620 S Build

[CarYenta]

4 hours ago, MV8 said:

This is all making sense now. With the key turned to RUN, you should have multiple indications of life such as working turn signals, the digital odometer showing in the speedo, warning lights in the tachometer, and the big red light between the two instruments. It sounds like one or a combination of factors related to power distribution or grounding; something is not fully plugged in, the ends of the battery cables are loose, or the battery is not fully charged.

The battery should be fully charged to the green lights (step 7 or 8), not just to the "ready to use" light (step 4). Small batteries reduce weight at the expense of allowed attempts to start the engine, so you may just need to charge. A few minutes running (alternator charging) is not long enough to replace a single cranking attempt, much less multiple. The battery is likely undersized to save weight (fewer amp hours) and less tolerant of failed start attempts. 

Battery terminals have a dull grey oxidation when new. This is removed with a wire brush before the cables are attached. I don't know what type of terminal you have but there are brushes for round post types at your local auto parts store.  

I see you have a short neck on the block where the thermostat goes instead of the long metal tube that did not accept a thermostat so you likely have the improved kit.

 

Also, never leave the key in RUN and engine off with out attempted starting within a few seconds. This can overheat the ignition coils and cause them to fail early.

Thanks for the help! I didn't know leaving the key in run could be harmful other than draining the battery, I will keep that in mind. I think maybe it was misread that I left the key in the run position for that 10 minutes, but I left it "key in but NOT on run". I probably shouldn't have written it that way oops.

After cranking over and over the very first time, I could start to hear the cranking slowing down, then waited 10 minutes for one last hurrah prior to giving up (upon which it started). My charger showed up the next day so I left it plugged in overnight to step 7 on the ctek. Following day, some thing. So it's not a battery charge issue I don't think. All electronics work with the key in run, and a few work when not in run like hazards (at least I think I recall them working key out).

 

So my guesses are either I'm soaking the engine with gasoline quite a bit cranking, then after 10 minutes of waiting it creates the perfect firing conditions and that red light is a red herring, or something is wired incorrectly and maybe that red light is trying to tell me something.

 

What happens with the computer during those 10 minutes of waiting? Is something resetting? Why does that light not turn on during all my cranking attempts initially, but does after waiting? Something must be different with the computer and it seems that may be related.

[CBuff]

I believe the 420 has a single shift light in that location. Mine has the sequential shift lights so it’s not there. 
 

I have a single light in the center of the dash that is the immobilized.  It is blinking until the immobilizer triggers it to disengaged and then turns off or goes solid (I can recall).  
 

could this single red light in the dash be immobilizer related ? If it only fires when this light is in a steady state then maybe that’s what it is.  Not sure what the different configs are between 420 and 620. 

Edited November 12, 2025 by CBuff

[CarYenta]

38 minutes ago, CBuff said:

this single red light in the dash be immobilizer related ?

My immobilizer light (which was initially popped back into the dash wiring so I couldn't see it, and earlier comments by myself were confused on this) sits right above the three gauges of gas temp oil. If I remove the immobilizer fob from the key chain, the car doesn't crank, so the immobilizer seems to work properly.

[MV8]

6 hours ago, CarYenta said:

Thanks for the help! I didn't know leaving the key in run could be harmful other than draining the battery, I will keep that in mind. I think maybe it was misread that I left the key in the run position for that 10 minutes, but I left it "key in but NOT on run". I probably shouldn't have written it that way oops.

After cranking over and over the very first time, I could start to hear the cranking slowing down, then waited 10 minutes for one last hurrah prior to giving up (upon which it started). My charger showed up the next day so I left it plugged in overnight to step 7 on the ctek. Following day, some thing. So it's not a battery charge issue I don't think. All electronics work with the key in run, and a few work when not in run like hazards (at least I think I recall them working key out).

 

So my guesses are either I'm soaking the engine with gasoline quite a bit cranking, then after 10 minutes of waiting it creates the perfect firing conditions and that red light is a red herring, or something is wired incorrectly and maybe that red light is trying to tell me something.

 

What happens with the computer during those 10 minutes of waiting? Is something resetting? Why does that light not turn on during all my cranking attempts initially, but does after waiting? Something must be different with the computer and it seems that may be related.

The ecu does nothing when off. Are you now saying the red light does not always turn on with the key in RUN with the other instrument lights? Did you clean the battery terminals and check for loose cables? Did you pull a plug to confirm flooding?

[Croc]

4 hours ago, CarYenta said:

My immobilizer light (which was initially popped back into the dash wiring so I couldn't see it, and earlier comments by myself were confused on this) sits right above the three gauges of gas temp oil. If I remove the immobilizer fob from the key chain, the car doesn't crank, so the immobilizer seems to work properly.

