VDO 120c Temp Sender ?

Hello
I am struggling to get my new VDO 120 degree celcius temp gauge to work correctly . The guage is wired correctly but not reading correct temp ? I am on my 4th different sender unit and my last attempt was to tap out the brass plug on thermostat housing to accommodate a 5/8 unf PRC2505.1 sender but even that does not register ? I understand VDO use specific senders for their individual guages? But I haven't a clue which one I need ?

RicardoRich said:

Hello
I am struggling to get my new VDO 120 degree celcius temp gauge to work correctly . The guage is wired correctly but not reading correct temp ? I am on my 4th different sender unit and my last attempt was to tap out the brass plug on thermostat housing to accommodate a 5/8 unf PRC2505.1 sender but even that does not register ? I understand VDO use specific senders for their individual guages? But I haven't a clue which one I need ?

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I presume you've been on their website to check the model gauge you have and what sender it needs?
You're not using 'plumbers' sealing tape on the threads by any chance?

Hi
Thanks for reply
I did look on the VDO website but they list so many 120 degree celcius senders it made me even more confused , not hard these days .
I understand the tapered thread on the body needs to earth , so no ptfe tape used.
I know the guage works as it shoots up when sender wire earthed.
Just frustrated that after trying loads different senders achieved nothing.
Maybe I will refit the original for now until I find someone that's done a successful guage/sender conversion.

RicardoRich said:

Hi
Thanks for reply
I did look on the VDO website but they list so many 120 degree celcius senders it made me even more confused , not hard these days .
I understand the tapered thread on the body needs to earth , so no ptfe tape used.
I know the guage works as it shoots up when sender wire earthed.
Just frustrated that after trying loads different senders achieved nothing.
Maybe I will refit the original for now until I find someone that's done a successful guage/sender conversion.

Click to expand...

Is the sender the correct length to reach the water floe stream? How long have you been running the engine for?
Were you earthing the sender wire just as a test?

Yes , just earthed it to test guage.
The sender is same as one PRC 2505.1
As fitted by a chap who wrote a web article about fitting VDO water temp guage. Somehow he managed to fit the larger 5/8 UNF threaded sender into a adapter nut in the standard place of the original .
I looked in Steve Parker's website and he sells a modified brass top plug in thermostat housing threaded to take new sender, I just drilled and tapped mine the same,
I also submerged the new sender in boiling water it moved the guage just under 40 degrees. ?

RicardoRich said:

Yes , just earthed it to test guage.
The sender is same as one PRC 2505.1
As fitted by a chap who wrote a web article about fitting VDO water temp guage. Somehow he managed to fit the larger 5/8 UNF threaded sender into a adapter nut in the standard place of the original .
I looked in Steve Parker's website and he sells a modified brass top plug in thermostat housing threaded to take new sender, I just drilled and tapped mine the same,
I also submerged the new sender in boiling water it moved the guage just under 40 degrees. ?

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Bugger, I was going to suggest you tried setting it up off the car using boiling water.
It must be an earthing problem from your gauge then? Have you tested that earth with a DMM?

I did fit a new earth wire on guage , but please explain the earth test ? I have a fluke DMM so interested in this check.

RicardoRich said:

I did fit a new earth wire on guage , but please explain the earth test ? I have a fluke DMM so interested in this check.

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Was thinking if you set the sender and gauge up with boiling water and it worked. You must have connected the earth terminal on the back of the gauge to the body of the sender. So, a very easy route for the signal to use. When you are fitting it in to your Landy the body and engine block fit in that part of the circuit. ie between the earth terminal on the gauge and where the sender grounds on the block. If it's not working when in the car, then that is probably where the problem lies.
Disconnect the live supply to your gauge and just leave the earth cable from gauge to wherever you have connected it to the body. If you find another bare bit of metal on the body somewhere and connect a DMM from there to the earth terminal on the gauge, you should get clear continuity. Because the DMM signal should travel down your gauge earth wire, through the body to where your DMM is connected. If you don't get clear continuity then the earth is the problem in your circuit.

I can't find sender PRC 2505.1 in the VDO catalogue.

Hi
Earth continuity is good.
Yes I couldn't find PRC2505.1 either ?
I guess the VDO guage idea causes more problems than it's worth .
Think will switch back to standard landrover parts and just be careful

RicardoRich said:

Hi
Earth continuity is good.
Yes I couldn't find PRC2505.1 either ?
I guess the VDO guage idea causes more problems than it's worth .
Think will switch back to standard landrover parts and just be careful

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Ok. Just to be sure, the earth wire was the only wire you had connect to the gauge?
TBH my knowledge is limited. @Kwakerman might be able to shed more light on it.

No
There are 5 wires to the guage . All wired correctly as per VDO wiring diagram .
My problem is finding the correct sender (temp sensor , thermocouple,wax plug etc)
That works with a 120 degree celcius VDO guage.
VDO guages are matched to senders apparently .

RicardoRich said:

No
There are 5 wires to the guage . All wired correctly as per VDO wiring diagram .
My problem is finding the correct sender (temp sensor , thermocouple,wax plug etc)
That works with a 120 degree celcius VDO guage.
VDO guages are matched to senders apparently .

Click to expand...

Then the earth test wouldn't necessarily work. I thought you had this gauge and sender unit working off the car using a bowl of boiling water?