gremlins..... electrical gremlins...

During the last week or so quite a bit has taken place, including the engine start, which thankfully went well. Covering the start in a blog and doing it justice is going to take time to compose, more time than I have at the moment. For this reason, I'm going to park the post for the time being.


What I'd like to cover now are a number of electrical gremlins which were spotted during the engine start and resulting movement of the car.


The gremlins included;

  • Number plate LED light not functioning

  • Tail lights of a differing intensity

  • No brake lights

  • No reverse light

Having no experience of automotive electrical wiring I've needed plenty of support to get to the bottom of these, thankfully the peeps on the Lotus 7 forum known as BlatChat were on hand to help.


Let's cover the most simple first. The non-functioning LED number plate light. Basically with the car's headlights turned on the rear number plate light should illuminate and it didn't. Caterham had shipped the light preinstalled on the car, although they also shipped a second unit too, which was confusing and made me wonder whether the unit should be swapped. Further, the 2015 assembly guide only covered the older bulb type unit. I guess I'd need to figure this out.


The light itself is fixed to a metal mount bracket, which is then fixed to the chassis tube at the back of the car using self-tappers. It's a fairly straightforward affair, but what was interesting was no plugs, the light was directly soldered onto the loom. The light has a black plastic shroud which covers the terminals and also mounting bolts, the shroud could be removed by simply removing a single screw. Once removed I checked for continuity from both terminals and the fuel tank which I already knew to be a good ground. Neither terminal had a ground connection, you'd expect the positive/red to be an open circuit, but surely the negative/black would be at ground. At this point, I became suspicious it was a grounding issue.


The light grounds itself via a short negative cable finished with an eyelet connector which one of the mounting bolts passes through along with a serrated washer and plain washer. Given this, the bracket should be a ground. I performed the same continuity test between the bracket and the fuel tank, again open circuit. I then started to work my way back, first checking the chassis tube, yes it was a ground, next the self-tappers holding up the bracket, yes it was a ground. At this point I figured it was probably too much powder coat on the bracket surface preventing the serrated washer achieving a ground connection. I took out a smaller file and removed a small amount of powder coat from the inside of one of the bracket's mounting holes, sure enough, a ground was present. So the bracket was a ground, just the metal surface wasn't reachable due to the powder coat. My solution was simple, file away more of the powder coat where the serrated washer clamped onto the bracket. This worked a treat, one down!


The differing intensity of the tail lights was a head-scratcher to say the least. A common view from BlatChat was that the issue was most likely another grounding issue, but the lights appeared to have a good ground when tested. One member of the forum however suggested the tail light which was brighter might be using the brake light filament instead, and hence looked brighter. The rear lighting for the Caterham has a single bulb with two filaments, one for the brake light and the other for the tail light. I tested the theory by removing the lens and carefully covering the bulb and squinting a lot, sure enough, the theory was correct. The left-hand side was transposed. I'm not sure at this point whether the light is at fault or the loom, my solution however was to dismantle the Econoseal plug on the light and swap the terminals for the brake and tail light over. I may at some point verify the loom's plug. I've yet to cover my headlight install which was the first time I'd assembled the Econoseal plugs, but this process required disassembly something not covered in the assembly guide. I think I'll cover both in a separate post. Once I'd swapped the terminals around the lights worked perfectly, well the tail lights did.

Swapping the plug terminals

You may have noticed in my above list I stated I had no brake lights, well this was true. This issue was quickly traced to a blown 10A fuse in the fuse box. The fuse at fault serviced both the rear brake lights and also the reverse light. Replacing the fuse and testing each function separately quickly showed the brake lights were fine, it was the reverse light or switch at fault. As soon as reverse gear was engaged the fuse blew. Diagnosing this one was complicated and needed a huge amount of support from BlatChat which I am eternally grateful for!

10A fuse used for brake and reverse light

Lots of work took place trying to isolate the fault, but a few key steps were used to identify what exactly was wrong.


Firstly it required the removal of the passenger seat and tunnel top to provide access to the gearbox and the reverse switch. Once the tunnel top was removed it was clear the reverse switch had been cabled correctly, so that wasn't the issue. As part of the diagnostic process, the switch was disconnected from the car's loom and the gearbox switch points tested. Both were open circuit so no apparent short was present. The loom was then tested and it appeared both were ground, this was not correct. To validate the switch was in fact not the issue, the loom connection used to connect to the reverse switched was jumpered with a piece of wire, simulating the closure of the reverse selector switch, and again the fuse blew. It would certainly appear a fault existed in the loom somewhere. In response, I jacked up the car removed the near side wheel and went digging around looking for an issue. Initially I couldn't locate anything wrong, but just in case something had changed whilst pulling on cables, I decided to check the loom between the gearbox switch loom connection and continuity with a ground, it had changed, only one point was now at ground. This was more like it. I reconnected the reverse light, shorted the switch loom connection and the reverse light work, the fuse remained fine. In conclusion something in the rear arch had changed as I inspected the loom. Eventually I found it, tucked right into the corner where the A-frame bolts to the car the loom had been caught and the wire damaged, it was shorting to the chassis. Maybe I caught it between the washers used to space the A-frame, I need to do more digging to understand how the damage occurred.

Damaged reverse light loom located by the A-Frame fixing

A repair is in order. I plan to perform a temporary fix for the test, and then in the summer with warmer days strip out the damaged section and replace wiring back the reverse light plug.


I've written the above blog with brevity in mind, with only the salient points included. It should be noted tracking down the reverse light short was only possible due to the support from BlatChat, particularly those who stayed online until midnight helping! Thanks guys! I really appreciate it.

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Meet Mark
Loves cars, light weight specials, jDM, Lotus, Caterham, Rallying and Trackdays