Replumbing 12V to the Instrument Cluster

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30 Jan 2013 21:14 #13414 by Rich928
My '79 928 suffered from a very dim back light, volt meter reading low (would go down to the red at idle with lights on) and the temperature gauge was reading too high (about 20% hotter than it should be).

I confirmed that the alternator output is proper with a volt meter and that the engine temperature was normal using a heat sensor gun.

I mentioned this to my shop, Exotic Motorwerks. Super mechanic Dan figured out a way to pull 12V directly from the Central Electric, used a relay to control switching, and I now have the full 12V to the instrument cluster.

The Voltage and temperature gauges are now showing true values and the back light is bright.

He also wired it such that I can switch back to the original "low voltage" circuit that Porsche used.

It is surprising how much better you feel when the gauges are where they should be. I knew that nothing was wrong ... but always staring at the wrong values was a downer.

Rich
'93 GTS cover girl
'87 S4
'79 5-speed
'79 Euro track beast

Rich
Membership Chairman
Charter Member
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast

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31 Jan 2013 16:07 #13426 by ROG100
Replied by ROG100 on topic Replumbing 12V to the Instrument Cluster
Rich,
That is very interesting and I have a similar issue on one of my 82 cars.
Any chance of more details for those of us who are electrically challenged?
Best,
Roger

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31 Jan 2013 19:13 #13430 by 928mac
Replied by 928mac on topic Replumbing 12V to the Instrument Cluster

Rich,
That is very interesting and I have a similar issue on one of my 82 cars.
Any chance of more details for those of us who are electrically challenged?
Best,
Roger


Or for a better term electoral difficulty :D

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02 Feb 2013 23:58 #13448 by Rich928
Replied by Rich928 on topic Replumbing 12V to the Instrument Cluster

Rich,
That is very interesting and I have a similar issue on one of my 82 cars.
Any chance of more details for those of us who are electrically challenged?
Best,
Roger


I'll see if my mechanic is willing to part with his trade secrets ... or provide some hints.

My '87 S4 also suffers from low battery voltage display - it seems that most 928s have the problem. Where and why the voltage drop, I don't know.

Rich

Rich
Membership Chairman
Charter Member
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast

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03 Feb 2013 09:46 #13450 by emmanuelli
Replied by emmanuelli on topic Replumbing 12V to the Instrument Cluster
my car showed a low voltage when i bought it last may. perhaps you have all done this but i was still going over things little by little then.

i took it to a radio shop to change the head unit etc and they did notice voltage in general was kinda low. a simple cleanup of the ground point in the rear brought the voltage all around up quite a bit and also allowed the head unit to play up to snuff.

still i do have some other electrical issues...

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03 Feb 2013 15:39 #13453 by pcar928fan
Replied by pcar928fan on topic Replumbing 12V to the Instrument Cluster
That is the most common cause of low voltage in a 928... dirty ground points and there are something like 13 of them in an S4!!! YOWZA!!! Find them all and clean them up and you will be MUCH better electrical power in a 928!

James
78 Silver / Black-white #295
84 Ruby Red / Black AO84
88 Dark Blue / Linen-Black
92 Polar Silver / Dark Blue 92EURO
93 Arrow Blue / Black

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04 Feb 2013 16:27 #13460 by emmanuelli
Replied by emmanuelli on topic Replumbing 12V to the Instrument Cluster
this is not new, i found it on a french 928 site:

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I have started on mine, doing one here one there as i can.

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30 Mar 2013 00:27 #14118 by Rich928
Replied by Rich928 on topic Replumbing 12V to the Instrument Cluster
I've written up a tech article for submission and publication to the OC. The fix is conceptually simple, but requires studying the schematics for each model year 928 as many changes were made over the years. My article doesn't provide exact instructions as they can be very different for different model years.

For my 1979 928, the instrument cluster is powered by an ignition switch controlled line (15 circuit in 928 schematic speak).

What the fix was for me was to cut that 15 circuit (wire) before it plugs into the instrument cluster. That circuit became the "low current control" voltage for a 12V relay (the other end of the low power relay control is connected to ground).

Next, a tap was made from a central "12 V always on" (30 circuit) spade that was available in the central electric panel (fuse panel), a 5A fuse was added in series for safety. This relay switched circuit now becomes the 12V circuit input to the instrument cluster.

The ground connection to the instrument cluster was cut and connected to a separate wire that was run to an available ground spade in the central electric.

It sounds easy, but it does require studying the schematics and finding the correct wires/circuits.

Rich
Membership Chairman
Charter Member
1993 928 GTS Cover Girl
1987 928 S4
1979 928 5-speed rescue
1979 928 5-liter track beast

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