Remote fan temperature switch

Fitting an adjustable otterswitch to a Lotus Excel

One of the Excel's many built-in flaws is the temperature switch used to activate the fan(s) on the radiator (one on a standard Excel, 2 on the SE version). For some reason, the switch is prone to corroding and sticking in the "on" position, which results in excessive cooling and a drain on the electrical system. Although the switch is relatively easy to replace, since it sits in a thread in a metal pipe in the top radiator hose, and is available (since it is shared with some models of the Reliant Scimitar), I decided to try an alternative replacement which would allow me to tweak the cut-in point at different times of year, and for different climates.

The switch I ended up choosing is a PACET CF300 capillary thermostatic switch. Unfortunately, PACET were bought by Kenlowe in 2014 and these switches are no longer available - but suitable alternatives are.

Capillary switches

The principle of the capillary switch is fairly simple. Unlike the original Excel switch, the bimetallic switch is mounted remotely from the water in hose, by using a long copper "pipe" to conduct heat from a copper block in the hose to the switch mechanism itself. In this way, the moving parts are kept away from possible sources of corrosion. They are also isolated, to some extent, from the vibrations of the engine, by being mounted on the bodywork.

Sensor inserted into top hose
Sensor in top hose at otter switch adapter. Note the orange instant gasket used to prevent leaks - much more effective than the supplied sticky pad.


Pacet supply a small foam sticky pad which is supposed to help seal leaks caused by the capillary tube. Throw this pad away and get some silastic or instant gasket instead.

Use the instant gasket to liberally coat the end of the metal tube and stick the capillary tube in place, before pushing the hose over the end. Once it sets, the instant gasket seems to create a good strong waterproof seal. (It's certainly been working well on the water pump on my other car for years). On my Excel, the "best" way to fit the "blob" seemed to be to have it pointing towards the engine block as this allowed the control module to be mounted conveniently on the inner wheel arch next to the VIN label.

Switch mounted on wing
Switch mounted on wing

Fitting the switch

Fitting the switch is very straight forward. All that needs to be done is to find a way of getting the copper "blob" at the end of the capillary into the hot water in the radiator hose, without causing a leak.

OK, now for the truth. First point - to do this on an Excel, you need to take the bonnet off. It's front hinged and nicely blocks access to the radiator & hoses in the up position. Fortunately, it's held by just two bolts or nuts on threaded rods + a hydraulic ram which should just pop off.

Once the bonnet is off, the radiator top hose is easily accessible from the front of the car.

Pacet's instructions suggest putting the "blob" into the section of top hose connected directly to the radiator, on an Excel this is easier said than done so I opted to insert it around the metal carrier for the failed otterswitch.


Mounting the controller is the most nerve-wracking part of the whole exercise. Having bent the mounting bracket back from a 90° angle to almost horizontal, two pilot holes need to be drilled into the bodywork so that it can be attached. Once these are drilled, the bracket can be clamped to the controller and then screwed, or riveted, onto the bodywork.

Two short extensions were needed for the wiring loom, to connect the original otterswitch blade connectors to the two connectors on the control module. Polarity doesn't matter as it's simply switching the power on or off.


Pacet's instructions suggest running the car until the temperate reads 5-10 degrees above the normal temperature and then turning the controller adjustment until the fan cut in and run for about 30 seconds to lower the temperature. That's exactly what I did, and it seems to be working. Recently I've encountered a lot of heavy traffic and the fans have been cutting and out just fine.

The whole system in place
The whole system in place - capillary tube routed over the air intake. I moved it lower down later.