I've poked around a little and can't see an obvious answer.

1990 740 GL Regina with the accessory configuration of a 740 turbo - alternator on top of the AC compressor.

Where is the other end of the alternator case ground supposed to go? I was doing troubleshooting on the No-start I have last night and bumped the ground wire I had made. Interesting thing happened... I had the key in KP2 and the fan set to defrost but the speed set to 0. Once the wire was disconnected, I heard the defrost fan start up. Wire connected = no defrost fan. Disconnected = defrost fan on.

I had a spare ground cable candidate that I pulled out of a recycling bin at work (shhh don't tell) and ended up putting it on the bracket that the IAC mounts onto on the bracket that connects to the engine manifold and to a bracket on the block near the engine mount. This is the same place it was mounted before, but I've never been entirely sure where it's supposed to end up.

Should it terminate at the engine block or the head?


Happy bricking
Will

One end of the blue ground cable off the alternator B- ground post normally attaches to the nearby top bolt of the strut brace for the intake manifold, but any connection to the block or head will do. Best to clean up the contact surfaces first. It's not required, but you can use something like a little De-Oxit paste if you've got it.

If I'm reading you correctly, that is indeed really weird why in KP-II having the alternator ground cable disconnected causes the fan to run and when reconnected to ground the fan is now off. Defrost mode also enables air conditioning and setting the fan on zero in defrost mode still has the fan on low, which will run with the igntion switch in KP-I or KP-II. The alternator ground is not involved in the fan circuit (the fan is attached to ground at the right A-pillar ground rail and the battery connected to the unibody frame member with one of the two blue cables). If it's as bizarre as you say, then something basic seems wrong. It's almost as if the alternator with its ground connected is somehow able to suck all the battery amps and leave not enough amps for the fan to run with the key on. How old and how well charged is the battery? What are the static volts across the battery terminals? It's almost like the alternator would have to be internally shorted to suck that much power if the battery is anywhere near normal. If so, I would expect arcs and sparks if you were to touch the ground cable to anything and cause this to happen.

At some point once you get it running half-normally, I would recommend checking the alternator output and the integrity of the ground and battery cabling by doing a voltage drop test to determine the amount of resistance in the alternator and battery cabling and connections. Switch off all unneccessary electrical loads (esp. DRL headlights, fan, window/mirror defrosters and heated seats). Measure well above idle, like up around 2,000 rpm (anything over 1,700). First measure the voltage directly on the back of the alternator between the B+ terminal (the heavy gauge red wire) and the B- ground post. You should be seeing a proper charging voltage, normally around 14-14.35V for a well charged battery with a healthy alternator regulator (no less than 13.75V for an older/partly charged battery). Now measure across the battery posts. Anything much more than about a 1/4 volt difference and your battery cabling and engine grounds should be attended to, cleaning up the connections.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Aha! Thank you!

that it happens to be exactly where I connected it, but I wasn't sure that was the place that it's actually supposed to go. I really appreciate you taking a look for me. I did happen to take a look at the voltage drop end-to-end on most of these cables and noticed a while ago that I had about a half a volt drop on this ground. it kind of makes me wonder if I have a bad connection somewhere else or like a poor connection between the engine block and or head to somewhere inside.

It didn't occur to me until the ground cable came off the alternator that the blower motor should always be running if it's set to the defrost position. I have done load testing with and without loads and it did well, but I bet it will do better with an intact ground cable!

I am hoping to replace the terminals on the positive post with new ones in the near future. I know 13.6-14.1V is the usual healthy charging voltage for cars, but I always thought that these 740s ran towards the low end of that like around 13.2-13.6. Is that not true?

Cheers,
Will

Your voltage may be running a bit low. The battery or the voltage regulator might be the problem in addition to cabling issues. From the Pocket Data book, measured at the back of the alt between battery B+ and the D- ground terminal, the Volvo spec at 2500 rpm for the later alts is 13.8V-14.6V on the car with a minimal battery load and 14.1V-14.9V on the bench with no load. More commonly you'd expect to see closer to 14.25V at the alt, a bit less with loads on or a charging battery, a bit less still if measured at the battery posts (cable/connection voltage drop). I usually want to see around 13.25V-14.25V measured at the battery at >1700 rpm with minimum loads and no less than 12.5V with all loads on and a battery in only average condition.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Hi Will,

Is this the same car that you suspected had water in the gas?
Did you abandon that thread? Any OBD codes?

Re the alternator ground
I looked in my 89 740 book and the case is Blue wire to an unknown ground.
In the 93 940 book it shows that case ground "D-" a Blue wire going
to ground 31/26 That's the good news.
The bad news is that they don't show where 31/26 is.

The battery ground goes to the front of the block just above the corner of the oil pan. That alternator ground should go to the block.

I'll look later if nobody chimes in.

Bill

Hey, Bill.

yes, it's the same car and I didn't abandon the thread I just didn't have time to reply to it today. I was at work and wrote this short post on my lunch because it was an odd thing that seems like it's potentially a rabbit hole. I'll be home soon and post a reply to the other thread.

Thanks for taking a look. The other reply confirms that I put it roughly where it's supposed to be put. The whole engine bay could use a cleaning heavy on degreaser, but I think these ground points are ok for the moment.

Cheers