Hi,
I have a 96 cinquecento sx and every time I turn on the ignition the 5 amp fuel injection relay fuse blows. I have disconected every sensor from the ecu and it still blows. I have also taken the whole ecu wiring loom out to check for chaffed wires but could not see any. The only time it does not blow is when I disconnect the ecu. Does that mean its the ecu and if so what do I need to replace? I know I will need the ecu, but will I also need the code box and a key from a donor car?
Any help appreciated.....Thanks Dan

Hi there have you been able to check for a possible Bad earth or possible a short on the Ecu Wireing loom as i would say that would be more likley rather than the ECU itself being duff. Did this problem just happen one day when you went to start the car.

Hi,
Thanks for the reply, I have completely removed the ecu wiring loom including taking off the outer sheefing to check all wires. Thay all appear to be fine. When I bought the car it did not turn over, which I found to be the main earth cable being snapped. Do you think this could have sent a surge to the ecu and fried it?
Thankyou for your info....

I would agree with marti155, it is proberly not your ECU. Check the condition of the fuse holder itself it might be shorting out somewhere. Check the condition of the relay socket and consider changing the relay itself.

Let us know how you get on.

Some earlier owner could have managed to connect the battery the wrong way round and screwed up the ecu?
There could be a problem with any of the ecu outputs but I doubt it.
If any of the digital or analogue outs were subjected to a short then an internal component would have been destroyed and the circuit isolated.

Hi,
I have swapped the fuel pump relay for the injection relay as they are the same part and I know the fuel relay works. But that did not fix the problem. Also if the fuse block had a short surely it would blow even if the ecu was unpluged? I have attached the ecu wiring diagram which might help? Also I can feel the relay clicking over when I turn the ignition on/off, even after the fuse has blown. I will probably replace the ecu as a scrappy has told me £25 for one, but I need to know what I will need to get it to work with my car? (code box,any key from the scrap car etc). One more thing what if the scrap car is a pre-immobiliser model, will it work?
Thanks for all your replies....Dan

Use a multimeter on ohms (200ohms range) setting and measure between pins 35 and 34.
12v/5Amp is 2.4Ohms so the reading will be less to blow the fuse quickly.
this should test the for an internal short.
If you get an un coded ecu then it will be plug and play.

On the circuit diagram you should notice that the fuel injection relays coil is fed from the unfused side of the + supply. A fualt in the ecu connected to pin 4 will cause the relay to work.

Have you tried powering up the ecu with the HT coil wires disconected?

Hi,
I checked between pins 34 and 35 the reading was 00.4 on 200 ohms range, does that mean 0.4 ohms? I take it that would be a bad thing(30A)? There was no reading between pins 34 and 04. When you say un coded, do you mean the other car has no immobiliser and I could use this on my car that has an immobiliser? Also if the scrap car has an immobiliser do I need the code box and a key from it?
I have not tried to disconect only the HT coil wires and power up the ecu, but I have unpluged everything icluding the HT coils, but not the earths or the ecu and it still blows.
Thanks again.....Dan

Sounds like a major short in the ecu if it's 0.4ohms. The only thing that worries me is what caused it?
An uncoded ecu means no imobiliser. If it's coded then you need the code box, (not 100% shure you need it but best to be safe) keys (inc: red one) with lock set and ignition barrel.
Failing that you can get the new ecu virgined (plug in and it learns your code) or unlocked (no imobiliser).

With ref to pins 34,4. Did you put the red probe on pin 4 and black on 34?

Hi,
Yes red probe to pin 4 and black to 34. Does that make a difference? (im not very good at all this electrical bussiness). I think I will have to get another ecu from the scrappy. So if the car at the scrappy is a non immobiliser model, I should only need the ecu?
Thanks for all your help its much appreciated....Dan

The pin 4 things nothing to worry over.
See what you can do at the scrappy.

Ok thanks for your help I will go to the scrappy on friday and get an ecu.

Have a little think before replacing the ECU. The ECU is basically nothing more than a switch, with complicated curcits that operate at very high speeds (Similar to the processor in your computer). When you connect this curcit by turning on the ignition the ECU receives signals from the sensors which switch on actuators/relays etc by switching them to earth. So if you have a malfunction in your ancilliary equipment. i.e. your fuel circut, as soon as the ECU makes a connection by switching to earth it is going to blow that fuse. The ECU is protected by fuses and should not blow providing these fuses have not been bypassed through jumping or other non standard testing methods.

As cc1 has correctly mentioned, if the ECU has been dammaged, you need to isolate the problem, this is becuase you may blow the replacement ECU.

Let us know how you get on.

Hi,
It WORKS!:slayer: I got another ecu form the scrappy out of an L reg cinquecento (a non-immoiliser one) that looked like it hand been there for years. I just put in the new ecu and all the wiring loom I took out for no reason, unpluged the code box and she fired up straight away! I figure when the main earth cable snapped it tried to pull all the amps for the starter through one of the smaller earth cables and fried the ecu(but then im new to electronics and im prob wrong). So if you get a low resistance rating on pins 34 and 35 susspect the ecu! One more thing for future reference the red wire in the pic is fused its not direct from battery(might help someone).
Thanks again for all your help, much appreciated....Dan

Well done, really pleased your back on the road. If you can open the ECU up and if it is fried I would be interested in seeing a picture, you don't see many of these that have blown.

Thanks! I did open it to see if there was any scorcth marks but all looked fine. I also checked the 3 resistors that were in there (2 were 100ohm and the other was a 0.1 ohm I think, but on my multimeter it showed 0.5 ohm) maybe that was the problem, but it read over ressitance so I doubt it. I will attach a pic of the ecu.

Glad the problems sorted and the cars ok now.