Bosch ECU issue

[bembo449]

Anyone know how these bosch ECUs work ??
Currently got a 30amp fuse that protects the power to the ECU , its got 12v on one side but when the ECU is plugged in there is only 4.9v on the other side of the fuse , ive checked the wiring for continuity and its all good , when i check power to the ECU plug from the fuse there is only 0.06v then it reverts back to rhe above state once ECU is reattached, surely it should have 12v in all tests

 

[LostKiwi]

Nothing to do with the ECU - you'd probably get the same issue with a light bulb.

From the description you have a bad connection either on the fuse, internally in the fuse or in the wiring from the fuse.

 

[M80]

Are you saying that the voltage on either side of the fuse is different when the ECU is plugged in?

As LK it sounds like poor or broken continuity somewhere in the line.

 

[bembo449]

exactly what im saying pal , ive checked continuity on one of the wires to the base of the fuse , the other side goes to a relay in the same fuse box , its a 4 pin relay and only one of the pins has 12v , 2 have 0v and one has 2.9v , i cant find where the 2nd wire to that relay goes as yet

 

[bembo449]

just to add , the ECU hast got 12v to it , the fuse ive been talking about is the PCM main load fuse , this explains why the car wont start as there no power to the injectors etc , im getting the ECU tested this afternoon just to be thorough but the car has been stood for over a year so im wonder if its worth uncovering all the wiring that runs through the car to check for rodent damage

 

[M80]

I would confirm at the fuse, to be sure you aren't testing 2 different lines.
You can test at each side of the fuse, there is a small exposed conductor on top of each side. If the voltage is different then the fuse is goosed.

 

[bembo449]

i tested the fuse and tried 2 others of the same rating so covered that , can i use 12v from a different place to power up the ECU ?? assuming it does actually need 12v that is ?

 

[M80]

But you said the ECU has 12 volts.

In truth your questions go beyond my pay grade.
I would assume the ECU requires a 12v feed.
I would imagine if permissives were holding out the start it would be signals to the ECU rather than the feed to it, but again you say you have the feed.

 

[bembo449]

to be clear , there is 12v to the PCM load fuse but not the side that goes to the ECU , thats the side that only has 4.9v

 

[M80]

If you can be sure that the +ve feed to the ECU is only 5v I would be following that back to see where it looses it's supply.

I wouldn't want to suggest applying 12v to it until sure it isn't another line.

 

[supernoodle]

A picture paints a 1000 words, so fag packet sketch attached.

The power circuit should look something like this. The numbers are the DIN norms. 30, the unswitched power, 87 the normally open output from the relay that goes to device being controlled (ECU here) , 86 one side of the relay coil, 85 the other.
86 is normally just connected to the 30 supply, 85 is pulled low by the ECU when it sees T15 (ignition).

I put the A and B on the fuse as you mention both sides of the fuse.

Might be easier to understand what's being measured and where if using this.