Electrics: Strange intermittent electrical problem

[Simon J]

My 2003 C70 T5 auto has a strange intermittent electrical fault (at least I think it's electrical). When stationary in traffic, or at low speeds, the car will occasionally seem to cut some of the electrics resulting in the radio going off, and three of the dashboard lights coming on briefly (brake circuit failure, ABS malfunction and STC disconnected). This is followed by a fairly violent ‘thump’ in transmission and the whole car lurches forward against the brakes. If I have the handbrake on the car seems to climb up backwards against the handbrake and then 'thumps' forward again. Then the radio comes back on an everything is normal again (until the next time).

It only seems to happen once the car is warmed up and it doesn't happen at anything other than idle or slow speed.

I posted about this before some years ago (where does the time go to?) but was never really able to get to the bottom of it. It seemed to go away but is now back with a vengeance and happens on pretty much every journey.

Any thoughts, anyone?

[byootox]

Do you also lose the blower motor, dash lights and indicators at the same time when it does this by any chance ?

[Simon J]

Good questions! Re the blower motor, I can't say that I've noticed as I leave the air con on automatic and I don't really notice the blower. As for the dash lights, this has really only started happening in the last few months and I haven't really been driving the car that much in the dark to notice but worth keeping an eye on. And I can't be sure about the indicators but again, I'll keep an eye on them as well.

Do you have something in mind?

[byootox]

Yep... if it's the same problem as what I had with mine.
Your symptoms seem very similar to the problem I had with mine.
Eventually... found that all the items going off including the radio etc were powered from position 1 (Auxiliary) circuit.
There is a relay in the under bonnet fuse box on the drivers side that powers damn near everything on this circuit.
Took me ages to trace it as it was intermittent and only really gave problems in the winter and would go away the rest of the year till it got worse.
If it is that, it's quick and easy to replace.
Check to see if you lose the blower motor, dash lights and indicators as well though.
Possibly even the wipers as well.
I didn't get the thump from mine as mine is a manual but it may well be the power to the auto transmission is being lost briefly allowing it to go into limp mode then back out again as well resulting in the thumping your getting.

[Simon J]

How very interesting! Thank you. I'll check this out as you suggest and keep an eye on the dash lights and the indicators. I don't hear the blower fan so no point in me trying to catch it.

I'll report back in a day or so (it happens all the time and I'll be out in the car tomorrow for sure!)

[Simon J]

As expected, the problem returned today when out in the car and for sure the air con didn't go off as the little green light stayed on. It was too sunny to tell if the dash lights were affected and it's over so quickly that there isn't time to check the indicators.

But I did call with my local Volvo specialist and his comment was 'there are about twenty relays - which one is it?'

It did happen once when I was putting the hood up and it caused it a bit of confusion as it temporarily stalled completely. Thankfully it completed the task and so far has been fine since. But I now avoid operating the hood with the engine running.

So I guess I need to peruse a wiring diagram and see if the brake electrics and the radio and the hood are on the same circuit.

[happy diver]

have you perused this

[Luxobarge]

As above - I'm sure I recall hearing about a known problem with the main battery feed into the fuse box going dodgy and causing some pretty random and weird electrical problems, well worth investigating in your case.

The other thing that comes to mind is that a common fault on these cars is wear in the electrical portion of the ignition switch, which can again cause a number of apparently random problems to occur. Only having the problem when running slow and warmed up doesn't really stack up with this, but may be worth bearing in mind. Try wiggling the ignition key around and back and forth slightly to see if you can induce the problem? If so, replacement of this part of the ignition switch is relatively simple and not too expensive I believe.

[Simon J]

Thank you both. The Matthews Volvo page gives the fuses but doesn't, as far as I can see, indicate which circuits might be with which relay.

I do sometimes find that the engine stalls immediately after starting and won't restart unless the key is withdrawn and turned over! That's possibly the antennae ring but I guess could also be related to wear in the switch.

I'll see if fiddling around with the key induces the problem

Thanks again - much appreciated.

[Luxobarge]

Originally Posted by Simon J

I do sometimes find that the engine stalls immediately after starting and won't restart unless the key is withdrawn and turned over! That's possibly the antennae ring but I guess could also be related to wear in the switch.

That definitely sounds like the antenna ring - well worth giving the contacts a thorough clean, a likely free fix apart from your time. Will be a separate fault from the one you posted though as this won't cause failure while running.