Getting hot
ONE other minor issue needing attention on the Series IIA was a non-working temperature gauge. The first step was to connect the sender wire directly to earth. With the ignition on, the needle rapidly moved into the red zone; it indicated a problem with the sender itself. I had a rummage around but the only senders I had were from military 2.5 diesels (of which I have several cluttering up the workshop). These senders are intended to work with the Defender-type gauge and have a totally different resistance profile to the Series items. I finally found a suitable sender attached to a 2.25 diesel head buried under a pile of junk at the back of the workshop and this worked perfectly.
Having finished off the Series IIA I turned my attention to the next job, which was transplanting an ex-discovery 300Tdi into a rather nice ex-military One Ten. I have done plenty of these conversions over the years and know which parts I will need: these include a new temperature sender. The Discovery sender has a lower resistance than the Defender version, giving a very high gauge reading. The correct sender, part number AMR3321, has a black plastic insulator, whereas Discovery versions are green.
I had a couple of senders already in stock. Both were labelled AMR3321 but one had a green insulator which made me wonder if it was a mispackaged Discovery item. The other had a black insulator and this was the one I fitted first. Even with the engine cold the needle sat about a quarter-way up the dial: after about half a mile the needle was almost in the red. I quickly drove back to the workshop and established that the gauge was telling porkies – engine temperature at this point was only about 60 degrees Celsius.
I swapped the sender for the one with the green insulator which gave the same result. Two new senders from different sources with the same defect seemed unlikely, so I swapped the temperature gauge for a new one. I have found a few times that the temperature gauges on older vehicles can read a bit high and need changing: Tdi engines run a few degrees hotter than the old 2.5 lump. In this case, changing the gauge made no difference. I double-checked the earth wire to the gauge casing. These gauges have an
➔ internal voltage regulator and will read high
if not earthed properly. This one was fine, pointing the finger back at the sender.
I ordered another AMR3321 sender, this time choosing a different brand. It was visibly different to the others: the terminal pin was bright-plated steel rather than brass and the overall shape wasn’t the same, so it had obviously come from a different factory. I had high hopes that this would cure the problem, but it made absolutely no difference to the gauge reading. I can only think that the various manufacturers are all drawing their technical data from the same source, and that an error has crept into that data. I now had a vehicle due to be collected the following morning, and no working temperature gauge. I needed a cunning plan, and fast.
On top of the 300Tdi cylinder head, about halfway along, is a threaded hole to take a sender. On early engines this hole was fitted with a blanking plug: later engines had a sender fitted as part of the emissions control system. I was hoping that the threaded hole would take a 200Tdi sender but it was too small. I then had an idea: I put a multimeter across the terminals of the sender I had removed and found the resistance was about double that of the duff AMR3321 senders I had been trying. I lashed up some temporary wiring to connect the sender to the gauge and was rewarded with a needle which sat bang in the middle of the gauge with the engine fully warmed up.
The only remaining question was whether the gauge would move into the red if the engine overheated. I tried a series of experiments involving a camping cooker, a pan of engine oil and a Defender temperature gauge, but the results were inconsistent, probably because of the small volume of liquid in the pan. The temperature needle certainly moved upwards, but I could not be absolutely sure that it would give early enough warning of an overheating problem. I will have to keep hunting around until I find a source of AMR3321 senders that work properly.