Temp gauge guru needed

So I am coming to the end of my mk1 caddy 1989 aaz project and the bit I thought was easy I'm getting stuck on

as soon as i switch the ignition on my temp gauge goes to 3/4 and then stays there. If i start the engine and get it up to temp the gauge goes to full and over heats. Warning flasher. It's like the gauge starts at 3/4. I have checked the sensors and bought a new one. So hopefully I can rule that out. It feels like a bad earth or something sending a signal to the gauge for it to read like that.

if anyone could help it would be greatly appreciated if i have missed some information out let me know
 

I have looked at the other posts on here about this subject and can't see anything that is like this

You mean the gauage goes into red but engine runs OK and not engine over heating?

What does it show if sender unplugged?

Have you checked the voltage coming from the 10V regulator?

When engine off it goes to 3/4. When engine on and started when it gets warm it goes into the red and warning light starts flashing. The fan comes on when it gets to hot. So engine is not over heating. If the sender is un plugged it sits at 3/4 on the gauge. I am starting to suspect the gauge but wanted to rule everything out first. Water pump changed. Had a go at bleeding the system so pretty sure there is no air pockets. The thing that's confusing is the 3/4 reading straight off before engine start

What readings would I get from which legs to check it
thanks for quick replys btw guys

When red and yellow wire shorted to ground gauge maxes out so gauge is fine.

the_whip said

Have you checked the voltage coming from the 10V regulator?

I think you got it there. I found some posts about the regulator so will check voltages tomorrow.  It threw me as the other gauges work fine.

look in the archive section or click on the link in my signature… there is a wealth of information on flakey water temp gauges, and how to fix, and diagnose them even with pictures.

Briano1234 said

look in the archive section or click on the link in my signature… there is a wealth of information on flakey water temp gauges, and how to fix, and diagnose them even with pictures.

Hi Briano
Yes I have seen them thanks very good guides   and I am hoping they will help me with this. I am going to test the voltage at the regulator and then look at the grounds. With it being at 3/4 level and I forgot to say oil light flashes but does go out after a bit so I discounted that as a problem. So yes it is pointing at the gauge or bad ground so going to look at them and hopefully it will sort my problem

A gauge that immediately goes to 3/4 full means to me that there is a possible grounding issue with the gauge, as the input is held in check, then by varying the ground the needle moves.

If you unplug your sender on the head what does the gauge do when you turn the car on?  

I would suspect the sender or the low/level control circuit before I suspect the gauge… If you unplug the sender, and it stays at 3/4 then I  would unplug the low/level controller and see what happens.

I mean the usual condition of the Gauge is that it sporadically works or it doesn't.  That is from usually cold solder joints on the wires that are off the studs where it draws power.

I know I can check the wiring on my 90's from the 2 prong connector (yours should have but one lead) using a 9V DC battery (ignition off) and I can power the gauge to go 3/4 to full scale and if the gauge is flakey then I will see it  go to low after about 2-3 minutes of battery….

So having it go 3/4 is like the sender or the low level controller is having issues.  I would check the input to the cluster pin 4 yellow/red to see if there is voltage there with the cluster powered prior to ripping the gauge apart…. as that would tell me that the Sender or the controller is iffy…..  Yes I have had senders go, and rarely have I seen the controller go iffy….but I suppose it could happen.

Ok thanks for the reply. I did a few tests. Fist checked voltage for the stabalizers and it was fine. Just about 10v only just though and 12v on the other. Cleaned all contact points on the gauge. I then took a direct ground from the cluster and it made no difference. So i then took a direct wire from the gauge replacing the red/yellow wire. The gauge then worked perfectly so the red/yellow wire must have been shorting to ground or another wire along the way. So i am going to just run a wire direct from the gauge to the sender. I was thinking it would be best to put an inline fuse on this wire as i assume it has one when going through the fuse panel? Maybe a 5-10amp fuse just to be safe

I have had a check of the wiring diagram and it is just a straight wire through so no point in fusing it. Thanks for your posts and briano thanks for your amazing guides vwry detailed and loads of help 👍