[968Syncro]

Hoping somebody is an EE, or remembers enough of circuit diagrams.



This is follow up from the ABS light problem I wrote about here: http://www.968forums...domly-comes-on/ where the !warning / ABS light comes on. On that thread "unknown4u" had an excellent suggestion that it might be the ABS relay.



I would like to either test or bypass the ABS relay to confirm:

- test: probably not possible, it is intermittent.

- bypass: it comes on frequently enough that a 15 minute drive would confirm that this is the cause. BTW, frequency and duration of failure both increase with driving time or perhaps engine compartment heat.

Reason: they are darn expensive: $140 at the local PCar dealer, about $70-$90 at various after-market places, $50 used at DC Auto. I'd prefer not to throw parts without a known root cause.



Take at look at the schematic I clipped from the wiring diagrams.

   



30 (7) - goes to the ABS fuse -- obviously power in.

31 (3) - goes to ground.

15 (2) - goes to the stoplight switch.

87 (5) - splits to (10) to the hydraulic controller, and (1) to the ABS computer.



I'd can't remember what the 3 "components" inside the relay do; the switch is obvious. So I was planning to bridge from 30 to 87 and see if the warning light stops coming on. However given the stoplight switch is a signal it makes me think that normally the ABS is active only when braking is active. If I bridge from 30 to 87, does that mean ABS will be active all the time? Perhaps this isn't a big problem, since the computer should sense the wheels are all rotating equally (except on a tight turn perhaps, I don't know its calculations). Or could this be a problem / risk in some other way?



Thank you!



Roland



PS: and then the bonus question; perhaps the contacts on the switch are just corroded. Why not open it up and try some repair?

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[MB968]

Not an EE, but an ME. But, if memory serves, virtually all 4 pin relays act the same. A low current circuit is closed by something like the DME, or another sensor, or a switch, or.. This in turn closes a high current circuit that powers things like the fuel pump, the starter, etc. Hope this simple explanation helps.

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[MB968]

And to your second question. If you have an old points file, give it a try, but if the electromagnet is shot, it won't fix it.



Plus, many of these points are plated so that arching won't cause oxides to build, causing a high impedance, resulting in failure to perform as planned. So, if you file them, you may have a much shorter life that the infinite life that we'd like to have on our car electronics. I'd just purchase a new one. But, if you want to play, have at it with the file. Or alternatively you can use a good very fine grit carborundum paper wrap on a thin parallel sided blade instead of the file. You can also check the electromagnet side of the relay by checking the resistance across those two pins. You should be able to find the range of resistance that implies a good winding on the electromagnet.

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[968Syncro]

Thanks guys, I guess I am still hoping for more insights. For MB968, yes I understand the basic 4 pin relay, but I looked up a couple of symbols, this one has more than a simply electromagnetic switch, it has a resistor, some sort of diode and a varistor and so am wondering what these other components might do in this case. The diode is probably there to make sure current doesn't flow back to ground ... but why is that connection even needed? I wondering if this circuit is creating some kind of "AND" or "OR" function.



But mainly just wondering if bridging (bypassing) from 30 to 87 directly might cause some other problem, and concerned that this circuit is providing some type of protection which I might screw up by bypassing the relay.



I guess the thing that makes me wonder is if this relay is active in some way without the stoplight switch being active. The warning light comes on while just driving without stepping on the brakes. So that would not be the case if this was a simple 4 pin relay of the usual type. And so wondering if this circuit is doing something else.

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[MB968]

This is likely the reason for the diode:





TRANSIENT PROTECTOR



http://www.technologystudent.com/images2/dode1.gif



When an ‘inductor’ device such as a relay is turned off a high voltage can be generated for a short time (Dia 1). This voltage ‘spike’ can damage the relay and other components. However, the diode does not allow current to pass through it in the wrong direction and short circuits this spike.



And, are you sure that #15 isn't the power in for the electromagnet. Looks to me like it would be. So, when you put your foot on the brake, and the brake light switch is powered, it in turn puts power on pin #15. This activates the electromagnet which closes the high current circuit to power the ABS. And, I think the resister in the low amp circuit is there (would be sized) to make sure the current flow in the electromagnet is controlled to the right range.



But, again this isn't my speciality so I can't speak with absolute certainty. Have touched some electrical stuff over the years, so I don't think I am too far off base, if at all. Like to see someone with practitioner knowledge chime in.

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[Peter B]

Have you done the basic checks on the ABS system?



1) Metallics get on the probe magnets. This deteriorates the signals.

2) The wires from the sensors are right there in the wheel well. They can fail.



If you haven't already,



1) Take the probes off and clean them.

2) Inspect your wires for dodgy-spots (near brackets, torn coverings, so on).

3) Disconnect the ABS leads from the ABS unit. (I don't know where to do this on a 968).

4) Hook up a volt meter to the two leads. As you turn the tire, you should see the voltage increase. I'm sure the "normal" range is in one of our manuals somewhere.

5) I'd then move your wire around a bit in the wheel well as you turn the wheel. If you can cause the voltage to drop, like the wire is open, then your wire is shot.



I used this method to troubleshoot an intermittant ABS light issue on an MB 190E 16V. I found a spot where the wire was bending over and over and was on the verge of breaking.



I hope you have a sense for which wheel is the issue.



Is the Porsche computer smart enough to isolate the failure location?

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