02/02/2026
420R oil temperature
I had a customer In recently that wanted to know what his oil temperature was while on track and on the road.
Anyone that knows me will know how fundamental I think this is and how often it's overlooked.
This car is less than a year old and it's pristine so I didn't really want to be taking the scuttle panel off to get access to the loom behind the dash and start splicing into wires.
I decided that the best plan of action would be to make a break out loom that would be plug and play and could be installed through the gauge hole in the dash.
The gauge on this year of car are super easy to remove, it's a screw on collar around the body of the gauge and can be undone with your fingers.
With the gauge removed it's simply a case of removing the plug from the back of the gauge and plugging it into one side of the breakout loom and the plugging the other side of the breakout loom into the gauge.
From there the singe wire that connects to a sensor in the dry sump tank needs threading through the grommet in front of the gearstick and then down the inlet side of the engine to the tank at the front.
I made back shells for the connector so that the wires could be sealed onto them. This is so that any accidental or deliberate tugging on the wires puts the stress through the connector housing and not the delicate contacts inside hence increasing reliability.
I also use spec 55 wire in almost all my looms for lots of reasons. Most of them are the same reasons it's used in top flight Motorsport and by the military but this is a long winded topic for another time.
The last thing to do was to fit the switch that would allow the coolant temp gauge to show either oil or coolant temps.
A small toggle switch was chosen so it would be usable but not get in the way. It also looks in keeping on the dash.
The main problem with fitting this sort of thing for most people is fear of drilling the carbon dash.
To make matters worse this sort of switch comes with a washer with a key and a tab on it. The key locates into a key way in the switch and the tab is meant to locate into another tiny rectangle hole in the dash. This is to prevent the switch turning in the hole and you never knowing which way to throw the switch.
To get round the need for this and to solve the issue of strain reliving the wires at the back of the switch I made a housing that the wires could be held to and have an extension to the top that would sit against the top dash hoop so that the switch couldn't rotate when mounted through the hole.
The final job was to make a little plastic jig that would mount through the gauge hole and clamp in place butted up against the top dash hoop to give you a guide hole to drill through and know it's in the right place so removing as much risk and fear as possible. Also the hole in the jig doesn't go all the way through to reduce the risk of damage from the drill bit to something you can't see.
The reports back from my customer were interesting.
He first tested it on track and initially thought it wasn't working only to realise that at speed the oil cooler drops the temperature of the oil to around 50 degrees but would climb a little in the slower section.
With this information he managed to blank of most of the oil cooler and start seeing slightly more sensible temperatures of around 80-90 degrees.
What a perfect example of why all Caterham's should be fitted with an oil temp gauge.
I can also see an oil stat in his future.
I made a couple of extra looms so if this is something you're interested in feel free to give me a call.
I might make more if there's enough interest.
