Intermittent charge light (idiot light) on the dash

[Luvmeadeadpedal]

I have searched a bit but not found an exact match (forgive me if I missed it).

I have earlier replaced my alternator on my 1991 and my charge voltage gauge reads right around 14v as it should and does not exhibit any strange behavior when I run the heater fan or turn the headlights on.

The last few times I have driven the car I get an intermittent alternator/ charging light on the dash (you know the idiot light) and I look over at the actual voltage gauge and it looks perfectly fine.

It is not a strong light and sometimes sorts of flickers and then goes away which I think might be related to something other than the actual alternator charging.

Give me your thoughts on this.

thanks

[Tuga2112]

I have searched a bit but not found an exact match (forgive me if I missed it).

I have earlier replaced my alternator on my 1991 and my charge voltage gauge reads right around 14v as it should and does not exhibit any strange behavior when I run the heater fan or turn the headlights on.

The last few times I have driven the car I get an intermittent alternator/ charging light on the dash (you know the idiot light) and I look over at the actual voltage gauge and it looks perfectly fine.

It is not a strong light and sometimes sorts of flickers and then goes away which I think might be related to something other than the actual alternator charging.

Give me your thoughts on this.

thanks

this is one that i dont have the time to explain in detail, but i can tell you something to look into in the forum that may lead you in the right direction (with a little luck)

what you describe seems to be related to the ouptut of the regulator not having enough current to power up the o2 sensor as well as stopping the light from coming on.,
being in the US. i assume your car has a o2 sensor.,

what your looking for is the threads in the forum talking about doing a closed loop conversion and after the conversion is complete people finding their alternator lights are always on.
swapping the regulator in the alternator is enought to mitigate the issue, but that means getting the alternator out. so i suggest you contniue and better inform yourself of the issue to be sure its relevant to you before going through that hassle.

a known (good) replacement for the regulator can be aquirred for less than 40 quid, however i wouldnt be surprised if the 15 quid alternatives also work, but i dont know of anyone who tried that, as the risk/reward balance has not been convincing for anyone to try it and document the results yet

[Luvmeadeadpedal]

Thank you for the tip on a possible cause.

I do have an O2 sensor and I just had my alternator rebuilt with new parts given I was previously seeing the light + showing low charge voltage on the gauge.

I new see steady 14v even at idle and the lights / heater fan running high so I feel fairly confident that I have the proper juice.

I will do some more searching to see if I can find some sort of a fix.

thanks

Dave

[Tuga2112]

Thank you for the tip on a possible cause.

I do have an O2 sensor and I just had my alternator rebuilt with new parts given I was previously seeing the light + showing low charge voltage on the gauge.

I new see steady 14v even at idle and the lights / heater fan running high so I feel fairly confident that I have the proper juice.

I will do some more searching to see if I can find some sort of a fix.

thanks

Dave

this regulator 100% works
its the one from simonP kit which i have installe in my car (with close loop conversion)

https://www.ebay.co.uk/itm/115767366218

if you can find the same part in the US, you will need to take the alternator out, and swap the regulator.

i think these 3 videos will cover more than enough about the job.

[RonR]

If you have Elanscan you can check to see if your O2 sensor's working properly, as the power supply for the O2 sensor heater is taken from the Alternator warning light circuit.
You should see the O2 sensor voltage alternating somewhere between 0.1 and 0.9 volts, a steady voltage around 0.45 volts means the O2 sensor or wiring to it is bad.

[Luvmeadeadpedal]

Update -

I did swap out the 02 sensor as I am 100% certain it was original. The light is now no longer intermittent but on fully all the time which seems odd.

The volt gauge is rock steady as just under 14 volts regardless of running the heater fan + headlights on high.

I did have an intermittent light previously + bad gauge readings at 12 Volts prior and I had the alternator rebuilt by a know local guy that only does these repairs.

Voltage - rock solid

Indicator lamp - currently on

Baffling at the moment as all indicators show OK otherwise.

Dave

[RonR]

My guess is your alternator has been refurbished with the wrong voltage regulator.

