Voltage drop

Has anyone experienced or got ideas to the following.
Oka 307 has a manual switch connecting the 2 batteries. It is earth switched with approx. 16 sq mm cable directly to each battery. I have 2 volt meters one connected directly to aux battery and the other connected to original Oka wiring.
Upon starting engine with both batteries connected , both voltmeters read 13.9 volts. After about 5 minutes the direct connected voltmeter reads say 13.8 volts but the Oka voltmeter reads 12.8. With high beam on voltage drops to about 13.5 and 12.5 respectively. If I stop the engine and restart to drive again they both start with the same voltage 13.9 and the Oka voltmeter dropping slowly back to 1 volt less again.
Voltmeters have been changed ruling that possibility out.
I will eventually directly hookup the dropping voltmeter to the main battery.
Peter

Peter
Assuming that both gauges are accurate, simple physics dictates that both batteries should read the same. The variable here is time i.e. Voltage changes over time which to me indicates a build up of resistance due to heat.
Check the resistance of your cables before and after running the engine. If you are pulling amps the it could be heat due to current flow or otherwise cables/connections near a heat source.
Pete

Peter, I'd agree with Pete F, resistance somewhere. I'd check the connections to the ignition and column switches as all those harness plugs have caused me grief at some stage.
If you are lucky it could just be a crook connection to the actual voltmeter.
Then check the ground points under the dash and the box behind the passengers seat.
The battery switches that Oka uses are also a known problem with age as the contacts become high resistance.
Your truck being ex police probably has plenty of non standard wiring just like ex RAAF 374 so there are plenty of extra wiring bits that don't appear on wiring diagrams ))

Peter D, it could be that good old white connector, if you have one, under the instrument panel. See this thread and this one .



After a few minutes with the headlights on, check the temperature of any connectors under the instrument panel as this connector passes all the normal operating current for the Oka and if it's high resistance/burning out (which mine was), you'll lose voltage and it will get hot. (1volt drop while carrying 10+ amps to headlights (2 x 55 Watt globes) = 10+ watts of power/heat in the connector, so be careful).

OKAs can certainly do strange things with electrics!
I agree that a high resistance joint (the connector is prime suspect) is the most likely.
However there are a couple of other things which can give strange voltage readings.
I have a house battery connected with a 'smart' solenoid, I had repeated problems with the battery not charging although the solenoid always tested OK (cut in / cut out voltage etc).
It turned out that under some conditions, there was insufficient voltage from the alternator to trigger the smart solenoid. However it had a (almost) new regulator which should have been within spec. The similar symptom to Peter's was that the main battery voltage slowly dropped after initial startup.
It turned out to be the temperature compensation built into the regulator - which is in the usual spot on the back of the alternator. However the alternator is close to the turbo so gets quite hot, especially after a stop. As it heated up, the voltage dropped. The cure for this was a diode (0.6 volt drop) in the alternator "sense" lead which bumped the voltage up enough to make everything work properly. I also suspect the regulator may have been on the low side of spec.
And another strange one - I have dual digital voltmeters (installed when I was tracking down the above problem) on the main and house batteries. As noted above these are linked via a smart solenoid so are nominally the same voltage when running. However, like Peter, the gauges said different! The gauges were checked OK, it turned out to be voltage drop between the main cab earth point (on the RHS firewall) back to the chassis. A solid connection here reduced the problem to a neglible difference.

Good luck!
John

Thankyou for the feedback. Tried to get on last night but oka site wasn't loading.
From memory the "oka" voltmeter is connected to a pink and black wire. I have tried another pink and black wires, but same result. I suspect an earth connection. The head and driving lights have separate relays and wiring from the original oka.