flickering Service Soon and no codes...
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HT4100 4.1, 4.5, 4.9 Discussion, flickering Service Soon and no codes... in Cadillac Engine Technical Discussion; I made another thread about this a while ago in the DeVille forum but I've made that one confusing with ...
  1. #1
    ehall's Avatar
    ehall is offline Cadillac Owners Connoisseur
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    flickering Service Soon and no codes...

    I made another thread about this a while ago in the DeVille forum but I've made that one confusing with extra wrong info so I'm going to start over in this forum.

    Back-story: my brother and I made a trip down south in my 85 DeVille sometime last year, and on the way back my smog pump seized up. This caused the serpentine belt to melt and break, and we overheated. I coasted it into a gas station that was luckily just a quarter mile away and called my dad. Between us we were able to find a smaller belt that let us bypass the smog pump, and got the car all the way back home. This worked well enough that I was able to forgo getting the smog pump replaced for a little while.

    Eventually I took it to the busiest shop in town, and had them replace the smog pump. Ever since then, I've had a sporadic Service Soon diagnostic that sometimes comes on when I accelerate, and then goes off when I let off the gas. This doesn't happen all the time or even most of the time, just every half hour or so. There's no correlation with velocity, just with acceleration. Worse is that it did not throw any codes.

    The last time it happened I checked my codes and there was an entry for shorted throttle switch (battery was recently changed so no other codes in my history). I figured that was the problem, and took it to another mechanic to have the idle throttle control motor replaced (it's where the defective sensor is located), and a few other things done (100k service, etc). However the mechanic just called and said that after doing everything he got the Service Soon flicker while testing the engine, and there was no code.

    He is going to start his diagnostics on Monday, but I thought I would ask the experts here if they had any ideas. What kind of soft error would trip the Service Soon light but not throw a code? Could a sensor be bad? Grounding error? Something to do with the smog pump that was replaced maybe, or is that just a coincidence?

    Any ideas greatly appreciated... Thanks!

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  3. #2
    ehall's Avatar
    ehall is offline Cadillac Owners Connoisseur
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    Re: flickering Service Soon and no codes...

    Talked to the mechanic today, he had it hooked it up to an analyzer and got the SS light to flicker, but his machine did not detect any codes either. He's stumped, and is calling in a buddy of his who used to be the service manager at the big local Caddy dealership.

    Anybody got any thoughts on this?

  4. #3
    peteski is offline Cadillac Owners Enthusiast
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    Re: flickering Service Soon and no codes...

    Possibly the SS lamp driver inside the PCM is toasted and it is making the light flicker. Since there is no code for this condition (and PCM doesn'e even monitor that ciscuit) no codes are set.

    Peteski

  5. #4
    ehall's Avatar
    ehall is offline Cadillac Owners Connoisseur
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    Re: flickering Service Soon and no codes...

    Thanks peteski, I was already beginning to suspect that the diagnostics itself was bad and asked my electrical mechanic to check the connections which he said were all good. I did not ask him to check the board itself. Seems like replacing it would be an easy way to eliminate the possibility anyway.

  6. #5
    peteski is offline Cadillac Owners Enthusiast
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    Re: flickering Service Soon and no codes...

    If your mechanic can replace individual boards inside the PCM then he is pretty good. Usually the entier PCM gets replaced as a unit. And yes, swapping a PCM unit is pretty quick and easy.

    Peteski

  7. #6
    ehall's Avatar
    ehall is offline Cadillac Owners Connoisseur
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    Re: flickering Service Soon and no codes...

    It's been a month since I got the car back and the SS telltale has not come on again... I didn't do anything to it either. I picked it up and was going to have my electrical mechanic replace the ECM but I wanted to wait for the light to come on again and it hasn't yet.

    What do you guys think, maybe just a weak ground on the module?

  8. #7
    NickySantoro is offline Cadillac Owners Fanatic
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    Re: flickering Service Soon and no codes...

    A dirty or "green" connection at the at the connectors on the module can do odd things. Sometimes just unplugging and replugging a connector can be the cure.

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