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Need help from car audio guys about car alarms

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  • rpb
    replied
    Originally posted by Dukk View Post
    I was a professional car electronics installer from 1990 to 2015 - full time for the first ten years or so and part time as a hobby mostly for the rest.

    Generally a car alarm will interrupt the starter wire. Now, in the mid-90s, people were trying to be clever so they may have interrupted power to the coil or fuel pump but I never did this as an unexpected failure in this circuit at highway speed could be tragic. An unexpected failure in a starter interrupt is merely an annoyance..

    It does not sound like this is your issue but it should be fairly straightforward to check the wiring under the dash to see if the starter wire has been interrupted. Most alarms from that era used outboard relays for almost everything and almost always for starter kill so it should be easy to find. 99% of relay failures are in the closed position and by far most systems rested this way, only energizing the relay and interrupting the starter circuit when the alarm was sounding.

    As an aside - if someone is afraid to let a shop work on their car, new or classic, you are in the wrong shop. A professional installer will perform work that will outlive the car.


    Thanks for the information!

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  • Dukk
    replied
    I was a professional car electronics installer from 1990 to 2015 - full time for the first ten years or so and part time as a hobby mostly for the rest.

    Generally a car alarm will interrupt the starter wire. Now, in the mid-90s, people were trying to be clever so they may have interrupted power to the coil or fuel pump but I never did this as an unexpected failure in this circuit at highway speed could be tragic. An unexpected failure in a starter interrupt is merely an annoyance..

    It does not sound like this is your issue but it should be fairly straightforward to check the wiring under the dash to see if the starter wire has been interrupted. Most alarms from that era used outboard relays for almost everything and almost always for starter kill so it should be easy to find. 99% of relay failures are in the closed position and by far most systems rested this way, only energizing the relay and interrupting the starter circuit when the alarm was sounding.

    As an aside - if someone is afraid to let a shop work on their car, new or classic, you are in the wrong shop. A professional installer will perform work that will outlive the car.


    Leave a comment:


  • devnull
    replied
    Originally posted by dwigle View Post
    I'm really glad I read this thread. I'm looking at an early 90's porsche that has an aftermarket alarm apparently installed just after purchase. I had no idea the alarm could be so intrusive to the internal components of a car. I can't imagine letting an aftermarket car audio/alarm shop work on a new porsche. I may stay clear of this one.
    The alarm shouldn't be hard to remove and wiring diagrams are available to put stuff back. I might be a little more concerned with the actual wiring. Some of the 80's and 90s Porsches had the fuse box located under a piece of plywood in the passenger side footwell. A lot of wire rot and corrosion from moisture.

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  • devnull
    replied
    No check engine light when key on and not running is probably ECU or power.

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  • rpb
    replied
    Originally posted by dynamo View Post
    I can’t recall if those are MAF or not, but if it has a MAF, try unplugging it and see if it runs.
    Will do. I may unplug the spout connector and try as well.

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  • dynamo
    replied
    I can’t recall if those are MAF or not, but if it has a MAF, try unplugging it and see if it runs.

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  • rpb
    replied
    Originally posted by dynamo View Post

    To answer your original question, yes old alarms had the ability to disable the vehicle, but iirc it was a starter kill, either by onboard or remote relay. The onboard relays were usually too small and burned up. Sounds like cranking us not your issue though. I suppose though the installer could however route any number of functions in place of or addition to starter kill on this circuit.

    You could look to see if the alarm has a valet mode switch or button under the dash to help bypass as many alarm functions as possible. I uninstall the alarm though to not only help your current diagnosis, but also eliminate future issues.
    That makes sense. All they would have to do is interrupt the signal wire to the starter relay.