 

I would zip tie the immobilizer to the steering column next to the sensor and leave it there permanently.  Your symptoms do make me pause to consider whether the immobilizer has unlocked the car to start - there is a delayed reaction as if it does not sense the key fob?

 

One question - can you hear the fuel pump priming on start up but before you crank starter?  It should be loud enough for you to hear.

 

 

i would check the battery terminals and cables like MV8 suggests.  I think your 620 should have a Odyssey PC680 (or similar) gel matter battery on the passenger side floor?  Check it is at full charge.  A failing version of this battery can produce some goofiness in the immobilizer.  

 

 

 

 

[CarYenta]

4 hours ago, MV8 said:

The ecu does nothing when off. Are you now saying the red light does not always turn on with the key in RUN with the other instrument lights? Did you clean the battery terminals and check for loose cables? Did you pull a plug to confirm flooding?

 

The red light between tach and speedo does not illuminate with a freshly charged battery. After cranking many times and letting sit, upon key in RUN the light appears dimly lit. At crank start, it goes bright red once and remains dimly lit.

 

Here you can watch me try to start it for 12 minutes yesterday (sorry for the fat thumb at the beginning)

https://youtu.be/KSHvOY9m6UA?si=pViO6Nx3fvYqQh4m&t=36

 

And here is 10 minutes directly after where it started immediately

https://m.youtube.com/watch?v=q3HjbjqVsdg

 

[CarYenta]

Here's a new one, now the speedometer is resetting when I start cranking. What??

 

Plugging in the battery overnight again.

 

https://youtu.be/CWvv53Uh0D0?si=XPpGfzV81u-INIFd

Edited November 13, 2025 by CarYenta

[CBuff]

Bizarre. That is your single shift light.  I don’t believe it should illuminate at all unless you hit your max rpm. 
 

here is the only thing somewhat related which unfortunately doesn’t have a clear resolution. But seems like a wiring fault. 

 

it’s a 420 but with a 620 dash. 
 

https://www.caterhamlotus7.club/forums/topic/278537-change-light-permenantly-illuminated/

 

I think Bruce would be your best bet.  
 

[NSXguy]

2 hours ago, CarYenta said:

The red light between tach and speedo does not illuminate with a freshly charged battery. After cranking many times and letting sit, upon key in RUN the light appears dimly lit. At crank start, it goes bright red once and remains dimly lit.

 

Here you can watch me try to start it for 12 minutes yesterday (sorry for the fat thumb at the beginning)

https://youtu.be/KSHvOY9m6UA?si=pViO6Nx3fvYqQh4m&t=36

 

And here is 10 minutes directly after where it started immediately

https://m.youtube.com/watch?v=q3HjbjqVsdg

 

 

First thing i always ask if i'm helping diagnose something is; has this always persisted or did you make a change(hardware or calibration) before the engine started the intermittent starting issue?

 

If your cold start and warm up enrichment is off, i would refrain from cranking too many times hoping for it to start. Cold starting is a high stress/wear state generally. If it's injecting too much fuel and washing cylinder walls without ignition you don't want to be cranking excessively. For an engine to start from cold, you need enough fuel enrichment(not too much, not too little), so enough prime pulse and cranking fuel, enough air and right spark and timing.  All those have to dance in harmony for start to happen so it's just a matter of finding which one of those factors is not showing up to the party or not dressed properly (malfunction or not getting signal). 

 

Verification (a Multimeter/Timing light works here): 

If your pump is priming, which it appears to be doing, you can verify that your injectors are receiving signal and spraying during cranking (can verify physically or check voltage at injector when cranking). i suspect they are working and flooding your cylinders due to all the cranking i saw in the video. 

 

Next you want to ensure the charge can be ignited; you can check your coils are firing during cranking (again, can do physically by setting up a test with grounding coil/plug to a ground; or if not comfortable with, check voltage at coilpack while cranking). Another simple issue could be fouled plugs from excessive cranking and enrichment (remove plugs and check to verify- see if they are overly sooty or wet).

 

I doubt the fuel system will drive the signal to the red light next to your speedo. It's more likely it has to do with ignition related status or  some other condition/signal .  I'm not intimately familiar with 620 architecture and glanced through the thread BUT watched the videos. Thats the basis for this reply. 

 

Another thing you can do is verify crank/cam sync - but it's a higher order test so i would suggest you do the basic ones first and report back. If you had access to the ecu programming via data connection/logging diagnosing would be easier. but some of the factory ecus come locked out. 

