Lucas 5AS fault?

Rover Mini Copper MPI 1997

I'm trying to work out if my Lucas 5AS is on the way outas I have been having an intermittent starting problem with my Mini MPI. The behaviour is that with the immobiliser off the engine fails to crank as if it is immobilised.

I've done some tests between the starter relay in the MEMS relay pack to the 5AS to the ECU. I have good continuity (less than 1 ohm resistance) on these wires but with the 5AS connected the resistance is high (2000 ohms) when the immobiliser is off.

This was puzzling me so I plugged in Pscan. Looking at the live data, I was getting a high/low alternating readout on the security line to the ECU. I did a short video to show you what I mean. Is this usual behavior? I don't recall seeing this before.

https://photos.app.goo.gl/oeLFg9sJyGETsdLYA

I do have a couple of spare untrusted 5AS units so I may try those over the weekend. I've inspected the wiring and plugs and I can't see any obvious breaks but in reality I could have a nicked wire somewhere in the loom

I'd be interested to know your thoughts.

When you say "untrusted 5AS units" are you implying that there such things as trustworthy 5AS units? :bust:

Whoops, that should have said 'untested' 😊

Later on today I will couple Pscan to our MPI Mini and see what our car shows.

John

Thanks John

A lot of Mini MPI electrical problems trace back to the big earthing point on the left side of the engine bay. Years since I had my Mini but a clean up of this did wonders. It's under something stupid I vaguely recall, which encourages rust......

Thanks.
Yes, I have been looking at that. I had previously taken it off and remade the lug on the end. It all looked good with a low resistance but I cleaned it up anyway.
I did find that two of the earth braids a n the screened cables had failed. The job for today is to remake these.

It's definitely a wire fault. With the ECU it situ the mini wouldn't crank. However if I held it in the crank position and moved the ECU then it started!

Okay so I've just put Pscan on the Mini and the security is doing as it does on yours it flicks from high to low every few seconds on live data and it does the same with the engine running . I took a video of it but the file is too big put on the forum .
John .

Thanks John.
That eliminates that from my enquiries then. I'll investigate the earths a bit more and report back.

Thanks again

Yes alternating between high and low is normal. That's the 5AS sending the EMS code down the wire. It doesn't mean that it's the right code but at least it's sending a code.

Mystery Solved!

After re-soldering the shield earths and cleaning up the main earth the problem still existed.

I decided to check continuity from the MEMS starter realy to the starter solenoid. Sure enough, the wire was broken at the starter motor end! When I was moving the ECU, it was moving the wiring loom which in turn allowed the broken wire to make the connection to the solenoid. I crimped out a new end, wrapped in heat shrink and it worked striaght away.

I've definetly learned a lot about how the starting system works - every day is a school day!

Thanks all for your help. I'll be uploading a video to youtube soon.

Well done for finding the fault . No prizes for guessing what I will be checking under the bonnet of our MPI .