The alternator warning light and O2 sensor heater supply are driven from a separate circuit to the Alternator output, so you will always see a steady voltage from the Alternator battery terminal if it's working.
However the voltage regulator in the Alternator needs to be able to supply enough current for both the warning light and the O2 sensor heater from the warning light terminal.
There are several versions of voltage regulator which look identical and have the same terminal names, but only some of them can provide enough current for the O2 sensor's heater, which requires extra current on top of the warning light current. If that current's not available the warning light output voltage will be dragged down thus keeping the light on. Your new O2 sensor probably draws more current than the old one, keeping the light on permanently.

As a test you could disconnect the O2 sensor to see if the light extinguishes as it should.

The correct regulator usually has something like "Will drive choke" in it's description. The subject's been covered before, have a look HERE for details.

[Tuga2112]

It's worth noting that although swapping the regulator is the preferred solution.
A circuit can be made (I think all you need is a relay) and installed in the binnacle that will recognize the alternator output and power the o2 sensor directly from the battery.
I never tried it but it was suggested to me by someone who's very well versed in the m100 electrical system and the theory made 100% sense to me when I looked into it.

[Luvmeadeadpedal]

A new circuit would certainly be easier than removing the the Alternator (again!) as I am still suffering PTSD on that one.

So if I just drive the car and ignore the idiot lamp and keep an eye on the volt meter am I doing any harm?

There is only so much summer left and I would rather drive it than repair it at the moment as this seems to be a good winter project.

Thanks all for your collective wisdom in solving the mysteries of Lotus. (cough cough GM)

[Tuga2112]

The honest answer is I dont know.

the fact the light is on, tells you theres not enough current coming out of the regulator to power the O2 sensor and have leftover to stop the light from lighting up.

the o2 sensor will be relying on current coming from the regulator as well as the bulb in order to operate. i dont know if a standard halogen bulb has a lot of resistance or not and if the "leftover power" after giving you light is enough to make the o2 sensor operate correctly. im sure some folk here know this stuff better but there is a certain minimum voltage at which the O2 sensor can operate. and the voltage being feed to it will be defined by a combination of factors. (power feed by the the regulator + "leftover" power after lighting the bulb)

TLDR
you can check the voltage at the sensor while its operating (car is running). check if that voltage is within the sensor operating range (id expect anything above 12v is good, but you may find details about the sensor from the manufacturer)
if the voltage is within range, i think you would be fine.

[RonR]

TLDR
you can check the voltage at the sensor while its operating (car is running). check if that voltage is within the sensor operating range (id expect anything above 12v is good, but you may find details about the sensor from the manufacturer)
if the voltage is within range, i think you would be fine.

As far as I can tell the power taken from the warning light circuit is only used for the O2 sensor heater. The ECU provides it's own reference voltage to the sensor, which the sensor then modulates to give the reading back to the ECU.

The sensor has to reach a temperature of at least 360C (600F) to operate properly, if it doesn't reach that temperature the O2 readings will be too low and the ECU won't allow closed loop operation.

So the question is: Can the warning light circuit provide enough current to get the sensor heater hot enough to to work properly?
Unfortunately the answer is: It depends! Different sensors will have different resistances and so will draw differing currents, and different voltage regulators will provide differing amounts of current on the L terminal.

The best place to check the voltage your sensor heater is seeing is at terminal B of the sensor connector block, the Brown & Yellow wire NY26B.

[Luvmeadeadpedal]

Wow - just wow!

Thanks to both of you for the detailed explainer on this.

I am shadetree ++ with a good set of tools so this is some next level ninja skills here folks.

I will go about checking voltages but as you say - best just to get it sorted correctly.

I had thought I had it fixed when I had the alternator rebuilt and was all well until the intermittent light which might have been the O2 sensor going out as it was obviously original. The new Bosch part seems to be drawing more juice and if I going to throw $$ at parts I might as well do the right thing an get correct regulator as much as it kills me to think about pulling that (*&%!! alternator out again.

thanks again!

[chrism]

Regarding O2 sensors, to make my '90SE 'go 1499' in early '22 I bought an SJ sensor, which seemed to be a Denso from the shape of it.

An EScan showed it to take getting on for 400s to 'get going' - shown by the blue trace - which I thought was typical and okay...

When the recent new EScan access for win11 stuff (thanks folks) became available I EScanned and found my air:fuel ratio had reduced. Realising that the extra fuel cons + higher MoT emissions meant that things weren't as well as before, I suspected the sensor and replaced it with a Walkers sensor from RAuto - only to find out afterwards that the cause was that the sensor wiring had de-pinned behind the driver wheel arch (And been like this for a couple of years )

Anyway, the new Walkers was already in place and an EScan (black trace superimposed) showed it to be operational in mere seconds. From memory its resistance was ~4ohms and the Denso was ~15ohms - so these sensors seem to vary an awful lot...

