Oh dear, here goes the Speedo…..

On the return drive home on Sunday in the mid-day sun, the Speedo started to go intermittent again. Lost about 60 miles when compared with the GPS.

Back to the drawing board I guess.

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Speedo officially fixed

I took the 7 out for an extended drive in very hot weather on Saturday with the NorCal drivers group. The Speedo behaved perfectly.

So, in conclusion, the speed sensor should have its own independent ground to chassis near the back of the car. And, cut the existing ground connection.

Speedo update 2

I got an email from James at Caterham with an official “how to fix” pdf. I almost had it right (see my last post), however I had not cut off the old ground point.  So I did that and went for a test drive. So far so good – its working correctly. I have to try again in the hot part of the day to give a final confirmation.

Erratic speedometer and warranty

If you have read through my blogs, you know that I have an intermittent speedo problem when the car gets warm/hot. So, as my 1 year warranty was about to expire, I contacted Caterham for assistance. They suggested a new speed sensor might do the trick, so they sent me one and I fitted it.

Its a trial & error process to get the right gap, so I recorded the settings with associated results. This time I used a feeler gauge to measure the gap.

Sensor                    Gap ”          Gap mm    Results


Initial setting     0.045″      1.143mm    Speedo works fine below 30mph, above 30 and speedo goes to zero.


out by 3/4 turn 0.067″      1.702mm   Speedo fails to register at any speed.


1 full turn in        0.037″     0.940mm   Speedo works fine below 65mph, above 65 and speedo goes erratic.

Indicator light ticking on & off cause speedo pertubations close to failure speed.


In 1/4 turn           0.027″    0,686mm    Appears to work at all speeds. However, on an extended drive, over 25 miles, speedo started going erratic at higher speeds.


With no further adjustment, I woke up early on Sunday morning and took the 7 for a drive. Ambient temp = 5C.  Drove about60 miles and speedo behaved perfectly.

On returning home, I left the 7 in the driveway for the morning, then about 2pm, ambient temp = 25C in the shade (hotter in full sun) I took the 7 for a short drive. The speedo was stuck at zero.

So, I’m going to get back to the Caterham guys…….

Technical update

The Speedometer Issue
Last time, I had concluded the speedometer was bad on account of when it went intermitant, a shot of freeze spray returned it to usefulness, at least for a few minutes. A faulty chip inside was my diagnosis. Caterham believed my story & sent me a new speedo, which I fitted & calibrated. And, it worked fine for 60 over miles- then proceeded to go intermitant.
Further input from Sean (CA Caterham Club) suggested that it was a bad ground to the instrument panel common grounding – which is bolted to the wiper motor mount on the LHS under the dash. I checked, read 1.4 ohms to the battery -ve. So I ran an extra wire from the battery -ve to the instrument panel common ground.
Yet to test it – stay tuned.

Over Cooling
After the chilly ride home on Saturday, I’m looking to get the heater working in cold weather. The problem seems to be that although the engine maintains a temperature of ~95c, when I turn on the heater, it provides extra cooling to the engine and the temperature drops to ~60C.
I got advice from Magnus & Doug (CA Caterham Club) to check out the thermostat.

FYI, here is my plumbing diagram……
7plumbing

I did a static test in the garage. Started the engine and set to 2400 rpm.
Indicated 60C Radiator cold
Indicated 70C Radiator cold
Indicated 75C Radiator cold
Indicated 80C Radiator cold
Indicated 88C Radiator starting to get warm at radiator top – this was about 5 minutes
Indicated 90C & holding
After another ~ 5 mins bottom radiator getting hot
Indicated 98C Fan on for ~ 1 minute
Indicated 98C Fan off
Indicated 98C Fan cycling on for ~ 1 minute each time
End of run.

Conclusion: the thermostat is working and opening at 88C.

After discussion with Magnus & Doug, we concluded:
-Oil cooler will have a significant cooling impact.
-An oil cooler thermostat should be investigated.
-Blocking off part of the radiator and all of the oil cooler in winter.
-The thermostat maybe opening at 88C, but what temp does it close?

So, I have blanked off part of the radiator for a test – see picture.

Cardboard blanking on radiator & oil cooler to prevent over-cooling in winter

Cardboard blanking on radiator & oil cooler to prevent over-cooling in winter