Fan resistor replaced but battery died disgracefully

[MikeBRG]

Having benefited from the excellent advice and information on this forum I wanted to thank all the posters and pass on my recent experience with my 6.5 year old CDTi ZT-T. You may be facing some of these issues soon! Much of this is doubtless being repeated all over the country but I could not find any help on the forum when I arrived at the last stage and hit a problem and thought it might be helpful to share an anxious moment and its outcome.

I have lived through the traditional fuel tank pump failures and last year, the secondary (under bonnet) fuel pump failure and thought I was on a roll.

With the first hot day of this year, I found the air-con was not working. Diagnosis from man-in-a-van specialist was coolant had blown out and system was over-pressurising like mad until trinary switch kicked in and fan came on at full speed. Removed coolant pending further diagnosis. May still have damaged compressor but quote of almost £1k suggested worth checking out true cause.

Turned to forum and find I am not alone - quickly find following sequence:
radiator fan not running at low speed (hence aircon failure)
identify two speed fan with silver resistor
check fan runs at high speed.
suspect resistor open circuit (it was, confirmed later on removal)
purchase resistor replacement kit from Jules in Wales. (excellent)

Decide to have a go at replacement myself (save £175 quote from dealer)
follow Jules and forum advice on removing bumper and crash bar
replace resistor - goes in fine. Fan now turns at low speed
follow Jules advice and check out other 'bumper off' issues (well documented in forum)
clean out wash bottle pump and filter
find damaged harness under driver side headlight (see attached picture)

repair wire to headlamp (cut through in picture) and protect harness
headlamp now adjusts up and down again

rebuild front end, checking fan and headlamp operation at each stage.

Up to here, things have gone well.

On final check just before screwing screws and buckles into underside of bumper, do one more fan check. All hell breaks loose....the electrical system goes ga-ga whenever I turn the ignition on. All the warning panel display lights flash on and off and relays are clicking all over. Only good news is there is no smoke or burning smell. No chance of starting engine. Suspect trapped or open circuit wire from my loom checking or repairs. Undo all my work, apart from new resistor, and uncouple all headlamps to try to isolate problem. Nothing has any effect - dashboard display flashes icons not seen before and relays behave like ticking clock - deep despair.

Contact Jules and check forum but no obvious cause - no trapped wires.
Contact AA with view to being towed to dealer who is already rubbing chin.

Experienced AA man finds source of problem in two minutes! It was the battery beginning to fail. A new battery has solved all the problems. What had happened was that over the two days including several fan tests and headlamp adjustment tests, I had taken the 6.5 year old battery to a low but not dead state, even though the volts looked OK on a meter, with no load. I had no indication of battery problems previously in normal driving.

The reason for passing this on is that the battery decline and failure mode on the 75 and ZT leads to very strange and alarming behaviour. I am used to graceful battery failure where lights dim and starter motors don't engage but not this type of computer driven mayhem. Apparently the ecu goes off if volts drop below 11.5V. I believe the voltage was fluctuating up and down around the cut-off as loads came in and out. At the worst moment I noticed the headlamps flashing on and off along with all the other random tickings and flashings and was convinced I was in deep trouble. I now have read and been advised that this is a good indicator that the battery is going down!

In summary, the point of all this is to encourage any happy 75 and MG owners to have a go at repairing some of the these problems - the guidance here is excellent - but don't overlook the battery. The way the car electronics behave as the battery volts drop is unpredictable. Its obvious now with hindsight that repeated testing of lights, motors and fans will drain the battery. Either run the engine to top it up or have a standby source to get going again. It was the last thing on my mind, having changed the car wiring. I think I was unlucky to hit the end of life and particular discharge point on my final check but it was a bad experience and I hope you avoid it.

On the safety front, I was working with the relevant fuse links removed but it would be wise to be able to disconnect the battery fully and rapidly when things go haywire. Be prepared.

Mike B