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 (?).
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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.

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Pat Moore did pass the time by typing:

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/productdetail.asp?ProductID0 http://www.partsexpress.com/webpage.cfm?WebPage_IDc (lacks pictures) http://www.circuitspecialists.com/prod.itml/icOid/5704
--
DougW



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DougW proclaimed:

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

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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

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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

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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!

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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, ill O|||||||O mailto: snipped-for-privacy@aol.com http://www.billhughes.com /
SteveBrady wrote:

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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!
http://www.billhughes.com /

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That is what VW dealer personnel tell me. If the heater blower pack thermo fuse blows = replace blower AND resistor pack.
You take a chance if you remove the fuse and jump the wires to get the other blower speeds. ;-)

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I am not sure what diode you are thinking of, there are a few I have mentioned.
The resistor pack either has a thermal fuse like most are saying or a diode. They serve different purposes. I haven't taken one apart to see it or I would know.
If it is a thermal fuse, it is there to blow before heat can damage the wiring, something like a fuse link wire. If it is a diode, it is there to direct the flow of power in one direction only. Without it something else can be affected when the heater turns on and something will likely let the smoke out and quit.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
SteveBrady wrote:

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Mike Romain did pass the time by typing:

It's also that a thermal fuse will often look like a switch in a schematic. i.e. _____/ _____
where a diode will look like this ---|>|----
--
DougW



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