Difference between revisions of "ECU or PCM diagnosis and repair"

Latest revision as of 01:56, 15 June 2011

This is how the fault presented in my car

I had driven about 100 miles and was accelerating up a hill doing about 70 mph when the engine ran rough for a couple of seconds and then died. Coasted to a stop. When I turned on the ignition all the dash lights go on like normal, but when I turn the key nothing happens. The engine does not turn over. I therefore tested the starter motor by running a wire off the battery terminal. It turned over just fine but the engine did not start.

Tested all the fuses and found number 67, alarm, was blown. Replaced it. Car started but was running rough, firing on only cylinders 1 and 4. AA man tested for fault codes. Reported fault on primary and secondary coils. Replaced the coil pack but engine continued running rough, firing on only cylinders 1 and 4. Replaced the new coil with the original. No change. When I use the dash diagnostics it flashes back and forward between "dtc" and d262. However, this code is not on any of the code lists where they all start with a letter P.

Generic Description of Fault

If the car runs rough on two cylinders or misfires you may have a problem with the coil pack. In the vast majority of cases failure of the coil is a simple matter addressed by replacing the coil. Sometimes coil failure "spikes" the car's computer, ie. the ECU or PCM.

How to Diagnose and Repair

The system is known as a wasted spark system and delivers sparks to cylinders 1 and 4 simultaneously, and to 2 and 3 simultaneously. On any particular cycle only one of the sparks fires the fuel. The coil therefor has two sides that alternate. With the car running, pull the HT leads off the coil pack one at a time. If there is a spark you will hear it and removing the lead will probably stall the engine, which is now trying to run on one cylinder. If there is no spark and the engine continues as before then you have identified the bad side of the coil.

The coil has to be replaced. If this does not cure the problem use a 12V LED or multimeter to test whether you are receiving the proper signals from the coil pack connector, which has three pins. The centre pin delivers +12V and comes from the alarm fuse, 67. Pins 1 and 3 receive the ground contact from the ECU. The ECU delivers a negative (ground) terminal alternately to pins 1 and 3. It alternates according to the speed of the engine. If the ECU is damaged it will not be delivering the ground to one or both of pins 1 or 3. Use a 12V LED or your multimeter to test for a signal between the centre pin and pin 1, and again between the centre pin and pin 3. If the signal is low or absent you have a defective ECU, assuming that there is not a broken wire between the connector and the ECU. A broken wire is unlikely if the sheathing is intact.

Removing the ECU can be problematic. There are two security systems. One uses a shear bolt in which case you have to cut a slot with a hack saw to use a screw driver on it. Another has a hidden bolt under the wheel arch. The one on my car required me to remove the suspension strut that passes across the top of the suspension spring housing. Seven bolts removed the strut to provide access, three of which were under a large plastic cover. Next, there was a riveted security plate protecting access to the top of the ECU connector. This had to be cut away with a hacksaw blade to allow the ECU connector to be removed and to access the 10 mm bolts that attach the ECU cradle to the bulkhead. The whole job took an hour after figuring out what needed to be done. The figuring out took two days, which is why I have written this wiki.

Don't forget you will need a new coil pack because it was failure of the coil that damaged the ECU in the first place.

Finally, I have prepared a Youtube video showing how to remove the ECU:

http://www.youtube.com/watch?v=E7NwZpAf2qU

If you want to thank me, then please thank ronniescot on this TalkFord thread:

http://www.talkford.com/topic/163204-is-my-ecu-gone/

Afterthought

I considered buying a second hand ECU together with the transponder and keys off ebay. However, I decided against this because you have to match your engine and the computer exactly. Any manufacturing difference, such as injectors or sensors, and the two will not gel and you might even damage your engine. Unless you know exactly what you are doing you might have to pay for reprogramming, in which case your costs will have exceeded by far the cost of reconditioning. In the worst case scenario it might not be possible to reprogram the second hand unit.

Wikis are supposed to evolve and improve as other users edit them. However, please do not delete this page and replace it with your own. If you want to make your own page, why not create a new one so that users have two sources of information.