mrdieseldude

The 928 uses two relays that are almost identical; the only difference being a resistor in parallel with the coil (terminals 85 and 86) in one of them. Porsche p/n: is 141 951 253 B is for the relay without the resistor, and p/n: 928 615 203 00 includes the resistor. Can anyone explain to me what the purpose of this resistor is? I do know that some relays include an internal diode across the coil leads to suppress noise/voltage spikes, and I do understand why this would be a beneficial feature in some circuits. The resistor seems like it would serve a similar, albeit less effective, function. To make things even more confusing, the wiring diagram shows p/n: 141 951 253 B (w/o resistor) as used in the Ignition System Circuit (relay XVI) for model year ’85-’86, and p/n: 928 615 203 00 (with resistor) for model year ’87-‘88. If anyone can explain what the relay with the resistor really does I would be grateful. Also, do the ’85-’86 cars have a different ignition system than the ’87-’88 models?

borland

The second part number is a diode protected relay, but also incorporates a LED fuse tester in the top of the relay cover. No resistor.

It only serves to allow testing CE panel fuses on later CE panels.

borland

Also, the WSM wiring diagrams do have errors and may show internal component parts (like in the relays) that are inconsistant with the wiring diagram legend.

mrdieseldude

borland,

The diagram on the wiring schematic shows this as a, well, a rectangle. I believe it is a resistor for these reasons:
1. Other diagrams in the manual do show diodes, and they are depicted like a diode should be... e.g. like a diode.
2. I do know that relays are manufactured with either:
a. nothing (between 85 and 86) - this is most common.
b. a diode (I know what this does)
c. a resistor (not quite sure what this does)
3. I have inspected one on the bench and it acts as a resistor.

Therefore, I have concluded that this "rectangle" is the German way to depict a resistor.

Why do you believe this to be a diode and not a resistor? Also, for the sake of keeping this question as simple and focused as possible, let's ignore the built in fuse tester of this relay as it has no bearing on the rest of the circuit (or my specific question/concern). Of note, however, and at the risk of adding confusion, this same relay, p/n: 928 615 203 00 is also used for the defroster relay (relay VIII), yet it does not have the built in fuse tester (but still the same p/n)... I don't know how they can have the same part numbers... is the manual wrong???

PS - the fuse tester does have a resistor (which is it's own dedicated resistor), but this is not the one in which I refer.

borland

OK, its a resistor as depicted in the wiring diagram.

I think a diode seems smarter, but a resistor would reduce back emf too. Maybe someone else has more knowledge about why they used a resistor.

The protection serves to protect the controlling semiconductor circuit that drives the relay coil. Something a mechanical switch doesn't usually need.

mrdieseldude

Yes, yes, and yes. You and I are in agreement, but I don't think that answers the question. At the very least, why is the non-resistor relay used in the '85-'86, while the resiter relay is used in the '87-88? Any other takers with a good electrical background/understanding out there willing to give this one a shot?

Alan

The only difference I see in the wiring diagrams between these years is that one is a fuse tester SPST (with additional test components: resistor, LED & socket) and the other is a regular SPST (...53b) relay. Both of these have resistors across the coil - what am I missing...?

On '87S4 - '95 GTS I see no protection diodes depicted used on any relay - for '86 I see 1 shown on the VI Window Regulator. The other diodes depicted serve different purposes.

In general both resistors and diodes serve to supress switching spikes. Diodes work better - either a standard diode or a Zener with reverse breakdown @ ~20v. better here means they limit the spike voltage better.

Resistors are less good at voltage limitation but are much cheaper and are still suitable in many cases. They are also are non-polarized so allow alternate relay coil configurations with possibly bi-directional activation.

If the relay coil is driven by a switch or a bipolar O/P stage - a resistor is likley to provide suitable protection. For a relay driven by an MOS device the better doide supression may be needed (but often these and/or supply resistors are built in to the drive stage anyway). Porsche driver designs in the 928 typically have bipolar O/P stages.

Alan

mrdieseldude

Thanks Alan,

My reference to the '85/'86 Ignition circuit not using the resistor relay vs. the '87/'88 using the resistor relay is from the relay/fuse chart as found on the 928International web site. I have not figured out how to include attachements, so this verbal description will just need to do. Thanks for the confirmation that the resistor is used for the same purpose as the diode, albeit less expensive and less effective. That was my hunch. That said, it seems like the resistor relay would be a suitible substitue in all cases, but not vise versa. For the pennies difference in cost, I don't understand why Porsche would consider using the relay w/o the resistor. Just the cost of changine the repair manual and schematics outweighs the small cost savings they would have in the relays. Is it possible the earlier car uses a bipolar output on the ignition brain, while the later ones use MOS technology, and therefore requiring the use of the built in protection in the relay? If so, this would be consistant with the schematics.

Are you saying that a MOS circuit can't handle the voltage spike of the relay's coil, whereas a bipolar circuit can? I just assumed neither would put up with it for long?

Every thing that I have said so far is from the reference manuals/charts. I am going to check what relay I have my '86's Ignition System circuit this evening. The book shows it as the non-resistor relay, yet I think I have the resistered version. Has it been changed or maybe Porsche just substitues the (better) relay. I will report my findings.

mrdieseldude

Alan,

Your comments are correct. Per the schematics, these years are essentially the same, yet, the parts list, refer to attached link below

http://www.928intl.com/repair/Relay/earlyrelay2.htm

shows p/n: 141 951 253 B (no resistor) for the earlier cars, and p/n: 928 615 203 00 (with resistor) for the later cars. Will this discrepancy cause any problems?

PLEASE NOTE: this is relay XVI per Porsche numbering scheme, and number 8 on the attached link.

Also, the schematics DO show relay XXV (LH Jetronic) as being different between the two years. Does this really make a difference? What is the difference between the two fuel systems that would really require different relays?

I know this is getting pretty detailed... thanks for listening.

Landseer

Yes, I think so.

Maybe some of your questions would be answered by reading year-by-year the "service information by year" technical documents in section 2.15 of the Moorehouse CD's.

Alan

I think in fact Porsche is just very loose with the diagrams here. A '94 Porsche uses many 141 951 253B relays (its the default relay) - and they all do in fact include a resistor. There are examples of relay positions that do use a 53b relay that are shown both with and without resistors in the diagrams - the car is equipped with the same relay in each case and this is typical of all S4+ cars I have seen.

Not so certain for earlier years but most also use 53b relays extensively (I'd assume the same diagram issues may exist).

The change in relay type for EZK was primarily driven by the relocation of the fuse tester relay - nothing more functionally (these fuse test relays weren't used before '85 - different fuses style). BTW my car came without a fuse test relay - just a 141 951 253 B in its place - I'm fairly sure it was delivered that way, The fuse test relay 928 615 203 00 is directly compatible with any other 141 951 253 B relay - mine (added later) is in fact now installed in the horn location...

Alan

mrdieseldude

I think that is the answer I was looking for... more or less. THANKS. e.g. loose diagrams, and most (all) of the relays include a resister even if it is not shown on the diagram. I think I know what you mean by a 53b relay, but perhaps you can confirm? Also, I would like to get a set of the Moorehouse CDs that Landseer refers to, but I haven't found any reference to them. Here is where the newbee gets chastised... I know... but I've searched and I can't find where to get them. Any help is appreciated.

Thanks

Landseer

The CD's used to be bought from Rog100, one of the sponsors.
You will find incredible information on them, in addition to the electronic WSM and PET and wiring Diagrams.

Alan

53b = 141 951 253 B (some are acually labelled with a big 53B on top of the can).

Alan