Volvo V70 Electrical Gremlin Help

OK, I have to admit to being totally flumoxed by this one.

The patient: 1998 Volvo V70 GLT

The symptoms: Rhythmic electrical semi-failure, usually when cold.

I've only seen it once, it does it to my wife fairly regularly. The headlights and dash lights go dim, the engine starts running rough. Then it clears and everything is fine. Then it all goes dim and rough, then clear. It will keep this cycle up on a roughly 1 second basis, with metronome like consistency. And after a bit, everything stays calm.

For the life of me, I can't figure out what would cause this to happen on such a smooth rhythmic basis. Like I said, I've only seen it happen once. It did it to me one morning while I was driving it. Started a minute after I started driving the car. No noise, no response to engine rpm, vehicle speed, load, etc. Went away on its own.

Fwiw, this is the same Volvo I brought up some time ago with the slow to start charging system. It takes the system a good 10-20 seconds to ramp its way up to charging.

I have tested the alternator (with built in regulator/rectifier), replaced the alternator, inspected and cleaned the grounds, replaced the battery. No change.

I can think of things that can a power drop, I can think of things that can cause sporadic and intermittent power drops. But not anything that can cause it to cycle with such metronome like rhythm.

Any of you folk have some notions of what can cause a metronome like rhythmic electrical power loss?

Wiper relay? Turn Signal Flasher? long shots, probably wrong, But I've owned an MGB with Lucas Electronics so nothing is out.

Chicken or egg? Is it losing power, then the idle speed drops, or is the idle speed dropping, causing the alternater output to drop? If it only exhibits the symptoms at idle, I'd be looking for a running issue, not a charging one. If it does the same thing at 2000rpm, charging may be the issue.

No Volvo experience, but modern BMWs sometimes have similar issues (though usually not as extreme), manifested by pulsing headlights. Mine started doing this under very specific conditions after replacing a failed alternator (with a Bosch rebuild): engine cold, ambient conditions damp or cold, high electrical load (rear defroster, seat heaters, etc. on). Adding electrical load makes it worse; once the engine runs for a few minutes, it gradually resolves itself. The problem persists across the RPM range. I checked the running voltage when this occurred and found it oscillating between the low 13s and high 14s, with a couple of spikes just touching 15.

I'm not entirely certain what the problem is, but I speculate that the VR may be slow to react under these particular ambient conditions.

Interesting points and good food for thought.

The one time it did it to me, I would say it was roughly at the rate of a turnsignal flasher. Interesting. I don't know where to go with that, but I'll look into that, and the wipers. And anything else I can think of that's on a flasher type circuit.

She describes it is going dim, and sometimes as dim and rough. Always that order. She never says rough and then dim. The one time it did it to me, as I recall the lights would dim and then the engine would roughen.

It's not simply at idle. I think it usually starts doing while running down the road. The one time it did it to me the car was idling, driving at about ~40 mph, in neutral and reved up. It kept it up for about a minute as I recall, and then either faded away or simply stopped, don't remember which.

I don't think I've had her cut the lights out when it acts up. I'm going to have to ask her to do that.

I've put a cigarette lighter volt meter in it to look at while driving around, and it's always struck me as showing a midly lazy alternator. But I am up the cigarette lighter circuit with that meter, so I've cut it some slack. A hand held meter across battery always looked fine voltage wise, but sluggish to start charging. But that's just sitting there with the hood up. I'm going to have to temporarily hard wire a meter or two, so she can have readings when it acts up and tell me what she sees. I figure one across the battery, and maybe a second across the alternator itself.

The car had a rebuilt alternator put in it about a year before I got it, so about three years ago. Don't know what the original problem was. I replaced it with a junkyard alternator that looked oem (not rebuilt). Both alternator units passed bench load testing and meter testing of the individual components. Installing the junkyard alternator made no difference in my voltage readings in the car, or the sluggishness to start charging. Problems with the dimness and roughness seemed to go away for a while, and have now come back. But I could be wrong on that. She may not have been noticing it (summer time, days are long and bright, so one wouldn't notice it as much) or not telling me about it. She is prone to tell me about a problem a few times, and then quit telling me about it.

You said you checked the grounds, but did you check the + line from the battery through the power junction box? Specificly the voltage drops between the battery + and the connections leading to the ignition electrics? If you had a loose conection anywhere in that series, you might get a temperature related intermitant open that might cycle your system voltage as it connects and reconnects with the charging load on the battery.

If the voltage drop produces heat, it can "fix" itself when things warm up. I know it sounds wierd, but I've seen this one before in cars with a loose nut in the power distribution box connected to the + lead of the battery. Put a VOM (volt ohm meter) accross the bolted connections. If you see any voltage reading on the 12 volt scale, you have a bad connection between the test leads. This can indicate where a loose conection is if you can't see the nut.

You might want to check the bolt torque on all of the + connections leading away from the battery, excluding the starter circuit.

Just a thought. Hope you find it.

I had the idle part of that problem for quite awhile on my 1995 855 t5r and could not figure it out. If the headlights were dimming I did not notice or think of it as a separate issue. It became an intermittent "the car stops running entirely issue" so i parked it and did a bunch of interwebs research and discovered that the fuel pump relay will cause the symptoms I was experiencing. I picked one up for $20 at the local yard with lots of volvos and that took care of it.