Help needed. Td5 90 suddenly stopped when driving, felt like a fuse had blown or a broken connection as the engine stopped dead.

Has no EML, no fuel pump or main relay click when ignition turned on. Water gauge doesn't move indicating a bad earth, oil and battery lights etc still work and starter motor works fine.
Plugging into a diagnostic machine the diagnostic kit can't see the ECU but can see the 10as alarm.

Tried a 2nd known working ecu which gives the same symptoms in my 90.

All fuses tested, relays swapped, impact sensor on bulkhead bypassed, main earth on chassis to battery box cleaned, earths ran from ecu case direct to battery. Bridging the fuel pump relay makes the pump run.

I'm only measuring 6v on the brown with orange tracer on the back of all the repays and the ecu. Is that my issue and how do I resolve it?? Battery isn't flat.

Sorry about the delay in replying - somehow I didn't spot this.

It all sounds very similar to what happens to me when I lose an ECU (It's happened to me several times now). But you've tried a known good ECU and it's still the same. I'm sure you've done this, but just a thought - have you synched the ECU with the alarm unit? On my Nanocom it's under 'engine' and 'utilities' and then 'learn security code' or on older models 'learn DDS code'. Otherwise the engine will turn over but won't start, but you don't get an immobiliser light. The 10AS unit also goes wrong sometimes. Does the donor vehicle you got the ECU from have a 10AS you can borrow in the interests of science? Another thing the TD5 is prone to is deterioration of the loom itself. The insulation gets brittle and flaky as do the wires themselves, with unpredictable and sometimes disabling symptoms. I'm pretty sure the supply to the fuses and relays in the seatbox is supposed to be 12v, so it's worth just bridging it to a 12v source to see if that makes a difference. It's only 12v so it's not going to blow anything.

It's perfectly possible, I've discovered, for an ECU to look fine on a Nanocom but be useless for starting the car.
The TD5 is somewhat sensitive to the part number of the ECU. Mine will run on an NNN500020 and nothing else. I've tried other ECUs from post 2002 models but it won't go. So make sure you're swapping like with like.
The casing of the ECU doesn't really need an earth - mine will run fine when it's not even screwed down. All the black wires in the multiplugs are earths, and that seems to suffice.
That's just what I've worked out from tackling faults on mine.

There's another couple of things that a lot of people mention so often it's hardly worth repeating, so I'm sure you know what they are. The crank position sensor, and related to this the possibility that the starter motor is generating interference on the crank position sensor. And fuel pressure, of course. Mind you, once started a TD5 will run with the fuel pump unplugged, just not very fast.

what year ,assume you have checked for damage cable entering the under seat fuse box and that there is no corrosion or loose connections on the earth header C291.
assume you have checked for a 12 volt supply on the Engine Control Unit connector C658 pin 33 fed on a white/green cable from the passenger fuse box fuse F5 this is assuming it's 1999 model
When you are checking the voltages it would be advisable to use a temporary ground cable connected to the battery to eliminate the possibility of bad or high resistance earth connections and to highlight them

ECU has probably failed. Check the earth under the drivers seat, but this is exactly what happened to mine and it was the ECU

Thanks for your help guys. It turned out to be a earth issue, main reason for replying is to thank you all and for info incase it helps someone else in the future.
On my 2002 I didn't have the earth under the drivers seat, It happened to be on the top right of the transfer case and in my case it was somewhere between the ecu and this point the wiring has failed.
With a new earth from the black plug going to a earth point i've made on the bulkhead where my ecu lives and direct back to the battery it runs fine.

Good, I'm glad you've made some headway on this.

Glad it wasn't your ECU.