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

 

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

 

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