[928uk] bulb monitor ecu

Mon Jun 3 18:55:57 UTC 2024

I make no claims to know anything about the older bulb monitor units. The info I gave was about the 87 onwards S4/SE/CS/GT/GTS bulb monitor units, because as I have said already that is what was being discussed since the start of the thread!

Out of curiosity I have now read a little about the older units and they apparently used current through counter-wound coils around a magnetic reed switch as a detection mechanism. If the left and right bulbs’ currents were sufficiently different the resulting magnetic field closed the reed switch. Unfortunately, the left and right bulbs comparison done by older units has become a repeated myth about the newer units working the same way. Let’s just blame rennlist, much as we may love it 😊

If anyone has a circuit diagram for the older type bulb monitor units or photos of the inside I’d be interested to see them.

Best regards,
Jonathan

1991 928 GT Amethyst Metallic

From: Paul R Smith <smiffypr at gmail.com> 
Sent: 03 June 2024 19:22
To: jonathan at rackowe.net
Subject: Re: [928uk] bulb monitor ecu

You didn't say which ones you were talking about at all, so it would be natural for anyone reading it to believe that you were referring to them all. 

And you referred to "the myth", which is not a myth, it is true, but does not apply to the ones you had in mind at the time.

If you knew that the old ones were different, would you not have worded it differently?

Smiffy

On 03/06/2024 18:29, jonathanrackowe at gmail.com <mailto:jonathanrackowe at gmail.com>  wrote:

You weren’t correcting anything, because I didn’t say anything about pre-1986 bulb monitor units. George was talking about a high level brake light and the bulb monitor in his GTS, which is the type used in all post-87 928’s, and those are the units I was referring to also.

Best regards,
Jonathan

1991 928 GT Amethyst Metallic

From: Paul R Smith  <mailto:smiffypr at gmail.com> <smiffypr at gmail.com> 
Sent: 03 June 2024 16:42
To: jonathan at rackowe.net <mailto:jonathan at rackowe.net> ; 928uk at lists.928.org.uk <mailto:928uk at lists.928.org.uk> 
Subject: Re: [928uk] bulb monitor ecu

You said that the balanced current to the tail lamps was a myth, I was correcting that, it is true, but only up to '86.

Smiffy

On 03/06/2024 14:51, jonathanrackowe at gmail.com <mailto:jonathanrackowe at gmail.com>  wrote:

But this discussion is about the 87 onwards S4/SE/CS/GT/GTS bulb monitor units and has been since the start!

Best regards,
Jonathan

1991 928 GT Amethyst Metallic

-----Original Message-----
From: 928uk  <mailto:928uk-bounces at lists.928.org.uk> <928uk-bounces at lists.928.org.uk> On Behalf Of Paul R Smith via 928uk
Sent: 03 June 2024 14:01
To: 928uk at lists.928.org.uk <mailto:928uk at lists.928.org.uk> 
Subject: Re: [928uk] bulb monitor ecu

Jonathan, you looked at an '87 onwards version, the earlier ones are 
different. The tail lamp warning does work on balance, and the brake 
lamp circuit needs a minimum current (not a window).  I have had my 
towbar fitted to my '80 and '84, and I would get a warning if only one 
trailer tail light was working, and I didn't get any warnings caused by 
the extra load.

Smiffy

On 03/06/2024 13:07, jonathanrackowe--- via 928uk wrote:

George, you are repeating the old myth that the bulb monitor compares the load one side with the other. As I have said before at least twice on this list, it simply does not work that way.

The bulb monitor separately monitors the current through each brake light bulb and through each set of tail light bulbs. The unit contains four 'window' comparators to check that each of the four monitored currents is within an absolute 'window' - i.e. not too high and not too low. That way it can detect bulb failure (open circuit or short circuit) or an incorrect bulb (too little or too much current flow).

You are correct that the 3rd brake light is connected in parallel with only one of the brake light bulbs (left side I believe) and so the bulb monitor is modified to have a different 'window' of acceptable current for that specific brake light.

There is nothing in the wiring diagrams to indicate that the Porsche mobile phone kit was connected in any way to the bulb monitor. I suspect I was repeating a rennlist myth myself there!

Best regards,
Jonathan

1991 928 GT Amethyst Metallic

-----Original Message-----
From: 928uk <mailto:928uk-bounces at lists.928.org.uk> <928uk-bounces at lists.928.org.uk>  On Behalf Of George Layton via 928uk
Sent: 03 June 2024 11:46
To:928uk at lists.928.org.uk <mailto:To:928uk at lists.928.org.uk> 
Subject: Re: [928uk] bulb monitor ecu

  From what I recall when I fitted a 3rd brake light to my GTS (it was
many years ago and my brain is now old) the light takes its power from
the circuit on 1 side at the rear. I understood that the bulb monitoring
unit had to be changed to allow for the now "unbalanced" load comparing
one side to the other.

Perhaps the mobile phone kit drew power from the circuit on one side of
a pair being monitored by the unit and so the unit had to be updated??

George.

On 03/06/2024 11:18, jonathanrackowe--- via 928uk wrote:

Yeah, I struggled to see the connection between the bulb monitor unit and the 90's mobile phone kit… but then who would doubt the infallibility of rennlist info!! 😃 (joking comment for those reading this in plain text without emoticons)

Maybe 928 641 603 10 flashes the brake lights if you're on the phone! 😉  (another joking comment for those reading this in plain text)

Best regards,
Jonathan

1991 928 GT Amethyst Metallic

-----Original Message-----
From: 928uk <mailto:928uk-bounces at lists.928.org.uk> <928uk-bounces at lists.928.org.uk>   On Behalf Of Paul R Smith via 928uk
Sent: 02 June 2024 16:12
To:928uk at lists.928.org.uk <mailto:To:928uk at lists.928.org.uk> 
Subject: Re: [928uk] bulb monitor ecu

I searched on this 928 641 603 10 in my PET, and it does find it, but it also shows

Market as USA.

It quotes this number for up to 1990 with or without option 612 (prep

for phone) and 1991 onwards with no mention of option 612.

I don't know what the link is with the phone kit, but it looks like it

is a red herring. 10 appears to be for all US versions 1987 onwards.

Smiffy

-----Original Message-----
From:jonathan at rackowe.net <mailto:From:jonathan at rackowe.net>    
Sent: 02 June 2024 11:53
To: phil rick <mailto:philrickse at gmail.com> <philrickse at gmail.com>
Cc:928uk at lists.928.org.uk <mailto:Cc:928uk at lists.928.org.uk> 
Subject: RE: [928uk] bulb monitor ecu

If anyone ever comes across 928 641 603 10, that was a version of the bulb monitor with modification for in-car telephone use.

928 641 603 09 Bulb control unit - rear lights (on left of passenger gloves shelf in UK)

928 641 603 10  Bulb control unit - rear lights with modification for in-car telephone use

928 641 603 11 Bulb control unit - rear lights with additional high rear brake light

Best regards,

Jonathan

1991 928 GT Amethyst Metallic