Voltage drop over coolant temp guage terminals

Hi guys, can anyone help me get my head around this one regarding my coolant gauge. 1989 90, straight diesel to 300 TDI conversion, you know the one It's got the right sender though.

I've got 14.5v on the green cable supplying power to gauges with engine running? So the green terminal has that supply. The coolant gauge terminal that sends to the Green/Blue Cable is at around 5.8-6 volts, with/without the sender wire connected to the terminal. I've read that the temp sender should receive a regulated 10v from that terminal?

So my guage input voltage is 14.5 and the output to sender terminal at 5.8v. Used multiple grounds and it's consistent. The sender connector was 5.8v too when measured in the engine bay through the engine loom. Is the gauge knackered? Am I an idiot Why would the output drop so much across the gauge hot or cold?

Doesn't the sender work through earth?

Sender is resistance to earth as mentioned. 14.5v doesn't sound right though, i believe this should be a regulated voltage. Maybe the wrong feed to the gauges.

Theres no voltage regulation in the circuit. It's a 1988. It does seem consistent with temp readings of the head. Pulls and runs great for miles.

Even with the sender disconnected I'm only getting 5.8 at the spade the sender connects to. But 14.5 on the other spade (green guage feed). The measurements are earthing through the multimeter so engine having a bad earth is out of the question I think.

I'll switch to capillary and see how she goes. I usually worry about nothing. Thanks for your replies guys, I appreciate it a lot.

So after some fiddling today at work it was the ground for the dash illumination stealing 8 volts from the sender spade somehow, note that the green supply spade was still recieving alternator voltage. With that illumination ground disconnected on the coolant gauge the sender receives the full alternator voltage and shoots up to the max position of the gauge when cold. It's wired on a separate cable directly to the sender so at this point I just put it all back together and drove home.

Will just strip it all and go over the loom next weekend. Making sure the feed is ok and seeing what is going on with the grounds.

Keep seeing them new Jimnys and am getting more and more tempted White ones look bad ass.