 

So I recommend working through the clues as they present, if it starts repeatedly and  immediately when the red light is illuminated, then look at the wiring diagram or trace the wires to determine if correlation=causation;  what it's for and what components are linked to it, then follow the clues from there until you arrive at the root and can either replicate the condition at will, or determine what controls the lights illumination. 

 

 

 

Edited November 13, 2025 by NSXguy

[CarYenta]

43 minutes ago, NSXguy said:

 

First thing i always ask if i'm helping diagnose something is; has this always persisted or did you make a change(hardware or calibration) before the engine started the intermittent starting issue?

 

If your cold start and warm up enrichment is off, i would refrain from cranking too many times hoping for it to start. Cold starting is a high stress/wear state generally. If it's spraying too much fuel and washing cylinder walls without ignition you don't want to be cranking for too long. For an engine to start from cold, you need enough fuel enrichment(not too much, not too little), so enough prime pulse and cranking fuel, enough air and right spark and timing.  All those have to dance in harmony for start to happen so it's just a matter of finding which one of that factors is not showing up to the party or not dressed properly (malfunction or not getting signal). 

 

Verification (a Multimeter/Timing light works here): 

If your pump is priming, which it appears to be doing, you can verify that your injectors are spraying during cranking (can verify spray physically or check voltage at injector when cranking). i suspect they are working and flooding your cylinders due to all the cranking i saw in the video.

 

Next you want to ensure the charge can be ignited; you can check your coils are firing during cranking (again, can do physically by setting up a test with grounding coil/plug to a ground; or if not comfortable with, check voltage at coil plug while cranking). Another simple issue could be fouled plugs from excessive cranking and enrichment (remove plugs and check to verify- see if they are overly black or wet).

 

I doubt the fuel system will drive the signal to the red light next to your speedo. It's more likely it has to do with ignition related status or condition . I have not looked through your whole thread in depth but watched the videos. Thats the basis for this reply. 

 

Another thing you can do is verify crank/cam sync - but it's a higher order test so i would suggest you do the basic ones first and report back. If you had access to the ecu programming via data connection/logging that helps too. but some of the factory ecus are locked. 

 

So I recommend working through the clues as they present, if it starts repeatedly and  immediately when the red light is illuminated, then look at the wiring diagram or trace the wires to determine if correlation=causation;  what it's for and what components are linked to it, then follow the clues from there until you arrive at the root and can either replicate the condition at will, or determine what controls the lights illumination. 

 

 

 

This has occurred since first start of the engine, no changes have been made since. 

 

There is spark and fuel eventually at least as it will eventually run, and rather smoothly now at that.

I agree a question is, is there initially spark and fuel? I haven't checked that - I can smell fuel odors during cranking from the exhaust, but I can't smell spark  I will check the spark and report back asap. For fuel injectors, I only know how to hear them while the engine is running with the old screwdriver stethoscope. During cranking, does one pull a spark plug and use their nose?

 

I also sent an email to build support to see if they can define what this light in question is for.

[MV8]

This looks to be an intermittent power distribution in RUN through a circuit that includes the #1 pin of the MBE and the speedo and tach power. The immobilizer is functioning normally. If it were only the shift light not coming on, I would suspect the MBE connector had a bent pin or was not seated but this includes the instrumentation. I'd start by reseating all the relays on the fuse block behind the panel. The shift light comes directly from the MBE and the speedo and tach needle swing/self test is internal to those components.

 

These cars have a calibration module to make different dash configurations plug and play, but that would only affect indications and not running. This car looks to have a combination dash (420 and 620?).

 

CAT does not share wiring diagrams. They used to share a harness layout in the back of the assembly guide. I assume this policy preventing direct customer support is to help support their dealer network for servicing.

 

Your dealer should be actively helping you with this.

Edited November 13, 2025 by MV8

[speedwagon]

back in the day, and in the military we did not accept any equipment that did not include a complete manual including schematics.   seems like a good practice to me, however I do not have enough power as a consumer to enforce this.  So I just try to avoid (when possible) not having them. in many cases I just back engineer something so that I have the info, or just don't do business with scussballs!!

[CarYenta]

Update - Caterham cars build assistance asked me to try, when the car won't start, to leave the key in RUN, switch off the FIA switch, switch the FIA switch back on and then try cranking.

 

I did as such -

 

FIA switch was off after having been off over night and recently after a fresh charge the night prior, I then turned FIA switch to on, key into cylinder and key turned to RUN, and everything initialized other than the shift light as per usual. Car did not start as per usual. With the key set to RUN still, I then switched off the FIA switch, then within 5 seconds turned the FIA switch back on, and the red shift light initialized immediately! Sure enough, the car started right away. Sent them this info, will see what they say and will report back.

