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  • Carb cooling fan problem

    I've got a problem with my carb cooling fan. The fan itself is fine, all wiring in the car harness has tested ok - there is voltage where there should be and the earth is complete. Now tried two delay units, neither triggering the fan.
    Does anyone have any idea where I need to look for fault or even understand the electronics inside the delay unit so that I can conduct further checks.
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  • #2
    Carb cooling fan only comes on if the coolant temperature is 90 degrees centigrade. Is your car in this cool weather getting to that? The cooling fan is operated by the thermostatic switch in the thermostat housing, have you tried that? If you bridge the terminals after removing the plug and the fan comes on when you turn the ignition on, that's your problem
    Last edited by Jeff Turbo; 26th March 2020, 20:57.
    1958 Ford Consul Convertible, new hood on
    1965 Ford Zodiac Executive. Back under cover
    1984 MG Montego EFi in black. Digital dash and bring worked on
    1984 MG Montego EFI in red. Digital dash and being worked on
    2004 MGZT cdti. Great workhorse
    2004 MGZT V8. Heater out as leaking

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    • #3
      I understand all that Jeff. Part of the test is to bypass the thermostat switch so the fan should operate as soon as the ignition is switched on. The fact it doesn't says the problem is elsewhere. Which is why I wrote that the wiring is all ok. If I manually make the switch inside the timer the fan works. The LT side is just not pulling the switch closed and this is on two timers. The fact that there is current on the two ignition wires at the connector and a circuit out to earth is what is confusing me.
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      Hyundai Tucson Premier SE (Daily run-around)


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      • #4
        I can't get inside your head to know what tests you've carried out if you don't put it on your thread on plain english Pete, I'm not that clever, I will have a look at my manual and wiring diagram when I get back from Manchester later to help you. From memory there is also a relay and fuse that I'm assuming you've checked, also the replacement delay unit was proven to be working?
        Last edited by Jeff Turbo; 27th March 2020, 08:00.
        1958 Ford Consul Convertible, new hood on
        1965 Ford Zodiac Executive. Back under cover
        1984 MG Montego EFi in black. Digital dash and bring worked on
        1984 MG Montego EFI in red. Digital dash and being worked on
        2004 MGZT cdti. Great workhorse
        2004 MGZT V8. Heater out as leaking

        Comment


        • #5
          Jeff, the timer delay unit is basically a latching relay that is held closed for a predetermined time using a small computer chip. The wiring for the fan involves an ignition controlled green wire going, fed from fuse C5, to the switch at the thermo housing. This closes when the coolant tempreature reaches 70 degrees. The other green/slate wire from this switch goes to the delay unit. This is the "LT" side of the relay circuit and is just used to energise a magnet that then causes a switch to close passing battery voltage to the item being protected. Meanwhile a purple wire (indicating battery feed and not controlled by ignition) is fed from fuse C8 to the fan motor. The earth side of the motor is a purple/yellow wire that goes to the delay unit. There is also a seperate purple wire from the same fuse going directly to the delay unit. This is to supply battery current to keep the relay energised once it has been tripped by the "LT" circuit. The final connection from the delay timer is an earth connection. There are no other relays involved in this circuit.

          For the purpose of the test, I have directly connected the two wires at the thermostatic switch, to prove if it is working or not. In this way, as soon as the ignition is switched on the fan should work. In my case it doesn't with either delay unit. This means that either there is a problem with the delay unit, the wiring or the motor itself. I have confirmed that I have 12v power to the green wires with the ignition on, and to the purple wires at all times. There is no break in any wires, so they are all good to the delay unit connector. By removing the circuit board from the casing of the delay unit, I have manually made the relay switch and the fan works. This indicates to me that the LT side is not working correctly and the fault is present with both delay units. As I'm seeing 12v at the green termination on the unit and I can see a complete circuit when connecing the green wires to the earth, using a multimeter, I am trying to understand why the relay is not being energised.

