940 High beams on = car shuts off

Voltage drop test your alternator positive and ground circuits. Results?

Voltage drop test you battery positive and negative circuits. Results?

This sure is a persistent issue. Is this the usual 100 amp alternator on the car? I'm wondering if there is a problem with the alternator when it gets hot during use? Possibly a diode opening when hot or a bad solder connection on the diodes?

Have you confirmed alternator output? Most auto parts stores will test for free (they want to sell you one after all).

Does turning on any other high load accessories (defroster, heated seats) change rpm or create similar symptoms?

ZVOLV, I know I need to follow up on the voltage drop tests, but I'm not 100% confident in doing it.
I'm starting to think it might make sense to look for someone who is more knowing and can do this for me or help me but being literally the only one around my friends who works on their own car doesn't make it easy. I might have to look for a shop.

I have the Denso 100A. It puts out around 13,50V (or something around that number) when running.
I changed the brushes recently and the regulator / diodes looked fine, visually.

Turning on other accessories affect RPM slightly with short dip, but not too much or up to the point that the engine stalls. Only the high beams do that.

You need to load test the alternator (ie test for current) not voltage test it. That's a far more reliable test. Or, check voltage while you turn on all the accessories; defroster, high beams, blower motor on full, turn on everything. If voltage drops too low then you know you have a charging problem.

If you load the car up with a bunch of things running like suggested above. The voltage should not fall below about 13.6v. If you load test the alternator at a shop. The total current output should be within 10% of claimed output.

Most auto parts stores will do a load test in the parking lot, at least they do in the US. Easy enough to put a clamp on the battery cable at the alt on these cars so no reason they shouldn't.

Thanks guys. So this could definitely be an alt issue?

Visually the Denso 100A looks like it has been replaced not too long ago (before I bought it though) and is not the original one my car came with I believe, as I have no AC or any other extra electrical equipment (like power windows) and it should be a Bosch 80A according to the parts catalogue.

Anyway, I'll have a look at it again. I'll measure the voltage with high load. Can I somehow test myself if the regulator could be bad?

Unfortunately we don't really have "proper" auto stores here in Austria where I could do a load test, only one bigger chain and they have abysmal service. For that I probably need to go to a proper shop.

A car that dies under high load could definitely have a charging issue. That could also be caused by a high resistance in the circuit but the first thing I would check is charging.

The stores I'm talking about here are the big chains, they sometimes have tool rental service or battery/starter/alternator tests.

All you need to do a basic test is to turn on everything you can and measure the voltage on the battery posts while the car idles. If it' is below 13.6vdc at idle with high beams, wipers, stereo, heater fan and anything else you can think of. Then either there is too much of a voltage drop on the wiring as Zvolv suggests or the alternator is marginal. When you rev it up and if it goes up to say 14v you should be ok but just know you are marginal at idle.

This is why I prefer the 100amp versions on efi cars because they are a bit more stable at idle. My kjet turbo only has a 55amp stock and I upgraded that to a 80amp denso which made that car more happy.

This is a classic euro car thing even with Volvos. They used to tell you it was ok if the red charging light came on at idle under high load. As long as it went out when you revved up and started driving. A beelte, a 122S and many other cars of those days had just enough generator/alternator power to run things.

I had something interesting happen yesterday.

I noticed when I started the car and drove off that when the quick self diag test of the ABS module kicked in (which is at I believe 5kmh), the power was gone for a split second (radio turned off an on again for example), much like what happens with the high beams. Also the ABS light briefly lit up during that.
Later I drove on a gravel road and braked hard to activate the ABS and guess what, the car sputtered and shut off.

So back to the ground point on the inner fender, as the ABS module connects to the same point.
I wiggled the 3 ground cables that connect to the plate while having the high beams on and they were flickering a bit, so it had to be still an issue there.

I removed the plastic connector on each ground cable and cleaned the connectors, they were all a bit oxidized with green patina.
I also thought they were a bit loose on the plate, so I pinched them a bit with pliers to have a stronger contact.
After that I couldn't get the car to stall, but I'm still not 100% confident about it. I'm still scared to switch the high beams on at night... I just hate intermittent issues like this.

The other thing is the alternator, I measured the voltage with pretty much everything turned on and I only measured 11,95V at the battery at idle...
It measures around 13,6V with the lights, seat heater and heater fan on though, which is pretty much everything I usually have on all the time.
So, I guess my alt is bad? Should I replace the regulator first before I throw a new alt at it, or is that not an issue that could be caused by that?

Thanks once again for all the replies in this thread, I'm pretty sure I would have lost track a while ago.
Still learning about all this, so excuse any noobish mistakes.

Good find on that weak ground location.

That low voltage could be just the regulator. I would try one of the higher set point regulators with the 14.6v setpoint and double check the brushes are in good shape and are making a good connection.

Yeah I really hope I got it now...

I replaced the brushes a few weeks ago, so they should be fine. Skandix only has this regulator: http://www.skandix.de/en/spare-parts/electrics/alternator/regulator-alternator-12-v/1040212/
They don't list any with higher set voltage like for the Bosch alts and I can't find a regulator anywhere else, so I guess I'll go with this one.

Here is what the ones I bought on ebay have on them.
transpo-cn
IN443-1768-12v

The IN443 is the main part of the number for the high set point denso regulator. I usually buy them on ebay. Give that a search if you haven't already bought one.

Just wanted to update and finish the thread, the high beams are working fine ever since I cleaned the corroded contacts.

I didn't order the regulator because the car is running and loading fine and the high beams work without issues.

The next issue I need to tackle is the pulsating idle dip.

Well here I am again having the same issues almost a year later.
Exactly the same things are happening, brief cutout during the ABS self-diag test and the car stalls again when switching on the high beams.

I cleaned the ground point on the inner LH fender again last weekend and all is looking well there (considering the small ground plate is 1 year old and all the connectors are clean and dry).

As I now mostly drive when it's dark on country roads, I really need my high beams to work and I don't have much time at the moment to work a lot on my car.
So stupid question but am I OK to wire the 3 grounds going to the inner fender directly to the battery negative as a temporary fix and see if it helps?
Until I can do some more diag work of course.

It almost seems like your system could use a large capacitor to help buffer the load on the system when you turn on the high beams. Like when you turn on an electric fan. That initial current rush is too much for your alternator to handle. Yes, it wouldl be fine to wire the grounds to the battery for troubleshooting. Just be careful as high beams do draw a lot of current.

Thank you!

I wired an extra ground strap I had in my parts bin directly from the fender ground connection to the battery negative as a temporary fix.
It seems to be working well for now, couldn't get the car to stall.