When replacing a temperature sender on a tractor, it’s easy to assume that if the part screws in, it must be correct. This is one of the most common causes of inaccurate temperature gauge readings.
When replacing a temperature sender on a tractor, it’s easy to assume that if the part screws in, it must be correct. This is one of the most common causes of inaccurate temperature gauge readings. While many tractor temperature senders look almost identical, the colour of the plastic surrounding the spade terminal is critical. The location of this plastic indicator is shown in the picture below. The colour options for the Nuffield, Leyland and Marshall tractors are white, black or red.
This detail represents the electrical resistance of the sender — and therefore how your temperature gauge reads.
What Does a Temperature Sender Do?
A temperature sender measures the engine coolant temperature and sends a signal to the dashboard gauge. This signal is based on electrical resistance, which changes as the engine warms up. A temperature sender (or sensor) works by converting thermal energy into a measurable electrical signal, typically by changing its electrical resistance based on temperature. Most automotive senders use a Negative Temperature Coefficient (NTC) thermistor, where higher temperatures decrease resistance, allowing more current to flow to the gauge.
The gauge is calibrated to work with a specific resistance range. If the resistance does not match what the gauge expects, the temperature reading will be incorrect — even though the engine itself may be running perfectly normally.
Same Thread Size, Different Electrical Behaviour
All temperature senders supplied by Tractor Spare Parts Ltd share the same thread size:
· 5/8" UNF
This means they will physically fit many different engines. However, they are not electrically the same.
The colour of the plastic around the spade terminal indicates the resistance curve of the sender. Different colours = different resistance values = different gauge readings.
Marshall and Leyland Tractors: A Common Question
Customers have the opportunity to ask a question on our website. This facility is available on each product page. I recently got asked this question:
“I have a Leyland 802 and I’m looking for a temperature sender. The one fitted has a black surround.”
The customer realised that the black temperature sender is usually only used on the Marshall 302 or Leyland 302 models, which are fitted with the Leyland 1800 engine, so they were wanting to check before purchasing. We would much rather speak to customers before point of sale to ensure that the correct items are being sent out – we don’t always spot mistakes in product purchases as we don’t know what model tractors many customers have. However, if our staff notice a product that doesn’t fit with the other products in a shopping cart do not be surprised if we contact you to check that you have ordered the correct parts.
In this case we were able to talk to the customer and establish that the customer did not need the black sender, part number MBU1244. Their sender had been painted over.
This is a great example of where confusion with temperature senders comes from. Harvest Gold Marshall tractors and Harvest Gold Leyland models, including the Marshall 802, would usually be fitted with a red plastic surround. This will be the same for the Leyland 802 tractor which is a cross-over model from when Leyland tractors became Marshall tractors and the Harvest Gold range was introduced with the Leyland branding for a few months.
If your sender appears black, it is very important to check whether it has been painted over during an engine respray. The engines on a Marshall tractor are painted black and it is easy to assume that the temperature sender is the paint colour on first inspection. You can read about the colours of the Marshall tractors in another blog that we have written – click here to read the blog.
Helpful Tip
Scratch a small area of the plastic around the spade terminal. If you see red plastic underneath, you have identified the correct sender type.
While a black sender may still operate, the gauge reading will be inaccurate if the resistance does not match the gauge — leading to misleading temperature readings.
Examples of Temperature Senders Available
Below are examples of temperature senders available from Tractor Spare Parts Ltd, all with the same 5/8" UNF thread but different resistance characteristics:
- Leyland Middleweight tractor and Marshall Harvest Gold tractor Temperature Sender – red colour
https://www.tractorspareparts.co.uk/leyland_tractor_parts/bmk1644-temperaturesender/ - Nuffield Temperature Sender – white plastic
https://www.tractorspareparts.co.uk/nuffield_tractor_parts/bmk1659-temperature-sender/ - Marshall 302 Temperature Sender - Black Plastic
https://www.tractorspareparts.co.uk/marshall_302_tractor/mbu1244-temperature-sender/
You will note that we have listed the models that each temperature sender is fitted to on the product page. However, it is always a good idea to physically check as engines have been swapped in some of these tractors during their life – they are 40, 50 or even 60 years old after all!
Each sender is designed to work with its corresponding gauge, despite looking very similar externally.
Choosing the Correct Temperature Sender
When selecting a replacement temperature sender, always consider:
- Tractor model and which engine is fitted in your tractor. Has somebody changed the engine or bodged the tractor to make it work with whatever they had available to them when something previously went wrong.
- Original gauge type
- Colour of the plastic around the spade terminal
- Whether the existing sender may have been painted over which may give you false first impressions.
Thread size alone is not enough. Matching the resistance to the gauge is essential for accurate temperature readings and peace of mind.
Final Thoughts
Temperature senders are a small but crucial component of your tractor’s monitoring system. Although they may share the same thread size, the colour of the plastic surrounding the spade terminal determines how the gauge reads.
If you are restoring or maintaining a Leyland or Marshall tractor, taking the time to identify the correct sender colour can save you from unnecessary worry, incorrect diagnostics, and wasted expense. If you are unsure you can always contact Tractor Spare Parts Ltd by email or telephone and our friendly and knowledgeable staff will help you decide which parts you need.
Always remember:
If the colour is wrong, the reading will be wrong.