          The replacement delay unit is unknown, and with the original one, the fan was working correctly but lately became random as to whether it worked or not.
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          MG Maestro Turbo #436 (To keep me occupied in retirement)
          Hyundai Tucson Premier SE (Daily run-around)


          You know you are getting old when your knees buckle and your belt doesn't

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          • #6
            Didn't need the lesson on how it works, just what you've tested so far Pete
            If intermittent it looks possibly like both units could be faulty if your replacement isn't known to be working, I have taken apart a delay unit many years ago and found a crack in a solder joint but that's not proven atm and/or wiring problem
            Now i'm home i've looked at the wiring diagram
            You are in error a couple of things though so maybe something else to try.
            There are five wires coming out of the unit as you say and three going into the fuse box.
            Pin 1. Green/slate to the coolant switch and on to green fusebox C5.
            Pin 2, Purple to fusebox A5,
            Pin 3. Green to vent valve and linked to C5,
            Pin 4. Purple/Orange to the fan motor and on to Purple fusebox A8,
            Pin 6, black earth.

            If you can manually make it work then either the unit is faulty or something is not triggering it as you say. Can you replace the unit with one known to be working so ensure that's not the fault?
            Last edited by Jeff Turbo; 27th March 2020, 14:25.
            1958 Ford Consul Convertible, new hood on
            1965 Ford Zodiac Executive. Back under cover
            1984 MG Montego EFi in black. Digital dash and bring worked on
            1984 MG Montego EFI in red. Digital dash and being worked on
            2004 MGZT cdti. Great workhorse
            2004 MGZT V8. Heater out as leaking

            Comment


            • #7
              Is your notation for the Monetgo turbo, Jeff? its just I don't recognise you saying Pin 3 goes to the vent valve, and I'm assuming that your purple/orange is purple/yellow on a Maestro. Likewise your fuse A5 is C8 on Maestros. But they are insignificant in this situation.
              I don't suppose you can recall which joint had the crack in it, as mine may be similar. Failing that I think I need someone who understands electonics. I'm fine with car electrical cicuits and ordinary relays, but with the array of resistors, condensors, chips and the where they connect on the PCB, on this delay unit, that is a black art to me.
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              MG Maestro Turbo #436 (To keep me occupied in retirement)
              Hyundai Tucson Premier SE (Daily run-around)


              You know you are getting old when your knees buckle and your belt doesn't

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              • #8
                Originally posted by Sussex Pete View Post
                Is your notation for the Monetgo turbo, Jeff? its just I don't recognise you saying Pin 3 goes to the vent valve, and I'm assuming that your purple/orange is purple/yellow on a Maestro. Likewise your fuse A5 is C8 on Maestros. But they are insignificant in this situation.
                I don't suppose you can recall which joint had the crack in it, as mine may be similar. Failing that I think I need someone who understands electonics. I'm fine with car electrical cicuits and ordinary relays, but with the array of resistors, condensors, chips and the where they connect on the PCB, on this delay unit, that is a black art to me.
                A long time ago Pete, so no can't remember that far back. Beg/borrow one thats known to be working.
                1958 Ford Consul Convertible, new hood on
                1965 Ford Zodiac Executive. Back under cover
                1984 MG Montego EFi in black. Digital dash and bring worked on
                1984 MG Montego EFI in red. Digital dash and being worked on
                2004 MGZT cdti. Great workhorse
                2004 MGZT V8. Heater out as leaking

                Comment


                • #9
                  If you are stuck enough to be bothered with posting them, I have a relay/timer fast check that would either exonerate or condemn the units... An I right in thinking that the fan should only come on when the ignition is switched off at coolant temp over 70, then run for the timed interval? If so, then the unit must have an ignition feed as described by Jeff. Minimum number of wires to make it operate is 5: permanent live, earth, output, ignition input (so it can detect ignition going off) and input from temp sensor.

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                  • #10
                    Originally posted by Doctordiesel77 View Post
                    If you are stuck enough to be bothered with posting them, I have a relay/timer fast check that would either exonerate or condemn the units... An I right in thinking that the fan should only come on when the ignition is switched off at coolant temp over 70, then run for the timed interval? If so, then the unit must have an ignition feed as described by Jeff. Minimum number of wires to make it operate is 5: permanent live, earth, output, ignition input (so it can detect ignition going off) and input from temp sensor.
                    Yep, exactly right, it shouldn't start working until its at that temperature. Won't work if the engine is cold but sometimes when you reconnect a battery it will come on. So I'm assuming pete is checking all this while the engine is warm or hot enough or with the connection unplugged and bridged. There again.............
                    Last edited by Jeff Turbo; 28th March 2020, 06:14.
                    1958 Ford Consul Convertible, new hood on
                    1965 Ford Zodiac Executive. Back under cover
                    1984 MG Montego EFi in black. Digital dash and bring worked on
                    1984 MG Montego EFI in red. Digital dash and being worked on
                    2004 MGZT cdti. Great workhorse
                    2004 MGZT V8. Heater out as leaking