 

For anyone wondering, I have a black ground battery cable running from passenger seat battery to the chassis below the coolant tank, and a second black cable goes to the engine block.

This cable then goes to #11 bolt on the block above the starter motor:

Edited November 14, 2025 by CarYenta

[MV8]

That is good news. If you pull the FIA switch to check connections, the battery negative terminal should be disconnected first. The FIA switch should have two large stud terminals and four male spade terminals for slip-on connectors. The spades are paired with the casing labeled 1 and 2. One pair is ignition (connected when the FIA switch is ON) and the other involves the alternator and a resistor (connected when in OFF). The connectors should not slide off and on without some effort. To improve the connection, with the FIA switch off, the connectors can be pulled off one at a time (to prevent mixing them up) and gently, barely squeezed so they are more difficult to install. You might also cycle the FIA switch off and on a dozen times or so to clean the internal switching contacts.   

[CarYenta]

6 hours ago, MV8 said:

That is good news. If you pull the FIA switch to check connections, the battery negative terminal should be disconnected first. The FIA switch should have two large stud terminals and four male spade terminals for slip-on connectors. The spades are paired with the casing labeled 1 and 2. One pair is ignition (connected when the FIA switch is ON) and the other involves the alternator and a resistor (connected when in OFF). The connectors should not slide off and on without some effort. To improve the connection, with the FIA switch off, the connectors can be pulled off one at a time (to prevent mixing them up) and gently, barely squeezed so they are more difficult to install. You might also cycle the FIA switch off and on a dozen times or so to clean the internal switching contacts.   

 

Thanks! This is good info.

 

Build support got back to me and said that at the beginning of 2025 (my car was built Jan 2025) they moved to on updated MBE ECU. They are "now becoming aware" that there was a batch of these which "struggle to come awake on ignition cycle and that could be overcome via power cycle" as was shown. They are going to send out a warranty replacement to Beachman.

 

"You can continue to use this technique to overcome the problem until your replacement ECU arrives. It's caused by a tolerance issue with one of the ECU chips. Once the ECU is awake, it will perform perfectly."

 

Whew! Well there we go. Thanks for everyone's help in this! Y'all are super knowledgeable.

 

In other news, longacre mirror is in and bracket mirror mounts are on their way for the spa.

 

The front frame and rear frame of the track day roll cage are 38 mm (1.5") and the curved bars and rear cross bars are 32 mm (1.25").

 

 

 

I also have some Westermann half doors to install, but this particular cage makes that really difficult to fit. I think I will have to make a big cut into the door. 

 

Anyone have a photo of westermann doors with this track day roll cage? I've only seen it with the FIA cage where it seems to fit easily.

 

Edited November 14, 2025 by CarYenta

[Croc]

That’s good news on the warranty repair for the ECU.  
 

separately - what brackets are you using for the Longacre mirror to hold it into the rollcage please ?

[CarYenta]

13 minutes ago, Croc said:

That’s good news on the warranty repair for the ECU.  
 

separately - what brackets are you using for the Longacre mirror to hold it into the rollcage please ?

They came with the 14" longacre mirror set from Pegasus - they can be purchased separately as well, they are these: https://www.pegasusautoracing.com/productdetails.asp?RecID=259

 

 

I bought a second pair so I can mount my SPA mirrors plus L brackets to orient it 90 degrees:

 

 

I might need to slot out the 1.5" a little longer we'll see. I'm hoping I can adjust the rear nut without removing the mirror this way.

[CarYenta]

One thing that is very obvious is the rear wheels on these 15x6.5" orcus are TINY looking. I like the idea of 13" wheels, but I don't like how there are (as far as I know) no tires which I wouldn't immediately hydroplane on. I run 17x9 245/40 PS4S on my nd3 miata in the summer and 215/45 crossclimate2 on the oem 17x7.5.

 

I get the feeling the avon zzs are probably fine for summer use, but is there a PNW winter tire out there for 13" or actually even 15"?

 

Edited November 14, 2025 by CarYenta

[Croc]

22 minutes ago, CarYenta said:

One thing that is very obvious is the rear wheels on these 15x6.5" orcus are TINY looking. 

 

What about buying the adapter that Beachman Racing sells to allow 15" CSR rears (8.5") on the back of a de Dion Caterham?  

[CarYenta]

35 minutes ago, Croc said:

 

What about buying the adapter that Beachman Racing sells to allow 15" CSR rears (8.5") on the back of a de Dion Caterham?  

What if I don't like how CSR wheels look 😬 

 

I guess I'm less worried about what wheels there are as I can get them custom made, but am more worried first about max tire sizes in useful tire models so that I can choose an appropriate wheel size, ideally a 13".