(In retrospect) I suppose I'm happier with the solid performance of the Walkers so I don't really regret the expense of replacing it...(anyone want my old Denso? )

[Luvmeadeadpedal]

So I went out and did the simple test an unplugged the O2 sensor and the light went away.

Culprit right there. Seems Bosch O2 sensors are power hungry beasts.

I did order the Wagner version from rock auto because spending the $28 bucks is better than suffering through a removal. It was worth an attempt anyway.

I will update as appropriate.

[chrism]

Just to confirm, the Walker sensor I received was this (lower) one:-



The old SJ offering is above it for wire length comparison (one of the Boschs was way too short )

I've just looked on RAuto and their offering is now different and now looks very much like a Denso profile (curiously, just like the pic on 'my' box - which I assumed to be a generic pic).

As you can see 'my' Walker looks very much like a Bosch, so if anyone's looking for similar EScan results to mine please be wary and, perhaps, look for this particular product code?

I can confirm that this Walker works fine in my SE, gets up to working temp very quickly and has the correct m100 wire length

Hope this helps - recently there seemed to be a shortage of sensors in UK...

[Luvmeadeadpedal]

So from what I have been sleuthing on the Internet three wire O2 sensors the two matching color wires are the heater.

I am trying to sort out which one is + and - upstream of the plug because after the plug the two white wires are the heater and the black one is the actual O2 sensor. I want to try and put in a relay directly to the battery to power the heater directly using the + side of the 02 sensor wire as a trigger for the relay.

A one wire O2 sensor will work but not get up to temperature as fast as one that has a heater. A one wire O2 will still function and give readings but albeit at a slightly delayed timeframe.

Given this O2 sensor is right on top of the turbo and directly in the path of the hot exhaust the 600-700C would likely come up pretty fast.

If I take the positive wire from the 02 sensor and use it as a trigger for a 30A relay with a direct path to the battery would this suffice.

I can't imagine the 1990's technology ECU had much other than some sort of timer to run the heater at startup for a set period of time because once the engine is up to temp and running the required temp for the O2 sensor to do its thing is already there.

I might be crazy but this may be a way to solve this issue and many are thinking just go get the alternator fixed (again).

Dave

[RonR]

The O2 Sensor heater is always on when the engine is running as it's normally driven from the alternator's warning light circuit. The two same colour wires just connect to the heater, in my Bosch sensor they aren't connected to anything else. The ECU doesn't control the heater at all, it just provides a small reference voltage on the 3rd wire (~0.45 Volts) for the sensor to modulate as the fuel/air mix varies.

The + wire you need to intercept is the Brown/Yellow wire NY26B running from the splice behind the dash to the sensor connector by the RH engine mount. Try to find a relay with a high coil resistance so it doesn't interfere with the warning light operation. A 10A relay would be enough, but I think Automotive relays start at around 20A.

Also - the warning light circuit voltage is variable as the light gets dimmer/brighter, so you need to be aware of the "ON" voltage of the relay, some trigger at around 8V.

[Tuga2112]

It's worth noting that although swapping the regulator is the preferred solution.
A circuit can be made (I think all you need is a relay) and installed in the binnacle that will recognize the alternator output and power the o2 sensor directly from the battery.
I never tried it but it was suggested to me by someone who's very well versed in the m100 electrical system and the theory made 100% sense to me when I looked into it.

your relay needs to have the coil energiged by the brown yellow stripe wire (the one that makes the light go off), and the "power path" goes straight from the battery to the wire that goes to the O2 sensor
you need to make sure the wire from the sensor is not connected to the brown/yellow wire for this to work and avoid a feedback loop (which would both drain your battery and keep the sensor permanently heating)

the problem with these circuits is that i can think about the way they should operate. but without actually setting them up, i dont know if there are risks in the way they operate which would be apparent after instalation.

ive onse added a parasitic drain to my car when installing a sidelights reminder buzzer. it turned out when the buzzer was not operating. it was draining power in the reverse direction to what i expected. adding a diode resolved the issue and its been running fine for more than 4 years now.