I have a '52 plate Mk 2.5 1.8 which recently suffered a flood in the passenger foot well. (Parked facing downhill in heavy rain!) Initially the engine would not start, but after some drying out the engine started, but ran roughly. Now after further drying out the engine and car run as well as before but the check engine light is on and the only fault code showing is P1562, “battery voltage low”. (The fault code returns immediately after clearing it.) This fault code seems to be a common problem but without a clear remedy. I wonder if anyone has ignored this fault and can say what happened? I don’t think my battery is in the prime of life, the off-load voltage reads 12.15 volts. Is this enough to trigger the P1562 fault? As yet I haven’t gone as far as unplugging all the cables to the engine computer and cleaning the contacts.
The p1562 has more to do with the wiring to the ecu. Your battery voltage is a little on the low side but given recent events, this code will be generated by damaged/corroded wiring to your ecu or possibly a residual ecu issue.
The MK2.5 car particularly tends to require attention to the ecu connectors after water ingress. The copper wire is very fine on the MK2.5 and perhaps half the ones I deal with where water has ingressed require one or more of the connector plugs to be replaced with associated stub wiring. Sometimes the wires actually snap out of the connector plug, typically the big one on the left but before that the wire will loose conductivity, signal voltages, etc will drop and your code results.
What did the pins on the ecu look like? If the plugs were particularly difficult to remove(corroded on) or the pins on the ecu look milky/corroded, it is likely that water has got to the fine wiring inside the plugs.
Many thanks, esp rhino666. I will remove the connectors and see what condition they are in. I wonder, do you have any experience on what can happen if the problem is not put right? Can it lead to failure of the engine management computer? I’ll keep an check on the battery voltage.
That code is for the battery back up for the PCM, the power to it with the ignition off. The fault code is stored when the voltage at PCM terminal 4AG drops to below 2.5 volts with the engine running. I’d suspect and wiring, connector or PCM fault before suspecting a battery fault.
Out of interest, the ROOM fuse hasn’t blown has it? Does the interior light work? The ROOM fuse protects PCM terminal 4AG.
Thanks for advice. Yes, the interior light is working so the ROOM fuse seems intact. If it’s dry this afternoon I will attempt to unplug the cables to the computer and inspect the pins.
I have now pulled out the plugs on the ECU with a view to spraying the pins with switch cleaner but have discovered that one of the pins on the large (left hand) socket has rotted away completely and remains embedded in the plug! This is the furthest pin on the left in the middle row with the ECU in its normal position. What does this pin do? Is the only solution a replacement ECU and harness?
(I have tried to add some photos but cannot find the “Attach File” button in the “Post a Reply” window.)
I’m not familiar with the plug or ecu, however, I did a quick google image search of an MX5 ECU, and I’d suggest that it would be possible to open the ecu, solder a wire on the pin from the inside, then splice this in to the corresponding wire on the outside.
Whilst it won’t be the prettiest of fixes, it will probably be the cheapest, and probably not that difficult.
As for images, you have to host these externally, somewhere like photobucket, and post a link
Sorry to hear this Alan but not surprised. I specialise in ecu parts so have dealt with similar situations quite a few times before. Was the broken wire blue by any chance?
Given the situation I would dismantle the ecu and have a good look at the PCB. Corrosion is usually very obvious so no need for a microscope or expert knowledge.
If the ecu pins are in good order and more importantly the ‘L’ shaped metal wires that feed from the pins to the circuit board are undamaged and in good order, the ecu may be salvageable.
In any event I would suggest that any of the three connecting plugs with obvious water ingress, to any extent be replaced. This really means cut the old plug and a few inches of wire off and solder/heat shrink on a good replacement. If more than one plug effected stage the solder points.
It is possible to short cut this process but if you do, don’t complain when the ticking time bomb of water initiated corrosion catches up on you and you or your wife are stranded in the middle of nowhere as a result.
Thank you all for such comprehensive information and advice. It is indeed the 4AG pin that has broken - battery pos, hot at all times. However, I presume this cannot be the only positive supply to the ECU as the engine still runs. I shall now spend some (enjoyable) hours with the ECU.
Just an update. Took the ECU circuit board out (rather surprised by such a lack of protection from damp and vibration) and discovered that the 4AG pin and its connection to the board had completely vanished (corroded away). From the junk box found some wire and connectors to bypass the plug and socket and connect with the blue/red tracer wire. Very fortunately there appears to be no further damage to the printed circuit board (despite cosmetic corrosion in some places) as now the engine and car run well without the check engine light on and the car is a joy to drive again. However, I am aware there could well be further problems. Thanks to all for advice.
