Fixing a Rover P5B Temperature Transmitter (Sender)

Fixing a Rover P5B Temperature Transmitter (Sender). As mentioned in a previous post, there are a number of jobs that need fixing in the area of the intake manifold, which I’ve left to do at the same time:

  1. Faulty temperature transmitter / sender. This is well and truly stuck – I’ve tried lots of different ways to remove it, but nothing has worked. Unfortunately it screws into the inlet manifold, so the only thing now is remove the manifold and get it sorted.
  2. Various weeping hoses. I’ve tried tightening the hose clips, but it looks like the previous owner has used generic hoses, especially for the heater, and they just don’t fit well – some of them taper slightly, so they generic ones don’t work very well. A few heater hoses look old as well, so need replacing.
  3. Fix the stuck heater valve which switches-off the rear heater. It’s a plunger-type tap on the side of the front heater box, and is seized with the rear heater turned off.
  4. Fix the leaking gearbox cooler pipes.
  5. Fix the hrs carb which is leaking fuel very slighty.
  6. Fit the correct choke cable and the correct choke knob, so I can get rid of the generic / BL item that’s fitted.

Shortly after buying the car the temperature gauge stopped working. Testing the gauge by shorting the cable showed the gauge was OK, and the wire to the transmitter (sender) was fine, so it must be the transmitter itself. The transmitter is screwed into the inlet manifold. Initial careful efforts to remove it failed – it seems to be seized.

While the car is garaged over the worst of the winter, I thought I’d have another go before the gauge became more necessary in the warmer weather. Despite repeatedly soaking it in releasing fluid, heating it, hitting it and trying with a large socket drive, all I ended-up with was a mangled transmitter.

There was nothing for it but to remove the manifold and try mole grips. I could also fix a few other things at the same time – replace the mangled carb screws, replace several weeping hoses, tidy-up the manifold, try to free the rear heater tap. The manifold came of easily, thankfully. More heat and mole grips failed to move the transmitter. So, the only thing seemed to be to drill it out and repair the thread with a thread insert.

Much discussion on the Rover P5 Forum and investigation revealed that transmitter thread is ⅝ inch UNF. Helicoil don’t do that size of thread insert, so a V-Coil kit was ordered, along with the biggest tap handle I’ve ever seen, and several drills leading up to 16mm.

I set about drilling-out the temperature transmitter today, starting with a small drill and worked-up to 16mm, the size required for the ⅝ UNF thread insert. Quite tricky, as the internals of the old transmitter rotated with the drill – there’s a brass plug, about 20mm long, a spring, and coiled wire (the thermistor itself, I guess). After a lot of drilling I managed to get all this out and fairly cleanly drill the 16mm hole.

I used a V-Coil thread insert, as Helicoil don’t seem to do ⅝ UNF. The tap that came with it required me to buy the biggest tap holder I’ve ever seen, but the tapping process was fine, with plenty of oil and going in 1 turn then backing-off half a turn.

The V-Coil insert also worked fine – went in very easily, and seems to fit well. The new transmitter screws in very easily as well, and hopefully the captive washer will seal it against manifold, as per the original. Below are photos of the whole process:

  • Drilling the old transmitter:
  • Some of the old internals that made drilling so tricky.
  • Tapping the V-Coil insert thread.
  • Inserting the V-Coil.
  • The V-Coil inserted.
  • And the new transmitter loosely fitted.

Next on the list is to strip and respray the manifold in the original grey colour using etch primer, then start putting things back together with new gaskets, hoses and clips.

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