V70 ETC / ECU / Harness Fault

Hi, my 1999 Volvo V70 T5 has a fault that seems to be fooling the main dealer. It started off when the ETS came up and the throttle wouldn't provide more than a tick over. Diagnostics revealed dead throttle block which was expensively replaced.

Almost immediately it developed a misfire which got worse with running. (no problem at all when cold). ETS and ECU both light up now. It is as if someone has turned the ignition off until revs come down to tickover speed. It ticks over very roughly. Best performance was found by keeping it at a steady 2000rpm. Diagnostics revealed throttle pedal defective, it was replaced, no difference. Now the dealer says it is a wiring loom fault which is 8.5 hours labour alone. The fault just doesn't seem to indicate defective wiring to me.

Any thoughts out there?

Colin N

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Colin N
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Colin,

I almost feel embarrased to reply but my '99 V70 T5 showed a series of weird problems one of which was the ETS staying on at start up. Sometimes it was cleared by re-starting but other times not and the running was rough if the engine ran up and down through the rev range.

Having read about the almost perpetual elictrical\electronic problems with Volvos I strongly suspected that from my career history in electronics that either the power rail voltages or the earth returns were allowing floating voltage levels. Before I had the chance to put a scope on the electrics, the battery did the decent thing and died. I put the best money (not a lot) could buy battery and that was the end of that and other problems.

If you wonder why a failing battery could cause all manner of problems then the logic I was going to apply is that all sensors and the ECM operate within strict volatge parameters and if a voltage is seen outside of parameters it is a fault which causes the ECM to make changes. If it's getting intermittent fault signals from all over the place and the ECM itself is fed by an out of parameter votage then nothing stands much of a chance.

When you hold the revs constant at whatever the alternator will hold the power rail at a steady voltage but when you allow the drop to tick-over the battery should be the lowest drop for voltage and if it's failing the voltage can drop below acceptable limits and the sensors give false readings.

My battery lasted 4 years and 2 months albeit I now realise that it was failing earlier. If yours is the original then it's in the failure window. Some batteries do the decent thing and die with an instant cell failure but others/most just become worse and worse until they fail to hold a charge over a reasonable period. In the latter case a daily user may not even know until the vehicle is left standing for a few days.

If it turns out to be your battery then the reluctance with which I post this may lead to justified anger which should be directed at the change everything in sight garage. At least it might save you a massive bill for re-wiring and the intense disappointment if that doesn't solve the problem.

Liam

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Noone

Liam

Thanks for the time you have put in to your reply. I avoided the rewire, it just wasn't the logical answer. It went to an auto elctrician with a good reputation in the trade and they discovered that when it played up cyliders 3,4 & 5 were not working. When they went to diagnose whether it was sparks or injectors that was causing the cylinders to fail, it stalled. Following a re-start it has run perfectly with no problems at all!

Colin N

Reply to
Colin N

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