Fix for repeated heater relay failure 77 Deville
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  • 2 Post By BRUCE ROE
500, 472, 425, 368 Discussion, Fix for repeated heater relay failure 77 Deville in Cadillac Engine Technical Discussion; Just thought I would pass on a possible fix for repeated blower relay failure on late 70's Devilles. After looking ...
  1. #1
    daniel58 is offline Cadillac Owners Fanatic
    Automobile(s): 58 Coupe Deville, 77 Coupe deville, 90 Deville
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    Fix for repeated heater relay failure 77 Deville

    Just thought I would pass on a possible fix for repeated blower relay failure on late 70's Devilles. After looking through the posts on here I see that others are having this problem. The only fix I have seen so far is to replace the blower motor and/or carry spare relays which I have tried. My 77 Deville burns out relays on a daily basis so having spares has been expensive. My fix (so far) is to make up a harness to use 2 relays in parallel using 10 ga wire, the heavy wire acts as a heat sink. This has been working for 2 weeks now, much longer than any single relay has lasted. I do carry a fire extinguisher just in case!

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  3. #2
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: Fix for repeated heater relay failure 77 Deville

    Why does this model have this unique problem? Special blower motor? Bruce Roe

  4. #3
    daniel58 is offline Cadillac Owners Fanatic
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    Re: Fix for repeated heater relay failure 77 Deville

    In looking through past postings, they say it is a design flaw. Who knows, I replaced the blower motor and checked and rechecked everything. The parallel setup was the only way I could get it to work other than a jumper wire. There may very well be a better fix but I need to move on to other areas as the 77 Deville is my dailiy driver now. I have a 90 deville that is not driveable after $3,000 in parts at least this 77 runs and is computer free!

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    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: Fix for repeated heater relay failure 77 Deville

    It sounds like the relay was too small. There are bigger relays, starting with the 30A or 40A relays sold for headlights, etc. One or 2 of them ought to be enough. There are bigger relays than that, if really needed. A kickback diode across the motor will also reduce contact stress. Bruce Roe

  6. #5
    daniel58 is offline Cadillac Owners Fanatic
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    Re: Fix for repeated heater relay failure 77 Deville

    You may be right Bruce, I'm no electronics wizard I will admit that right off. This fix is working so far for me, its hot here in central Ohio right now... so whatever works! I tried to use a generic 40 amp relay but could not figure out the connections (4 pin) to make it work as it should, the blower ran constantly even with the ign off. I would like to get this "rolling restoration" working as it should eventually.LOL! Moving on to mode door not working correctly, window switches, paint. stuff like that. Kickback diode, man you are beyond me with that!!! I thank you for your comments however, I have seen some of your posts, you are somewhat of an electronics wizard it seems. Can you offer advice on my 90 deville,4.5, distributor 180 out (posted in 4x) problems. I know now its not out 180 degrees at this point but still have the rough idle, 1or 2 cyl miss on accle and lack of power on top end with much money invested in parts.

  7. #6
    awadecki is offline Cadillac Owners Enthusiast
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    Re: Fix for repeated heater relay failure 77 Deville

    If your timing is correct, the rough idle/miss could be dripping fuel injectors. If the exhaust smells rich, this is likely the cause. As to low power, that could be so many things.

  8. #7
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Fix for repeated heater relay failure 77 Deville

    While I don't know much about the 4.5, some of those engines had a "cam sensor" under the distributor rotor. Later engines had no distributor and a real cam sensor. On my 70s distributor with EFI, you could put the distributor in backwards, move the plug wires around half way, and it would still run. This would be fine on a carb engine. But on EFI it reverses the the timing of the fuel injectors, because of the "cam sensor" being turned around. Later engines than mine could have a similar problem, which would be most obvious at low rpm.

    There can be some tricks to adding relays. But since your car already has one, it should just be a matter of identifying the connections. The basic relay has 2 coil connections, interchangeable in this case. And 2 connections to a normally open contact, also interchangeable. Probably your original relay has these, or sometimes they tie a coil and a contact terminal together inside to reduce it to 3 wires. The 4 wire can do the same externally. If there is a normally open AND a normally closed contact, its a little more complicated.

    If you have the 4 wire situation, the common 12V "headlight" relay has pins

    85 and 86 coil

    30 and 87 normally open contact Bruce Roe

  9. #8
    daniel58 is offline Cadillac Owners Fanatic
    Automobile(s): 58 Coupe Deville, 77 Coupe deville, 90 Deville
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    Re: Fix for repeated heater relay failure 77 Deville

    Thanks for getting back to me guys. I though I had an understanding of relays, basically a remote switch. I did identify the slots to tag in the generic where the relay went but the blower would still run with the ign off. That may be a clue as to why the relays are failing in the 1st place.

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