"It was quite humorous that the boss had carped about the cost of a gauge from Hyster so the cheapo gauge was purchased. When the boss was informed about the new gauge reading too high........he stupidly remarked "at least it's reading something now". The concept of why there are gauges clearly shot far over his head. "

so i guess it would be safe to say the guage is an aftermarket unisource guage?
i'd be willing to bet thats part of the problem
guages and senders now have to match certain specs to work together properly
if your mixing aftermarket and oem parts together this might not be such a good thing.

"replaced sender twice and gauge once".
Again we are faced with the possibility that the gauge in the instrument panel might not be correct.
All gauge system components, be they analog or digital, require some kind of "voltage regulating" component as part of the circuit in order to be able to display a CONSISTENT value over a range of voltage fluctuations (fluctuations which are normal in an automotive setting). Vehicles once had a separate "instrument panel constant voltage regulator" which distributed a regulated, "lower than battery" voltage supply to analog gauges in the instrument panel. The regulated voltage was usually at about 8VDC for 12 V systems and 4VDC for 6V systems. This was to assure that gauge readings would not fluctuate with normal drops (or peaks) in general system voltage.
Today's automotive world uses a similar system but to a much greater degree and the regulated voltage is often called "reference voltage" but the aim is the same.........keep the playing field level despite fluctuations of system voltage level.

Back in the day when ALL gauges were analog type, you could buy individual, "stand alone" gauges. These gauges were often available with their own built in voltage regulator circuit so that the gauge would operate properly when added to an application where no instrument panel regulator was present, or as a "work around" in cases where the OEM dash regulator was defective (and costly). Aftermarket gauges could also be obtained that DID NOT have a built in voltage regulator for use in applications where an instrument panel, constant voltage regulator was already in place.

I would wonder if perhaps somewhere along the way the gauge was replaced with with a non regulated dash gauge.
It might also the possible that if the Hyster was built having a constant voltage regulator for analog instruments......it has failed and now supplying a higher output voltage than it is supposed to resulting in the gauge showing a higher value. Typically when those regulators would fail it would be a "no output" failure but I supposed they could "short to full voltage".
I will admit that I am not familiar with that 1985 Hyster but many years ago we had a similar situation on an even older Hyster where the temperature gauge dash unit was replaced with a "looks like it" gauge from a local parts house and that gauge also displayed an overheat value at normal operating temperature.
It was quite humorous that the boss had carped about the cost of a gauge from Hyster so the cheapo gauge was purchased. When the boss was informed about the new gauge reading too high........he stupidly remarked "at least it's reading something now". The concept of why there are gauges clearly shot far over his head.

You could go to a local Napa store and get a gauge and sender unit. I'm quite sure they could also give you the resistance values you require (assuming you have Napa's across the pond). I still think a ground strap from the engine to the battery would be a good idea.

Hey guys thanks for your replies, they are all very much appreciated.

Following conversation with my engineer (and his insistance that he has eliminated the loom already), this morning we have rigged an independent set up to test the guage and the sender. We still don't know which component is incorrect but at 65 degrees C, the guage reads 7/8 toward the red and at 83 degrees reads well into the red. I understand the thermostat operates at 82 degrees.

In the UK the main Hyster dealer has been replaced by Briggs Equipment in the last two or so months and the organisation is currently in total dissaray (they don't even answer their phones anymore).

UK customers had for some time been directed by the former Hyster agent Barloworld to use TVH (formerly Intrupa in the USA). As you can imagine it was TVH who supplied the (well said swoop) "crappy" parts.

So back to my original question, does anyone know the resistance readings we should be getting through the sender....

where did you get replacement sender from?
dealer? or local parts store?
is it OEM or aftermarket?

could be crappy aftermarket junk

I would agree with 7777, I'd wire the gauge normally, through the wiring harness, then an additional bypass wire from the engine directly to the battery negative and check for any difference in the gauge reading.

Maybe you have a poor ground to the machine from battery. Have you by-passed all the wires to the gauge, and hooked up direct from bat (+), grd (-) and sender. I'll be very surprised if its a gauge or sender issue.

Hi 7777, thanks for your reply.

Correct, the truck is NOT overheating.
We have checked resistance / short circuit through all of the wires relating to the temperature guage system and have indeed already by-passed the trucks wiring harness like you suggested. The new guage does exactly the same as the old guage (so doubt very much it's the problem).

I was hoping someone from Hyster might see this on here and could tell me the correct resistance range of the sender (so we can test it properly on the truck) and the correct resistance across the guage (again so we can test it to eliminate the problem). Hyster in the UK (now Briggs) don't appear interested in the problem....

The truck is not over heating, right? The gauge may be grounding on the sender wire some how. Seperate all the wires going to the gauge, run temporary wires to the gauge. This will isolate the harness, and see what results you get. The gauge also may be no good.