                    Comment


                    • #11
                      Originally posted by Doctordiesel77 View Post
                      If you are stuck enough to be bothered with posting them, I have a relay/timer fast check that would either exonerate or condemn the units... An I right in thinking that the fan should only come on when the ignition is switched off at coolant temp over 70, then run for the timed interval? If so, then the unit must have an ignition feed as described by Jeff. Minimum number of wires to make it operate is 5: permanent live, earth, output, ignition input (so it can detect ignition going off) and input from temp sensor.
                      There are really only 4 wires connected to the delay unit. Permanent live, incoming earth from fan, connection to body earth, ignition feed from temp sender which is also looped to a second pin, making 5 connections.

                      Originally posted by Jeff Turbo View Post

                      Yep, exactly right, it shouldn't start working until its at that temperature. Won't work if the engine is cold but sometimes when you reconnect a battery it will come on. So I'm assuming pete is checking all this while the engine is warm or hot enough or with the connection unplugged and bridged. There again.............
                      Ahem! I did write in posts #3 and #5 that I was bridging the thermo switch wires to take it out of the equation.
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                      How to add yourself to the M&MOC Map:http://www.maestro.org.uk/forums/sho...703#post185703

                      MG Maestro Turbo #436 (To keep me occupied in retirement)
                      Hyundai Tucson Premier SE (Daily run-around)


                      You know you are getting old when your knees buckle and your belt doesn't

                      Comment


                      • #12
                        Originally posted by Sussex Pete View Post

                        There are really only 4 wires connected to the delay unit. Permanent live, incoming earth from fan, connection to body earth, ignition feed from temp sender which is also looped to a second pin, making 5 connections.
                        It can't work if the only ignition switched feed is from the temp sensor, it has to know that temp is high enough and that ignition has been switched off, can't do it on one wire, maybe somebody has altered the wiring?



                        Ahem! I did write in posts #3 and #5 that I was bridging the thermo switch wires to take it out of the equation.

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                        • #13
                          Originally posted by Sussex Pete View Post

                          There are really only 4 wires connected to the delay unit. Permanent live, incoming earth from fan, connection to body earth, ignition feed from temp sender which is also looped to a second pin, making 5 connections.



                          Ahem! I did write in posts #3 and #5 that I was bridging the thermo switch wires to take it out of the equation.
                          That's my lot then, can't help anymore as everything i've said is obviously wrong for your car and I'm no help at all, as the saying goes, Im out. You might wanna take John up on his offer
                          Last edited by Jeff Turbo; 28th March 2020, 18:40.
                          1958 Ford Consul Convertible, new hood on
                          1965 Ford Zodiac Executive. Back under cover
                          1984 MG Montego EFi in black. Digital dash and bring worked on
                          1984 MG Montego EFI in red. Digital dash and being worked on
                          2004 MGZT cdti. Great workhorse
                          2004 MGZT V8. Heater out as leaking

                          Comment


                          • #14
                            Bearing in mind I said the other day on fb I'm no good with electrics, but could it be a poor earth, in that it shows continuity but won't pull enough current to make the relay work??
                            midget1380@btinternet.com

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                            • #15
                              Originally posted by mgdavid View Post
                              Bearing in mind I said the other day on fb I'm no good with electrics, but could it be a poor earth, in that it shows continuity but won't pull enough current to make the relay work??
                              That's quite a good thought. Two issues with it, for me. One, doesn't completely explain why it was working fine until just before I SORN'd the car and two, the earth is one of the bunch connected to the scuttle behind the dash, so if it was problematic I would have thought other faults would be making themselves known. Oh, a multimeter test shows no resistance between cable in the connector and another earth point.

                              I'm still thinking there is fault in the electronics, so I may send one of the timers I've got to John and see if the Fast Check picks anything up.
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                              How to add yourself to the M&MOC Map:http://www.maestro.org.uk/forums/sho...703#post185703

                              MG Maestro Turbo #436 (To keep me occupied in retirement)
                              Hyundai Tucson Premier SE (Daily run-around)


                              You know you are getting old when your knees buckle and your belt doesn't

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