356 Oil temp sender and gauge ramblings

[Kneeslider]

Hello!

After getting my 356 project running and then mobile, thoughts turn to making the irritating details better.

One such thing was that the oil temp gauge never worked, and since the engine is effectively air and oil cooled, then knowing what was going on behind was a good idea. I thought that it would be a straightforward investigation into a failed sender or somesuch... Hmmm, that's fatal, isn't it?

So, I started by going through the procedure on the Registry website about how to test the gauge and the sender, and found out that there are two systems. The original, and a modified system. This has come about because original Porsche senders are very expensive used, and non existant new. Basically, the Porsche OE system, with the power off, the pointer on the gauge shows fully to the right, i.e. hot, and on turning the key, drops fully left, i.e. cold. According the the Registry article, the original Porsche sender should have a reststance of 25 ohms or so at room temperature. The gague checked out OK because the pointer moved when switching on, and responded in the way you would expect when grounding the sensor wire in the engine bay.

So, to business. I pulled out the sensor, and checked it with the meter, and it was mysteriously much more than 25 ohms. More like 200 ohms, so the first thing to do would be to obtain a proper, and expensive OE sender. Ebay came up trumps, and I sourced one from the USA, which worked out at about £40 by the time shipping had been added in. I then waited a fortnight or so for it to turn up.

So, I was a bit miffed when I fitted it and it didn't work! There must have been a reason for someone to have replaced the sensor in the engine bay with a different one... I tested my newly purchased sender and it did indeed read the appropriate quantity of ohms at room temperature, so there must be a problem with the wiring or the gauge. Wiring is easy to sort, gauges less so.

I took the gauge out of the back of the combo instrument by unscrewing the 4 screws, and took the battery out of the car so that I had a 6v source to play with, and purchased for a few pence (Ebay again) a couple of variable resistors, and connected one in seriex with the gauge and battery so that I could wind the resistance up and down, to see what happened to the gauge needle.

To my surprise, the needle would only stay at fully cold or fully hot, and flick between the two, it would never rest at any point between. I tried to work out why this should be, expecting the mechanism of the galvanometer to be sticky, or have debris in it, but moving the needle from side to side showed this not to be the case. My best guess is that the wire wound coils in the meter must have had their insulation break down just enough to effect the resistance. Everything in there is very delicate, and there are no visible means of adjustment, so repairing the meter would be beyond my capability. A few calls to instrument specialists didn't get anywhere, and lots of them were less keen when I told them that it was a 6v instrument. Hmm.

An alternative would be to buy a gauge and sender from PRS, who list exactly the parts I need, but unfortunately, they want £220 or so, which is a little salty for a tight northerner, so it was time to break out some ingenuity...

[Kneeslider]

So, I bought one of thses for a whole £7 inc P&P from Ebay:-



It comes with its own sender, in a plastic case, with a soft aluminium bezel.

So all you have to do is to prize off the bezel, and undo the screws on the back, to release the mechanism. Then take off the needle pointer, and undo the screws holding the scale on.

Next, I did the same with the Porsche instrument to get the dial off, and was surprised to find that it had exactly the correct hold spacing to screw on the Porsche dial to the new meter parts.



The screws are the ones from the new gauge, but you get the idea. Its important to realise that while the new meter is notionally 12v, this doesn't matter that you are going to be running it on 6v because what the meter is doing is measuring the resistance of the sender, which stays the same whether you are powering the gauge with 6v or 12v or whatever (within reason).

The back plate that mounts the gauge into the combo gauge looks like this when you remove it.



I then marked the back plate up so that it can be drilled to take these pins.



But you have to be a little careful, because the new gauge has a plastic body, and it has to be bolted to a metal plate, and you don't want the pins to short out, or worse for 6v to ground out on the back of the combo gauge and fry the wiring. I drilled the centre pin hole at 4mm as this is the ground / earth, but drilled the other two holes at 5mm, and used fibre washers over the pins.

[Kneeslider]

The only remaining thing to do is to wire it all up, and see if it works, and drop the sender into a pan of boiling water. I stuck the old needle on to do this, and all was OK.

Another slight problem was that the Porsche needle has a hole in it which is too small for the spindle from the new instrument. I had to open out the hold in the needle to 0.5mm with a drill in a pin chuck, but this is properly fiddly, as it is very hard to hold the delicate little pressed aluminium needle without bending or snapping it.

Another pan of water was used to boil the new sender, and when the instrument had stopped moving, I placed the needle on, so that it pointed straight up. This is apparenly the correct position according to the wisdom of the internet.

I fitted the gauge back into the combo guage, and wired it all up, and it works when grounding the wire in the engine bay.

This is where I am at the moment, but there is one final problem, that the new sender is 10mm with a 1mm pitch thread. The oil distribution block it screws into is 14mm with a 1.5mm thread pitch. I'm going to have to do a spot of lathing to sort this out, and time has gone for this week, so work will re commence soon.



New sensor, with wire warpped around to test it in boiling water.



Old sensor with much bigger thread size.

[Hugo 356]

Nice work. If you want the original gauge repaired at some point try North Hollywood or Palo Alto - both get well written up in the 356 registry:

http://www.paspeedo.com/about.htm

http://www.nhspeedometer.com/

Cheers