1998 V70 What's killing my ECU's?

kjhiggins · Post by **kjhiggins** » 10 Jul 2014, 23:59

I have been having a no-spark problem with my '98 V70 (287k miles) - see related threads 1 and 2 - and it's puzzling even trained Volvo mechanics to the tune of $360 and still the same problem. It's died three times in the last two weeks, the latest happened today, 6.2 miles after I left the shop. I called the shop and they said 'there's nothing more we can do'!

All the usual sensors have checked out okay and the only way it's started after dying is to have a new ECU put in, in which case it runs a few miles and dies again. The shop replaced the B+ cable to the fuse box because it was corroded and put in a loaner ECU and it started but it wouldn't start with the other two ECU's I had been using the first two times it died. So it seems whatever is happening is killing the ECU, or at least killing the circuit that controls the spark. I can still connect with it using my code reader and it doesn't indicate anything is wrong so I don't think the ECU is completely dead, just the spark circuit.

I read in another post here that the coolant temp sensor, the throttle position sensor, and the cam sensor share the same ground in the ECU and in that case a short in one of the sensors was causing too much current to flow through pin 18 and burning out the pin. I've opened up my ECU and there is continuity from pin 18 in through the nylon connector.

Any ideas on what is killing my ECUs? Is there a way to fix the ECU's? Any and all help is appreciated.

Kelly

Post by **abscate** » 11 Jul 2014, 03:17

I would do a comprehensive list, and then systematic removal, clean to shiny metal, replace with new hardware, of every single ground wire on the engine.

Post by **Ozark Lee** » 11 Jul 2014, 07:48

What codes are you setting?

The sensor ground circuit board trace would never burn out due to a shorted sensor but I suppose that it could burn out if the wiring harness had a short between one of the grounds and another device that has battery voltage, not supplied by the ECU, on it. The common wires to pin A-18 are all Brown/Black so you can split the conduits apart and inspect the wires. If you have EGR that also shares the same common pin.

As far as repairing the ECU goes, if the trace is burned you can typically scrape the coating off of the trace and solder a strand of a small gauge wire on the board to bridge the burned trace. It takes some soldering skill and you need to make sure you don't bridge any adjacent traces but it is definitely doable in most cases.

...Lee

kjhiggins · Post by **kjhiggins** » 11 Jul 2014, 08:39

There are no codes being set, which is part of the puzzling aspect of the problem. When it first happened there was a stored code for the ECT but the 'Check Engine' light was not on at the time. Previously, it had been on for a week or two and I knew it was the ECT but I didn't have a chance to replace it. The 'Check Engine' light turned off by itself so I thought perhaps the problem with the ECT had corrected itself.