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Thread: Relays

  1. #1
    Fellow Frogger!
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    Default Relays

    Not electrical minded at all but can I use 30amp 5 pin relays to replace the missing 25amp relays on my mi16 loom if I upgrade the fuse?

    these are missing (not my pic)



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    Can I use these?


  2. #2
    1000+ Posts Richard W's Avatar
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    I'm not sure that all 5 pin relays are wired the same. You need to confirm that the diagram on the top of that relay and the pin layout matches the Pug ones that you are replacing.

    30 amp refers to the maximum current the relay can carry. However, the fuse is not there to protect the relay - it should be matched to the maximum load likely to be drawn by whatever the relay is running. Changing the relay doesn't change that load, so don't change the fuse.

  3. #3
    1000+ Posts robmac's Avatar
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    There are so many variations in pin layouts between manufacturers and relays.

    I'll almost like a conspiracy against car owners.

    You need check the pin layouts on the original and replacement relays are the same.

    Some relays have supression diodes on their coils, some have resistor/capacitor network and some have no protection at all.

    Unless you are electronically savvy, I'd raid pick a part for the relays.

    Surely some Afer has a few spare relays ?
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  4. #4
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    Bugger I was hoping they are all the same and I could just match the pins up. I was hoping to tick this off the list today on the way home from work. No matter I will ask about shortly. Wish the local wrecker had one

  5. #5
    COL
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    I would get the same relays that you have already from the wreckers and you should not be far off the mark.
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    1000+ Posts schlitzaugen's Avatar
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    Rob is right, there's a zillion options, but if you match the pin diagram it's no problem. Also, the type of relay you're looking for is the most common layout (you didn't expect Peugeot to use something exotic, did you?).

    But rather than using old, unknown bits or expensive cheap bits (like that Tridon), I would visit jaycar and get the cheap bits cheaply. They come from the same factory as the Tridon and have the same quality.

    I have a shitload of those relays from cars I have raided, including the very ones you're looking for, but it's a 50-50 chance they work.
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  7. #7
    1000+ Posts PeterT's Avatar
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    The relays we're referring to here are M744, M743 and M783 as per the XU9J4Z wiring diagram.

    M744 is the fuel pump relay. It needs to be a double outlet 5 pin type, where the both 87a and 87b outputs are switched on together, in parallel. If you look at your picture, most Peugeot relays switch the input from pin 3 to pin 4 OR pin 5. Thus this type is not suitable (pins 4 and 5 being equivalent to 87a and 87b on generic relays).

    M743 is the A/C compressor relay. It can be any four or five pin relay. Leave it out if you don't have A/C.

    M783 is injector supply. It can be any four or five pin relay.
    Last edited by PeterT; 8th October 2016 at 12:10 PM.

    '92 205 Mi16
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  8. #8
    1000+ Posts robmac's Avatar
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    Quote Originally Posted by PeterT View Post
    The relays we're referring to here are M744, M743 and M783 as per the XU9J4Z wiring diagram.

    M744 is the fuel pump relay. It needs to be a double outlet 5 pin type, where the both 87a and 87b outputs are switched on together, in parallel. If you look at your picture, most Peugeot relays switch the input from pin 3 to pin 4 OR pin 5. Thus this type is not suitable (pins 4 and 5 being equivalent to 87a and 87b on generic relays).

    M743 is the A/C compressor relay. It can be any four or five pin relay. Leave it out if you don't have A/C.

    M782 is injector supply. It can be any four or five pin relay.
    So Peugeot don't suppress the back emf from the coil with a diode or a capacitor ?

    They must have pretty hardy outputs on the ECU
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  9. #9
    1000+ Posts PeterT's Avatar
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    Only on M783. Thus as Rob has suggested, use a green one for this.

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  10. #10
    1000+ Posts robmac's Avatar
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    Quote Originally Posted by PeterT View Post
    Only on M783. Thus as Rob has suggested, use a green one for this.
    Yet so many manufacturers play this game with different colored relays denoting certain coil suppression technique.

    You would think common sense would dictate to use as many of same relays as possible, when there is no special function required like timing or fuel pump relay function.

    It just seems such a stupid methodology to customize relays to locations when in most cases it's totally unnecessary.
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  11. #11
    1000+ Posts PeterT's Avatar
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    A pic tells a thousand words.......
    Attached Thumbnails Attached Thumbnails Relays-xu9j4z-loom-relays.jpg  

    '92 205 Mi16
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  12. #12
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    It is cool that people get so passionate about tiny details! If the ECU is only switching earth on the fuel pump relay, then the back emf from the collapsing magnetic coil is next to meaningless given it switches only once per engine run cycle. There are a very limited number of configurations possible with a 5 pin relay.

    They are:
    SPST (single pole single throw) like the green one in Peter's photo which will mace the circuit on pin 3-4 when energised.
    SPCO (single pole change over) like the black one, circuit from pin 3 to 4 in normal (de-energised) state, and 3 to 5 when energised.
    SPDT (single pole dual throw) which will connect pin 4 and 5 at the same time.
    The first and last configuration above can be normally open or normally closed which means there are really on 5 configurations possible.

    Other than that, the electric specification on the relays may vary with coil impedance (current required to pull up), inrush current (negligible in a coil this small), current rating (capacity of the contacts, not the coil) and possibly with a freewheeling diode (usually including an LED for active indication) or a capacitor to suppress the voltage spike as the magnetic field collapses when the coil switches off.

    You can re-configure the plug to match your required pin configuration, but I wouldn't bother. Just replace it with an original, it will probably last another 25 years.
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  13. #13
    1000+ Posts robmac's Avatar
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    If the ECU is only switching earth on the fuel pump relay, then the back emf from the collapsing magnetic coil is next to meaningless given
    Tell that to several generations of electronic engineers, who , without a second thought, fit a suppression diode to each and every relay.

    FYI a 12 volt auto relay can deliver a back EMF of a several hundred volts worst case. That is far from "meaningless" when the relay is being switched by 50 volt open collector transistor or chip array.

    https://www.youtube.com/watch?v=43HFng0CVKg

    This video is worth watching and one should note the relay coil in the demo is much lower inductance than one should expect in a auto relay, according the Back EMF is likely to a lot lower in the demo, that with an auto relay.

    Suppression diodes installed to relay coils perform a vital function.
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  14. #14
    1000+ Posts PeterT's Avatar
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    Note relay M783 (with diodes) gets earthed by the ECU M58. No idea why fuel pump relay M744, which also gets earthed by the ECU, doesn't have diodes.
    Attached Thumbnails Attached Thumbnails Relays-image.jpg   Relays-image.jpg  

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  15. #15
    1000+ Posts robmac's Avatar
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    Perhaps then relay has a R-C network or a MOV across the coil ? Have you ever opened one up ?

    Or the switching transistor is considered robust enough to cope with several hundred volts.

    Whatever the reason a diode, costing milli cents in quantity, seems a futile saving, considering damage potential of a 300v-400v back EMF let loose in the ECU electronics.
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