Hi everyone. So this has been something that it has been doing forever, but I figure I should ask some questions to maybe figure it out. At Idle, especially at night, I can see the cluster and the head lights fluctuate in brightness fairly rapidly. While driving or revving, no problem. With my fairly nice fluke digital multimeter I measure between 13.5 and 14.2 volts on the battery, bouncing around between the two. My battery is an optima, never a problem starting. Always tests well over 12v with the car off. My voltage regulator is a year old (crapped out on me once). I have a few of those bosch alternators floating around. Would it be possible that the alternator is the cause?

Heres some more stuff...
The car has sort of a random miss at idle. Not terrible, but enough to be annoying. The misses seem to follow the dimming pattern slightly. So far I have installed new, correct plugs, wires, cap, rotor, injectors, and I cleaned that suppressor disk up. My fuel pressure is good and I've installed a new fuel filter. I replaced the EHA and to my crude understanding of how this system works, I have adjusted it correctly. It feels like at points, the alternator puts a significant load on the motor. The Ac compressor pretty much robs all of its low end power. My low end power really sucks, but I can pass someone going 70 like its nothing. My city mileage is terrible, but I get almost 27mpg on long drives. My exhaust in general smells very gassy. I also didn't pass NOX. The guy said I need a new cat, which I believe, but I think there is something else going on and I don't want to muck up a new cat.

I guess what I'm wondering is, is the alternator causing the engine problems, or the engine causing the voltage weirdness. I've run out of things to try that I know of.

By the way it is a '92 2.6. Thanks for any suggestions!

I've been meaning to get a DC ammeter to easily test the alternator. The auto parts store will do some kind of quick test. Not sure exactly what they do. A simple voltage drop test should tell you if you have a loose wiring/ground issue. Google "alternator voltage drop test" and the first link is a youtube video.

I think you're looking at a symptom, not a cause.

Alternators don't put out a lot of current at idle, so a jumpy idle causes fluctuations that you can see in the lights, etc. If it's bad enough in newer models you'll get voltage alerts and systems alerts in the cluster.

Once you fix the idle issues, I'm guessing the light issue will rectify itself (no pun intended).

If you wanted to do a quick test, just slip off the fan belt when it's cold and start it up (assuming your lumpy idle is there all the time). If it is the alternator, your idle will be glass-smooth, but I'm guessing it will be just the same.

And a partially-blocked cat will do exactly what you're describing, because the engine is running so slowly that occasionally the backpressure of exhaust will keep a cylinder from emptying on the exhaust stroke, so the next ignition stroke doesn't really ignite...misfire. That's usually followed by a rumbly kind of running, then it picks back up again. Pick up the rpms enough and the engine pressures are enough to get around it.

If you aren't in California, there are inexpensive aftermarket cats you can have welded in, Magnaflow makes a wide assortment and allexhaust.com has excellent prices and free shipping on them.

I'm not saying it is the cat, but it sounds like a good suspect.

Good luck.

Good luck.

I'm sure my cat is all worn out. Probably need a new one. Too bad I'm married. (wahka wahka)

I never really thought of it that way. I wonder if there is a good way of determining if that cat is the problem. Do you think if I took the oxygen sensor out it would let enough pressure out to smooth out the idle?

Test the O2 sensor and the OVP.

__________________

1990 190E 3.0L

Indeed OVP seems to be the most popular thing to check / replace on BW

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I just gutted the cat today. Figured there is no harm since I need to replace it anyway. Throttle response seems way better, but its hard to tell. Still idles weird and the lights still flicker.

My O2 sensor is ok. It makes no difference if its plugged or unplugged. I also replaced it recently. My OVP is also fine. Swapped it out with a known good one. No difference. Didn't get around to testing the alternator yet.

Just thought I'd check back in. I pulled the alternator off of the car and cleaned and blew it all out. The car holds a steady 13.8 with the headlights on now.