I'm currently unable to clear the codes from a Bendix/Rex/Regina 740 and am in need of advice regarding swapping the computers. I'm hoping that someone has either had this problem before or can offer suggestions on how to proceed, something that I may have missed, etc, otherwise I'm going to have to replace both computers. The car that I'm working on is a '92 740 wagon (automatic) and has a Siemens ECU part# S10159020A (Volvo # 6842882), and the ignition computer is a Bendix S101529102C (Volvo # 3507696).

I've never had the need to replace a computer in a Rex/Regina equipped Volvo, and thus am not familiar with the compatibility between ECUs and Ignition controllers of different part numbers. Does anybody know what changes took place and what units are compatible/replaceable with others? There are several of them on eBay and in the local salvage yards, and I'm wondering if it's possible to swap in a unit with different part numbers if I can't find a direct replacement.

Some quick background, a friend of mine bought a used '92 740 last year after I did a compression test, gave the car a decent checkup, and deemed it a good purchase. Everything was fine at that point (including the computers) except for an issue with the heater core which we're still working on. During the winter, he had the exhaust system replaced at a local Muffler Man and this is where the troubles begin.

After the exhaust system was replaced, his fuel mileage took a dive. -terrible at best. During our diagnostic work last week, we found some troubling issues with the new exhaust system that had been installed. The shop had completely cut away the old exhaust starting with the cat and everything behind it. This included the O2 sensor and we found the cut pigtail dragging below the car, near where the Cat should have been. They welded together a new exhaust system, doing the welding while the components were held in place under the car. They did not replace the O2 sensor, or even install a fitting for one to screw into.

After discovering the missing O2 sensor, I checked the trouble codes expecting to find an O2 sensor error, but instead found a 2-2-1 (rich/lean warning) stored in the ECU, and also 1-4-3 and 2-3-4 stored in the Ignition controller (knock sensor and throttle switch). When I tried to clear the codes, the systems would lock up and I could no longer read the codes or communicate with the computers. The results are the same for both the ignition and the fuel injection, -trying to clear the codes results in the unit going brain dead and completely unresponsive until the key is cycled to off and then back on. I have other clients with Rex/Regina cars and this is the first time I've ever experienced this problem. I already know for certain that the ECU isn't ok because the lack of an O2 sensor failed to trip a 2-1-2 code.

I called the exhaust shop and asked them if they'd disconnected the fuel injection computer and the ignition computer before doing the welding, and they said no. I asked them if they had read any of the warnings about doing welding work on a car with electronic systems (Volvo has tons of warnings about disconnecting the computers) and their reply was that they'd never had a problem in the past so it wasn't something that they were concerned about. They did get a little concerned when I told them that I was looking at a car that was missing a cat, missing the O2 sensor, the computers were semi-functional, and they were the last people to work on the vehicle.

Starting tomorrow, my friend needs to take this car on a round trip to Kentucky and back here to Michigan. We have a new/replacement O2 sensor on hand but do not wish to install it until we have good working computers installed (in order to avoid the risk of fouling a perfectly good/new O2 sensor). Any advice that can be offered would be greatly appreciated. I'm pretty sure that this car isn't the same "Fang" that Chrissij dealt with a few years ago, but I'm going to scrub the ground connections again (we did this at purchase time last year) before we put the new computers in, assuming we find a pair to install. However, when you compare the inductive spikes from a poor ground against an arc welder, I think the welder wins for the most damage done.

God bless all of you, and hopefully this car too,
Fitz Fitzgerald.