Stupid Question! Rear Lights - Which one is which?

[AJSGSE1990]

Really sorry to have to ask, but completely forgot to write down what the three red rear lights do and am now wiring the new PDM.

On the Left Hand Cluster, going from Left to Right, this is what I think:

A: Position Light - on with low beam
B: Rear Fog - on with low beam + fog light button
C: Large Metal Unit - brake light

Is this correct?

MaxxECU is brilliant, in about 30min fully up and running on the bench, major functions are all precoded.

WhatsApp Image 2026-06-12 at 21.30.29 (Small).jpeg

WhatsApp Image 2026-06-12 at 21.30.30 (Small).jpeg

WhatsApp Image 2026-06-12 at 21.30.29 (1) (Small).jpeg

[peteoddcar]

You have the brake and fog incorrect
Fog is the large metal with the 55 w halogen bulb, brake light is the 21watt

Seems daft but the lamps are not Lotus but Renault Alpine, as far as know, these are the only cars with 110w rear fogs !
You could configure it so the brake lights are 55w but thats not how things were legally homologated.

[AJSGSE1990]

You have the brake and fog incorrect

A: Position Light - on with low beam
B: Brake Light
C: Large Metal Unit - Rear Fog - on with low beam + fog light button

This is crazy, the fog light is brighter than the front low beam! They are all going onto LED equivalents in the end but need to set max current per 12v line on the original equipment first.

Thank you.

[peteoddcar]

Are you building a wiring harness for the car ? I can’t understand why you would need the fuse ratings? If you have access to the electrical manual and check rear fog circuit you’ll see that the rear fog has a relay, protected by an 15 amp fuse and the brake lights not.
As a guide, the fuse rates are in the fuse list and in line with normal automotive electrical design the fuses are rated at approx 150% of the normal working load.Just around 8 amps for the fog lamps. 3.6 for the brake lights,
Am genuinely interested in your project - are you building a light weight M100?

[Tuga2112]

theres no project thread. but questions scattered around the forum giving hints...

last i am aware was the hint about changing a ECU for a aftermarket ECU
but there was some original issue which was not fully diagnosed as far as im aware.

now that is the main concern i have about that "not project" happening.

but im not asking questions. i am just checking the new posts and making guesses about one (roadworthy) trackday toy coming up which may be quite an interesting one to see in person when the "big reveal" happens.

i shall be observing (like many others) the usual places.. its good banter for the CC campfire evening

[AJSGSE1990]

theres no project thread. but questions scattered around the forum giving hints...

Right did not quite post a project. Might as well do that. To answer your question there is no Lotus wiring left on the car, other than a few tails. I want a daily driver and lessons learnt from my last 3 classics is that electrics are almost always the source of all "difficult" issues.

Put it this way, I have now done 1000 miles in the 1969 Alfa since I fixed the last issues caused by the "professionals". Horrible Essex -> London commuting and a few longer trips. I'm going again in a couple of hours. I drive it like petrol costs 2p a gallon and am not afraid to crank up Beastie Boys Sabotage when needed...! Impossible without proper modern electronics.

Sorry to say it's not the most exciting project, no track day hero.

Cheers,

Andrew.

[Tuga2112]

To answer your question there is no Lotus wiring left on the car, other than a few tails. I want a daily driver and lessons learnt from my last 3 classics is that electrics are almost always the source of all "difficult" issues.

i have also found myself in the same position as far as vehicle wiring ans specifically the jobs from other people in any car. (a 97 alfa spider that had interior lights working when the fuse was removed being a prime example)

however from experience in both Elan, MR2 and Celica gt4 what i have grown to believe is that the factory instalation is reliable. the issues i came across are always the aftermarket stuff.
also.. what you just said may not come in the best of light when the guy who drawn the wiring diagrams of the M100 visits the forun regularly... and even helps others.. he's the kind of guy that is genuinely interested in seeing a project building a light weight M100

i certainly have added to the M100 wiring. and in the process updated my own print out of the wiring diagrams. i find that to work for myself. it helps me ask for help as it stays in a shape that others are familiar with, and if needed i can publish my own files to help others help me. in the event i need that.

[dapinky]

Sorry to say it's not the most exciting project, no track day hero.

Cheers,

Andrew.

No such thing as an unexciting project - those of us who like to change our cars to make it "easier" to live with love to see new ideas......

..... so I'll start with a question which is probably not in the right thread (but I'm sure a Moderator will move it if they want to )....