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  • rpb
    replied
    Originally posted by dwigle View Post
    I'm really glad I read this thread. I'm looking at an early 90's porsche that has an aftermarket alarm apparently installed just after purchase. I had no idea the alarm could be so intrusive to the internal components of a car. I can't imagine letting an aftermarket car audio/alarm shop work on a new porsche. I may stay clear of this one.
    I don't know that it's the problem. I'm just trying to find out if I need to dig it out from under the dash. I did see mention in my Ford EEC-IV book that this kind of thing can happen.

    Leave a comment:


  • dynamo
    replied
    Originally posted by rpb View Post
    I have a 30 year old car that I bought used from a dealer. I've owned it over 20 years. The first owner was into car audio, and had some kind of alarm. I've never used, or had issue with it, but I think it's still installed. My question is...Do these alarms disable the vehicle? I have been trying to fix a no start condition, and wondered if an old failing anti-theft system could be the problem.
    To answer your original question, yes old alarms had the ability to disable the vehicle, but iirc it was a starter kill, either by onboard or remote relay. The onboard relays were usually too small and burned up. Sounds like cranking us not your issue though. I suppose though the installer could however route any number of functions in place of or addition to starter kill on this circuit.

    You could look to see if the alarm has a valet mode switch or button under the dash to help bypass as many alarm functions as possible. I uninstall the alarm though to not only help your current diagnosis, but also eliminate future issues.

    Leave a comment:


  • dwigle
    replied
    I'm really glad I read this thread. I'm looking at an early 90's porsche that has an aftermarket alarm apparently installed just after purchase. I had no idea the alarm could be so intrusive to the internal components of a car. I can't imagine letting an aftermarket car audio/alarm shop work on a new porsche. I may stay clear of this one.

    Leave a comment:


  • rpb
    replied
    Originally posted by devnull View Post
    It's been a while since I've worked on a ford that old. If I remember correctly it doesn't have OBD2 but you should be able to pull the codes with a jumper wire and count flashes on the check engine light. That might help narrow down the search. Good luck.....
    Tried that yesterday, but it did nothing. I don't even see the check engine light! That might be a clue, I don't know.

    Leave a comment:


  • devnull
    replied
    It's been a while since I've worked on a ford that old. If I remember correctly it doesn't have OBD2 but you should be able to pull the codes with a jumper wire and count flashes on the check engine light. That might help narrow down the search. Good luck.....

    Leave a comment:


  • rpb
    replied
    Originally posted by devnull View Post
    What kind of car? Does it have an integrated ECU or separates for fuel and ignition? Does the ECU have good power and ground? Battery fully charged? Does the voltage when cranking stay above the factory spec?
    It's a 91LX 5.0 Mustang. Mostly stock with a few little bolt ons. One computer controls everything. I need to check power and ground on the wiring harness, and maybe a relay, or fuse for the ecu. The fuel pump does come on, which I don't think can happen without the ecu being powered up. Not sure though.The system reference voltage is supposed to be 5v. Currently, I measure zero volts at the tps. (Throttle position sensor) Battery is fine. I have a second Mustang that I can compare with, and swap parts to test. I've ruled out quite a bit this way.

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  • devnull
    replied
    What kind of car? Does it have an integrated ECU or separates for fuel and ignition? Does the ECU have good power and ground? Battery fully charged? Does the voltage when cranking stay above the factory spec?

    Leave a comment:


  • Steve Lee
    replied
    Actually I didn't mention compression for the same reason you didn't check it - unlikely to be the cause - it should at least run, maybe not well but . . . .

    Man - I had an '85 BMW 325e that did this to me and after replacing the ECU twice, the idle controller module, idle injector and timing position sensor I finally sold the vehicle as is.

    That buyer had a mechanic friend who worked on BMW's for a living and the tools/lifts to actually work on it where I did not.

    Turned out to be a $0.50 part.

    The TDC lobe/pin on the flywheel had fallen out and the ECU could not find TDC so it just sputtered sometimes, ran others and mostly left me where I last turned the ignition-off.

    Check the simple stuff 1st.

    Good luck, Sir.


    Leave a comment:

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