. I wouldn't even replace a diode in an alternator, it's
cheaper to buy the rebuild. And we haven't figured out what caused the
Original Poster's to pop, like an internal short in his resistor pack?
God Bless America, ßill O|||||||O
http://www.billhughes.com/

SteveBrady wrote:
>
> I've heard Bill and Mike both talk about a separate diode that's been
> replaced at times....what is this diode for?
> Again, I can find the replacement instructions for it in my service manual,
> but haven't been able to find a description of what it's for!

Sorry Bill....I know I had heard it on here a couple of times and assumed it
was one of the more knowledgables (Mike, you, etc..).

Bill is right....if the diode blew...it did for a reason.
Check the heater fan rotor....it should be easily turnable with your hand.
Give it a hard tug and see if it spins (have to take belt off of course--or
gear...); it should turn a couple of times.
If the bearings go, then the motor might be drawing more current than
normal, which would kill your diode.

You might check the diodes ratings (look up it's part number) and also hook
up an Ammeter in series (in place of the diode) and check for the current
draw and compare this to the diodes rating.
Be aware that the diode will not be in place at this point so be sure to
disconnect your probes before turning off the switch as the collapsing
magnetic field will have a free path back up the line and thru the meter and
on....(disconnecting the probes break the circuit before the field can
collapse back in to the line.)

Be sure you use the proper scale on your meter too....most regular meters
are only rated for 10A, which may not be all that much....so definately
check the diode part rating first!



"L.W. ("ßill") Hughes III" > wrote in message
...
> Hi Steve,
> Not I. I wouldn't even replace a diode in an alternator, it's
> cheaper to buy the rebuild. And we haven't figured out what caused the
> Original Poster's to pop, like an internal short in his resistor pack?
> God Bless America, ßill O|||||||O
> http://www.billhughes.com/
>
> SteveBrady wrote:
>>
>> I've heard Bill and Mike both talk about a separate diode that's been
>> replaced at times....what is this diode for?
>> Again, I can find the replacement instructions for it in my service
>> manual,
>> but haven't been able to find a description of what it's for!