- My model of MX-5 is:Mk1 __
- I’m based near: Bristol
- I’m looking for technical help or recommendations on: Non start. I have no spark or light at the Diagnosis LED. I have changed the ECU and checked all I can. I have only 1.2 volts at the white/red wire which I believe should show 12 volts._Advise please. Thanks
Do the ignition lights come on? Have you checked the 80 amp main fuse? Which white/red wire are you refering to.? Does the engine crank normaly?
What year is you car and is it a 1.6 or 1.8?
I did write up on how the power was routed in a previous thread with a no start problem but that seems to have dissapeared with the move to the new forum software.
I could go through it again if needed but check the basics first.
Good luck.
D.
Hi D. Thank you very much for your reply. 80 amp fuse is ok. Engine cranks normally. The red/white wire is a common link between the ECU, ISC, Fuel pump relay, CAS and the diagnostic connector. I think this should show 12v without the engine running but shows 1.2v. The car is a 1.6 eunos 1992.
The car did run but would not start when hot. I changed the coolant thermo sensor but this made no difference. I turned my attention to the throttle body and attached components. I changed the ISC for a used item. From that point car will not start - no spark. I changed the ECU for a known working item but no change. I am mystified as to why I have no light at the diagnostic LED check. Initially the light came on and then went off, which I think is normal. Following the above changes it does not come on at all. I have checked continuities and earth connections, fuses ok. The CAS was replaced by the previous owner. Does this provide any clues? Thanks Geoff
Hi Geoff, thanks for the comprehensive reply.
I agree the red/white wire should have 12 volts on it. It is the supply for just about everything in the engine management system.
From looking at the diagrams ( though to be honest there are so many variations I can’t be sure this 100% relates to you car ) the 12 volt permanent feed comes from the battery to the 30 amp injector fuse ( labled INJ I think ) in the main relay and fuse box. After this fuse it goes to the " Main Relay " also in the same box, and when the relay is energised ( in the active or closed state ) this passes the 12 volts via the red/white wire to all the stuff you mention above.
So, check the 30 Amp injector fuse, if this is intact is 12 volts on both sides of the fuse. Next, check if the Main Relay, is this activating? The Main Relay is supplied with 12 volts to activate it from the ignition switch through the 15 Amp " Engine " fuse. Is this fuse OK and does it have 12 volts on both sides when the ignition is on. If not the ignition switch may be faulty. If all these voltages check out ok you may have a faulty relay. Can you hear it click when you turn the ignition on? Take it out and clean the connections. Try tapping it with the ignition on in case its a bit sticky. Can you borrow another one to test?
If you could check these things and let me know what you find we can go from there.
Good luck.
Dave
Sorry, but wondered if there was any update?
Hi Dave.
I’ve just finished testing and general fiddling.
OK - here’s the update:
30 amp inj fuse ok. 12v on one side of fuse, ok?
Main relay bench tested and ok.
15 amp engine fuse ok and showing 12v.
10 amp Room fuse ok. 12v at ECU
Fuel pump opening circuit relay bench tested ok.
Still 1.2 v at white/red wire.
Haven’t tested ignition switch - don’t know how. With ignition on the only light on the dash is Battery -?
Does all this help - no particular problem identified it would seem. Question is when does 12v at w/r wire become 1.2v?
I look forward to hearing from you.
Many thanks
Geoff
Humm. If the Engine fuse has 12 volts on it that would indicate the ignition swithch is working.
Unfortunately my mk1 is in Devon and I’m in Berks at the moment so I can’t go and do any checks on that.
I’m guessing you have a DVM and know how to use it as you are able to do these readings, which is good.
I’m begining to think this may be some sort of earth problem, this would explain the 1.2 volts. I think there is an earth block connection just below where the winscreen wiper motor is in the engine bay, check that’s all clean and tight. Where are you connecting the negitive lead of your meter?
My feeling is the Main Relay is not energising or is faulty.
I can’t remember, but if you unplug the Main Relay are you able to get your meter on the socket connections? I believe the relay should have a diagram of the connections on it, and if you’ve bench tested it I guess you have worked this out. If you can get your meter onto the connections check for a 12 volt supply accross the coil sockets when the ignition is on, and 12 volts on one side of the switch contacts all the time. Can you hear the relay click when you turn the ignition on? If you feel brave you could put a link across the switch contacts ( not the coil ) of the relay and see what happens. I’m not recomending this but it’s something I’d try.
Got to go, dinners ready. Let me know how you get on.
Dave.
Just thinking over dinner, even if the relay contacts tested ok on a bench test the contacts may have become burnt/damaged so they read ok with a meter but are resistive when a current load is put on them. This would explain the 1.2 volts as well.
I can’t remember but is the same type of relay used somewhere else in the car so you could do a swap? Headlight relay or something.
Just a thought.
D
Also does the voltage on the red/white wire change from 1.2 volts when the ignition is on to 0 volts when the ignition is turned off?
Hi Dave. Thanks again for your thoughts. I will check things out and get back to you. Hope I didnt give you indigestion over dinner! G
Hi Dave
Update:
Checked all earth connections I can find and all ok. Some are grouped, others are a strap to the engine. Earth wires at ECU are ok.
I rechecked the Main relay, seems ok. I bridged the relay contacts in the fuse block to bypass the relay coils with no change. I tested sockets as you suggested and it checks out. The relay clicks. Its unclear to me what is triggered when the Main relay is activated. I am not an electrician so try to use the wiring diagrams and a bit of logic!
The red/white wire goes 0v with the ignition off.
The room fuse has 12v as does pin 1A on the ECU - they are connected directly.
The engine fuse shows 12v also.
Still getting 1.2v.
I’m running out of options, Im hoping you can make some further suggestions. Once again I look forward to hearing from you.
Cheers
Geoff
PM sent