Faulty ECM

michaelstevens32 · March 10, 2022

Hello, I have a jag s type 3 litre v6 auto, 2003 model. Car is perfect all over. But, I know have an issue with the rear lights. Now what is happening, when I indicate the rear indicator Bulb flashes not so bright and the reverse light flashes with it. The other side is fine. I’ve changed fuses, all bulbs, battery has full charge and all earths are clean. Sometimes the warning light comes up to say the Bulb has failed. Then it disappears. Any ideas?

mrmike · March 10, 2022

Normally two dimmer lights flashing together means the that the earth side of the chosen Bulb is missing and for whatever reason the Earth return for that Bulb is via the 2nd Bulb. If the 2 bulbs were equal brightness then there would be a S/C on the + side. Check the bulb holder carefully and measure with a tester from the body of the bulb to chassis. I just had another thought. I think the side pins are weird on the bulbs and someone may have tried to fit a differently oriented one, It can work temporarily as I have done it too in an emergency.

Good luck.

M

michaelstevens32 · March 10, 2022

May I ask what you mean by ‘sc’. This is the strangest thing this problem started after a night of heavy rain on the motorway. But this problem started on the right side. So replaced everything on the right, I even took apart the ECM cables and put it back together. Now the problem shifted to the left side. But taken the cables out of the ECM caused my parking sensors to constantly beep whilst driving. So I took the live cable off the battery for 30 mins and that seemed to if cured the parking sensors. I use to here about jags and electrical problems when I was a kid. But I never believed it.

The Voices · March 10, 2022

S/C = short circuit

JOE-DOT-COM · March 10, 2022

HI

usually this is a fuse, theres 2 fuses to each light, so if 1 fuse goes it makes both flash dimly

its so easy on these, the bulbs have a permanent feed on one side of the Bulb, so when you remove the Bulb cluster, if you touch the body, it pops a fuse every time

cheers

Joe

michaelstevens32 · March 11, 2022

I’ve replaced all the fuses. That was my first job, I’ve checked them again.

michaelstevens32 · March 11, 2022

One thing I’m thinking that I will check later is when I re fitted all the connectors back into the ecm, I may of refuted the tail lights connectors the other way round. Hence why the problem shifted from right to left

michaelstevens32 · March 11, 2022

Bang goes that theory, I open and closed the boot and now both indicator lights don’t work plus parking sensors are constantly making noise.

mrmike · March 11, 2022

8 hours ago, JOE-DOT-COM said:

HI

usually this is a fuse, theres 2 fuses to each light, so if 1 fuse goes it makes both flash dimly

its so easy on these, the bulbs have a permanent feed on one side of the bulb, so when you remove the bulb cluster, if you touch the body, it pops a fuse every time

cheers

Joe

Hi Joe,

Hadn't realised that the bulbs were always live and were decked to turn them on. What sort of devil thought that out. Bit like the Spanish mains system. Pull their fuse and power still goes to your house because the fuse is in the return/neutral line.

mrmike · March 11, 2022

5 hours ago, michaelstevens32 said:

Bang goes that theory, I open and closed the boot and now both indicator lights don’t work plus parking sensors are constantly making noise.

Get some "Good" contact / switch cleaner/lubricant, not cheap Chinese stuff. and spray and open and close the connectors a few times. Check also for bent pins/contacts.

JOE-DOT-COM · March 11, 2022

hi

virtually every thing on the S-type works this way, 12v feed one side and the other side goes back to the body module and switches the earth to bring it on

cheers

Joe

michaelstevens32 · March 11, 2022

Now my rear right indicator is working. I’m thinking something is being shaken when driving or closing door/boot lid. MOT tomorrow but the garage said ti come 1 hour early so that they look into this. I’m at a dead end with this

JOE-DOT-COM · March 11, 2022

hi

the other thing that causes this is either putting the wrong Bulb type in, or putting it in the wrong orientation(brake light bulbs) the pegs on the bulbs are in a specific place in put in wrong, the short the tabs at the bottom of the Bulb out

cheers

Joe

mrmike · March 11, 2022

I concur with this having done it myself in an emergency. The correct bulbs are not always easily available especially in the local (Spain/Italy) Chinese shops.

Mike

michaelstevens32 · March 11, 2022

The bulbs I bought fit correctly. But I did consider if they are still not correct for my car. I’ll leave this with my trusted garage. I like to go through as much as possible myself. It’s always good to be armed with knowledge around garages. They won’t perform MOT if it’s going to fail. So they won’t charge unless they can figure out what is happening. I’m hoping they can plug into the ECL and get some data.