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1979 Sedan Deville
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Discussion Starter #1
The blower motor relay on my 1979 Sedan Deville keeps blowing out. Upon close examination it is the power running to the motor that is getting hot and causing it to blow.
 

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94 Fleetwood Brougham
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7,534 Posts
Which side of the relay? The coil side or the contacts side?

If contacts side, sounds like too much current being drawn. Suspect bad blower motor, or one that is failing.

If coil side, suspect bad batch of relays, and that is quite unusual.
 

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1979 Sedan Deville
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448 Posts
Discussion Starter #3
It's the contacts side, and I've replaced the blower motor. I'm suspecting a short somewhere in the wire between the relay and the control head because the control head and the programmer box work fine, and have also both been replaced recently, which made no difference. A new relay will last a day or two before burning out though, and if it was a short, I would think it would happen right away. I guess I'll have to tear the whole dash down to find the problem. I took it to the shop, and the guy said he thought I knew more about it than he did.
 

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1979 Sedan Deville
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Discussion Starter #4
Okay, here's a little update. A while back, someone on here told me to replace the original factory style relay with one that could take more power, so I did. Shortly after it stopped working again and I assumed that relay had also blown. Today I discovered that was not true. The relay is fine. So, I plugged it back in, and listened carefully. The only time I hear the relay click on is when I go to defrost on the control head, but the blower motor is still not coming on. I ran the battery to the motor and it works just fine. So this would lead me to believe that the control head is not connecting the orange wire from the relay to the tan wire that goes to the motor.
 

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1979 Sedan Deville
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448 Posts
Discussion Starter #5
Okay here's another update. I took to control head out and took apart the contact wheel, and it looked fine so I put it back together and put the control head back in the car. Now I don't even hear the relay click on when I put it on defrost.
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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2,096 Posts
The laws of physics have not changed since I first explained how to repair your problem. You will make yourself crazy by avoiding the fix which I have suggested. Consider listening and saving yourself torment. Benifit from the experience of others. Have a nice day.
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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Replace the undersized factory blower relay with a pair of paralleled Bosch cube relays. It's still in the archives of this site.
 

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1979 Sedan Deville
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448 Posts
Discussion Starter #9
I just said I replaced it in one of the above messages. I didn't get a pair of Bosch ones, but again, if you re-read my messages above you'll notice I say that the one I used to replace the factory one did not burn out, and while it solved the problem for a few days, it stopped working again. I busted the relay open to see if it was fried, and it was not, so now something else is messed up. I think what I'll do is hotwire the connectors on the control head to see if I can make the relay switch on, and then I'll hotwire the lead from the relay to the blower motor as if it was on defrost or hi, and if that turns it on I'll know the control head isn't making connections properly.
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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Good luck. When you get finished checking every other possible failure out just try what I suggested 6 months ago.
 

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1979 Sedan Deville
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448 Posts
Discussion Starter #11
DUDE! I JUST TOLD YOU I TRIED IT! Sorry to seem like I'm losing my temper, but didn't I just say I replaced the factory relay with another kind? Okay, well just in case I didn't say so, I'm saying it now, and just like you said would happen six months ago, the relay stopped blowing out! Ape Man I appreciate your help, but now that I've tried what you said, only to run into further problems, could you drop it and help me with what's happening now? Please?
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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Please post a picture of your heavy duty relay and maybe we can help.
 

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1979 Sedan Deville
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448 Posts
Discussion Starter #15
Ok, I'll get a picture when I can, but for now lets assume the relay I have solved everything, and everything was working fine. Continuing under that assumption, why, when the relay is still functional, is my blower motor not coming on? I've checked out all the wiring, and verified that the motor itself runs, and I have a brand new programmer and fairly new head unit, and while I've taken the head unit apart and put it back together, it seems to only go one way. So why doesn't this work? Lets just say you approve of my relay Ape Man, and I manage to convince you that it is still functioning, what will you tell me to do next?
 

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'80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
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I need a picture of the relay. Assuming just leads to guessing. You want me to guess and have you waste even more money on parts that won't fix the problem???
 

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1979 Sedan Deville
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Discussion Starter #17
No I want you to guess and then I'll decide whether or not to waste money on more parts that may or may not fix the problem. I'd like to add that the only reason I'm trying to fix this myself is because all the mechanics I've taken the car too told me it sounded like I would have a better chance of fixing it than they would. Basically I'm looking for as much information as I can get. Just because you tell me a bunch of stuff doesn't mean I'm going to do all of it, it just means I'm going to consider it, because if you know as much about the systems as people say, I'm sure you can think of a lot of things it could be that I haven't thought of.
 

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Ok now we can get someplace. Get a test lamp or Voltmeter and a paper clip to hook to the probe. Ground the negative of ground lead of the test instrument and check that it works by testing a known 12 Volt source. Leave the relay plugged in and use the paperclip to probe the terminals. The terminals are all labeled 30, 87, 87a etc. Turn the heat on 90 degrees on the economy setting with the car running and warmed up. What do you get at each terminal?
 

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1979 Sedan Deville
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Discussion Starter #20
Okay here we go. 87a is not used. On terminal 87 (orange wire) I measured 0.03 volts. Terminal 86 (blue wire) was 0.02 volts. Terminal 85 (brown wire) was about 13 volts. Terminal 30 (orange w/ black stripe wire) was 0 volts.
 
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