940 High beams on = car shuts off

I voltage drop tested again today between battery negative and the driver's side ground plate on the inner fender where the ground of the lamp housing connects to.

When I switched the high beams on, the car again shut off and I saw over 5v there! Definitely not good.

I then inspected the bulb. It had no obvious flaws and the little braided cable looked fine as well (H3 bulb), but I changed it anyway for a new one.
Started the car, switched high beams on and wiggled all cables and connector that go to the lamp housing. Car kept running this time.

I might disassemble the lamp housing at a later point just to be sure there is nothing wrong with the wiring inside.

Good find! That sure was a sneaky problem.

Weird how it coincided with the removal and refitting of the valve cover ground strap though, so I really hope the bulb was the issue.

5v drop is HUGE. 250 millivolts is about all you wanna see.

Yeah this can only mean I had a short, right? Really hope it was indeed the bulb itself.

Fa182 said:

Yeah this can only mean I had a short, right? Really hope it was indeed the bulb itself.

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5V delta from "ground to ground" means you have a broken ground connection OR at minimum an extremely resistive ground connection. As the TECH said, the delta should be very very small when grounding thru copper cables and steel. I'd disassemble and clean all ground connections where you took those readings.

Those grounding connectors on the 7/9 cars are prone to corrosion. They are usually screwed to the fender using serrated washers on the fastener.. Cleaning that up shoud help a lot.

Fa182 said:

Yeah this can only mean I had a short, right? Really hope it was indeed the bulb itself.

Click to expand...

It means you have excessive resistance in the ground circuit. That 5v "chose" to go thru your meter instead of fighting thru a bad ground.

I already cleaned those plates, but I ordered some new ones anyway, they were cheap enough on Skandix.
I noticed there are two on each side stacked on each other, but I only have 3 (or maybe 4 on RH, can't remember) ground connections there per side, so I'll only put one of those plates per side, I'm sure it will help to have a proper and good ground connection.



I only know some of the very basic electrics, so I'm still getting to grips on such electrical gremlin stuff.
So, thank you guys so much for the help. I really appreciate it and that's why I love TB!

My theory is you have a bad ground and the current is backfeeding thru a different circuit and causing the stall.

Did you do a voltage drop on the battery ground cable? Put one probe on the battery negative and the other to the engine. Disable the ignition coil and crank the engine. Approx 250 millivolt max should be seen.

I did the drop test from battery negative to engine while running and it was somewhere between 200-250mV if I remember correctly, but I didn't to the test while cranking with the coil umplugged.

Will do this and report back.

So I finally did a voltage drop test from battery negative to engine with the coil unplugged and it read about 0,65V, which tells me it's probably too much than what it should be.

I did clean all grounds that I could access and installed the new ground plates on the inner fenders before I did the test. Not sure where to look next to be honest.

The high beams seem to work now though, I've engaged them quite a few times now driving in the dark and there were no issues so far.

When I used to have an A1 76 Golf aka Rabbit over here. It had issues like that. I ended up adding another ground cable from the engine to the body. The car had guages added and an electric fan. When the fan would run the gauges would go crazy. The extra ground cable sorted that out.

Good idea, I have some extra ground cables that I could add and see what happens.

650 millivolts is too high. Under 100 millivolts is ideal. And your 5000 millivolt drop on the chassis ground is very bad.

I recommend a new battery ground cable.

You should also do the positive circuit. And voltage drop the alternator ground circuit if you havent.

I wasn't able to replicate the 5V drop on the chassis ground ever since.
It seems to be okay now but I still notice a dip in revs when I switch on the high beams during idle, despite voltage drop of only 20-25mV.
Not sure if this is how it's supposed to be or not when putting load on the loading circuit.

The alt circuit is fine when I tested it and showed around 20mV.

Any clues where I can get a new battery ground cable (for 97 945T)?
I have the one with the 90? connector and am not able to find anyone that carries them. Neither do I find the 3523986 cable that the parts catalogue specifies.
Probably all NLA...

When I looked around for the positive cable for my 93 245. It is the one with the extra fuse box on the positive. The fuse box was nla but the cable was available for $400 list or some crazy number. Probably better off buying a high end set of terminals and using those to make your own better larger cable. To get fine strand cable for that application you can buy welding cable.

Mine drops for a milli sec when I power up the 100w low beams so what you describe now seems normal.

Thanks, good to hear the drop is not only on my car.

After looking through the catalogue again, I think I got the part no. wrong.
I need a 9148323 cable, but the only one I find is the 9148322, which is the one for the NA engines with the battery on the passenger side. Not sure what the difference is, but I guess it's the length of the cables. Well..

In the volvo parts webstore they show that number supersceded twice to a newer discontinued part number. 9148323 to 9130254 to 3523989.

Alright so the issue came back today on my way back home from work.

I switched the high beams on and the car lost all power for like half a second (the radio switched off an back on again), but it kept running.
The remaining drive home, I did not switch them back on, but I noticed that whenever an additional load was put on the electrical circuit, my headlights dimmed a little bit.
So for example when I stepped on the brakes and the brake lights came on, the headlights noticeably dimmed a little and back as soon as I let off the brake. Same was whenever I had the turn signals on.
Before I was home, I stopped in a quiet spot and turned the high beams on again and the engine wanted to stall as long as I had them on. I switched them off immediately and the engine came back to the usual idle.

What's weird is that after I switched off the car and started it back up, the high beams worked fine and I could not recreate the issue until I was home.

I'm really lost right now what to do next.

Could some electrical component or something in my car cause an intermittent issue like this?
Should I open up the headlight assemblies and inspect the internal wiring there? The only time I had one side open is when I replaced the broken lens in 2017, so long before the issue started.
It must have to do something with the headlights??

What I've done so far is:
- replaced the ground strap from valve cover to firewall
- replaced the ground strap from intake manifold / injectors to firewall
- replaced the alt ground strap to engine
- replaced the ground plates on inner fender
- replaced the high beam bulbs
- cleaned the ground point on the frame rail where the negative cable from the battery goes to.
- cleaned the ground point on the engine where the negative cable from the battery goes to.
- tried switching the low and high beam relays (they're the same)
- inspected and replaced the fuses (even though they were not broken)