I may add another information:
I took off the Battery cable and tried to measure which voltage arrives at the battery while the engine is running. There are 0v at the battery cable while the engine is running. This sounds verry wrong to me.
I also measured the voltage again at the alternator plug on the picture above, with the battery cable off, and there are also 0v.
When i put the battery cable back on, there are 7v to 32v again.
I repeated the process 4 times just to make Sure I didn't confuse anything, but results stayed the same.
How is this possible?
I ran out of Explorations...

 

Do you have any 12 volt accessories wired across one battery?
Going by the voltages you posted the alternator is charging fine. One battery being drained by a 12 volt accessories could explain the higher voltage on the second battery.
Not sure on this 7-32 volt reading you are seeing. Can you post a photo showing where you multimeter is connected (when posting photos click on full image. Makes it easier to see the picture on phones). Even with that plug disconnected ( alternator sense wire) I think the alternator would struggle to get to 32 volts.

 

The diesel solenoid is wired only to the left battery, dont know why the previous owner did this. Maybe it is a 12v solenoid and will break when 24v are applied?
I bought a clampmeter today and measured the amps right at the alternator outlet and even 6nder high load and higher rpm it wont reach above 8Amps. Seems a bit less to me, as it normaly should be somewhere about 60A right?
The 7v-32v voltages on the alternator were measured at the outlet plug, see picture below.

Considering the strange voltages and the low Amps I suppose the voltage regulator is broken.
Does anyone know, if it is integrated in the alternator or is it external?

 

The plug in your photo is not the outlet plug. Those two wires are for the warning light in the dash and where the alternator measures the battery voltage. The outlet wire is the large white wire, bottom right in your photo.
With that plug disconnected the alternator will overcharge . So it’s possible to reach 30 odd volts.
I’m not sure what rating those alternators were but 60 amps at 24 volts seems high. Possibly only a 25-30amp alternator. It should be stamped on the alternator. Regulator is internal.

 

The white cable in the loser right of the picture i send is in fact connected to this exact plug on the picture. Isn't the voltage measuring whire the thin white/Red one?

 

To sidestep a plethora of these problems leave all existing wiring intact and consider running a oversized amperage fused cable from alt directly to battery terminals. Made my alt so much happier at least.

 

I gave it a try, nothing changed. At least I know now that the wiring is not the issue.

 

Sounds good enough to give it a try. Where do you suggest to take the Power from?
Directly from the "Battery (+)" marked pole on the picture below?

 

Second try for picture:

 

I had similar battery discharging issues to you many years ago. I was drawing too much power from the factory 35A alt. I replaced it with an 85A one.
Is there a battery isolator/split charger before the 2nd battery. If so and its a single diode type that explains the overall lower voltage due to voltage drop over diode.
I also replaced the 2nd batt feed with an ign switched 100A relay and an isolator switch that allowed me to put both batts in parallel for emergency starting and winching thru an old winch solenoid.
I also replaced my isolator with a dual diode one and put the red/white wire on the battery output side so it kept charging voltage higher without drop.
Not all details above are 100% as I cant remember exactly what I did at the time but it worked and was simple...

 

There is no isolator/Split charger between the Batteries. Seems like a simple Wire all the way.
I will get the alternator Rebuild tomorrow, hope that will solve my issues.

 

Batteries should be replaced as a pair, ideally same make and with very close serial numbers, otherwise the internal resistance characteritics of the old one will drag down the new one.

Pretty simple 3-diode bridge rectifier in these, but that doesn't soud like your issue. Sounds more like the field-sense isn't getting correct voltage.

Is the ~24V measured between the two battery posts, the same as measured at the back of the altenator?

 

The original ones that i startet my journey with were bought on the same day in the same store, same Model and everything.

 

As another possible diagnostic, if you swap the batteries, left for right, do the voltages move with the batteries, or stay on left/right side of vehicle?

 

Voltages stick to the batterys

 

I've also heard some vehicles that had split 12/24 systems, not familiar enough with these to comment, check a tail-light bulb is it 24V?

On a true 24 you should have a 24-12V buck converter in the passenger side kick panel, which feeds most of the dashboard.

 

I've also heard some vehicles that had split 12/24 systems, not familiar enough with these to comment, check a tail-light bulb is it 24V?

On a true 24 you should have a 24-12V buck converter in the passenger side kick panel, which feeds most of the dashboard.

Patrols were either 12V, or 24V with the converter for the dash. Landcruisers had the 24V start/12V run system on some models; not sure what other makes may or may not have had.

 

I took out the alternator and brought it to a repair Shop in Sacramento. The guy over there testet it and said that the stator has a short circuit and is corroded.
He will try to rebuild it and ill test it again.
Hope that this solves the issue.

 

Short in the stator will cause low output from the alternator. It doesn’t explain the different voltages across the batteries. You need to remove the 12 volt feed from the battery. Either purchase a 24 volt solenoid or a 24-12 reducer

 

Either purchase a 24 volt solenoid or a 24-12 reducer

I dont't think a fuel solenoid could draw enough power to cause a couple volts difference?, especially since it would only be powered while engine running/ charging.


Depending results of alternator rebuild, probably time to clean and grease all connections, terminals, grounds, and test cable runs for continuity and shorts.