.... I have followed your other posts with a mixture of "Why?" and "Wow, that'll do it!" - either way, it is going to be your car, so it isn't up to me to demand "Why?" to anything - after all, I've been known to spend a long time sorting out things on my cars that were never really an issue to start with, just because I wanted to see how it would work. (Not just the usual things like brake upgrades, or changing some minor electrical/cosmetic stuff to how I wanted them.... but stupid things like Cruise Control which may well have no place in a sportscar!)

So, my big interest at the moment is the MaxxECU. It isn't something I know anything about (despite reading much of the website last week when I read about it from you). I have previously used Megasquirt, and have that on my Dolomite (just a basic system as it retains Dell'Orto carbs) and I have known a few other Elan owners using Haltech, albeit that they were projects from 12-15 years ago.

Technology moves on at a faster pace than ever, and I like to look to the future wherever possible. These cars are getting to the age where many of the original control systems are somewhat antiquated (in the ECU concept), and use a variety of sensors to achieve the results of smooth running. I would love to see a "better" system which uses more standard sensor inputs, and is easily variable with a laptop and some knowledge. Every car is different, and a 'standard' map is really nothing more than starting point to get the best, safe, setting for their own circumstances.

So, my question is - which ECU are you starting with? When looking at the comparison tables on their website it seems to me that many will do the job, but only a couple will work with low impedance injectors...... so are you keeping the OE injectors, or changing them out? Which Temp sensor are you using? Does it retain the original CAS with position sensor or does it need a new crank sensor? ARe you keeping the MAP sensor fromt he car, or using one integrated int he ECU?

Lots of questions, which you may or may not have sorted already, but I'd be interested to know,

Oh, while I'm at it - what is the relevance/usefulness of changing out the fuseboxes for a PDM? (I've never seen them before, they didn't exist back in the day when I was building my competition cars!)

[AJSGSE1990]

No such thing as an unexciting project - those of us who like to change our cars to make it "easier" to live with love to see new ideas......

That is the general idea, the M100 is a brilliant car, even in its less than ideal state on B roads it is just amazing.

So, my big interest at the moment is the MaxxECU.

As you said, things move so quickly. I have 3 cars on Motec which is too hard for me to deal with, now that Bosch bought them its getting worse as they have zero interest in customer service of any kind. MaxxECU is very well set up out of the box, a lot of functions are built in, from simple stuff like a whole menu set for indicators and blink rates. Makes at least the body loom so much easier.

So, my question is - which ECU are you starting with? When looking at the comparison tables on their website it seems to me that many will do the job, but only a couple will work with low impedance injectors...... so are you keeping the OE injectors, or changing them out? Which Temp sensor are you using? Does it retain the original CAS with position sensor or does it need a new crank sensor? ARe you keeping the MAP sensor fromt he car, or using one integrated int he ECU?

Short answer is I do not know as the engine I am going to need a lot of help. Probably about 3-4 months away before we get to that stage, but definitely electronic throttle and coil packs. Both Lancia have those and they work fantastically. Alfa is moving to coil packs shortly. Just to put this in context, today I was doing "ahem ahem" miles an hour on the M11 hit static traffic for 20-25min as people were "creating content" due to a minor accident and the car sat firmly at 85 degrees. Imagine that with a 1969 aluminium block and standard cooling, throttle and more. In fairness it does have a PWM core radiator which is just spectacular, but I had to completely redesign the cooling circuit to match the original Alfa 75 from which that engine head came from. The very expensive specialist got it so wrong, three times, it blew the gasket.

Oh, while I'm at it - what is the relevance/usefulness of changing out the fuseboxes for a PDM? (I've never seen them before, they didn't exist back in the day when I was building my competition cars!)

The PDM is basically a box with a bunch of solid state relays which can be controlled/rated electronically. They are literally magic, I'm not joking. No relays, no fuses, nothing. Just a USB cable and a laptop your kids think is too pants to run anything.

[dapinky]

Thanks for all that - I'll continue to follow your progress.

[AJSGSE1990]

also.. what you just said may not come in the best of light when the guy who drawn the wiring diagrams of the M100 visits the forun regularly... and even helps others.. he's the kind of guy that is genuinely interested in seeing a project building a light weight M100

I don't doubt it was an excellent installation at the time, but apart from technology moving forward and costs collapsing, wiring and especially relays on this car are 35 years old. There is no escaping that. My personal opinion is the car is so good for British bumpy roads that I want to keep the dynamic feel of it, however I need to make some changes to seating position, dash (cut out the area to the left of the steering wheel for my knees), and put at least a 100mm spacer on the steering wheel to fit my driving position. The one thing Lotus did badly was not serve the 6 foot plus community. So once the dash goes back in, I would like to avoid ever needing it to come out again.

Cheers,

Andrew.