Bill Ball

I had speedo and other instrument problems when the ignition switch was starting to fail. It took me a while to discover that wiggling the key would temporarily alleviate the problem. Replacing the electrical portion of the switch (less than $60) permanently fixed it.

Jim M.

II just had an ignition switch fail on my 93 GTS and didn't have any speedo problems. What would happen is I would lose Cruise Control and wipers. No warning lights on the dash!

I also had a ABS warning with loss of speedo about a year ago. Turned out to be the ABS relay on the central electric panel. My RDS has been disabled and removed long ago so I didn't get that warning. Greg928GTS try to swap the relay with a known good one. All of the S4's use the same relay.

Jim Mayzurk
93 GTS 5-spd

worf928

Yes.

(I'd prefer that Dr Bob or Alan explain this, but, since they haven't weighed-in, I'll give it a shot...)

A relay is, for the most part, a simple switch. A low-current signal is used to mechanically switch a high-current load. So, to test a relay you need 12v to 'jump the switch.' I use a 2A 12v power supply.

The process is pretty simple:

1) examine the wiring diagram for the relay and determine which pins are the 'switch signal'

2) apply 12v with the correct polarity to the switch pins.

3) listen (or feel) for a 'click'

Using the 'bosch 53' relay as a simple example:

Pin 30 is the high-current positive.
Pin 87 is the high-current ground.

When current flows through 30/87 the relay has 'clicked' closed.

Pin 85 is the switch-circuit positive.
Pin 86 is the switch-circuit ground.

So, when grounded at 86 and with 12v at 85, a working '53' relay will click. A multi-meter will show continuity between pins 30 and 87.

This is the simplest test. It will tell you if the relay is dead. It may not tell you if the relay is able to handle high-current through the 30/87, but, to date, this simple test has been sufficient for me.

For the more-complicated relays, look at the diagrams on the central electrics page to figure out any operative differences from the '53' relay. For the '53' relay you can get the same behavior with reverse polarity on 85 and 86, but this is not true (or perhaps safe) for the complex relays with diodes and other stuff in them.

dr bob

Dave C nailed the relay functionality.


My simplistic test protocol for the '53B relays is to plug in a new one. and see if it fixes the problem. As much as I love diagnosing with jumpers and meters and stuff, a new relay ($7-15 each depending on quality and source, plus time-of-day, location, and air temperature), is cheap enough that you should have one or two new ones riding in the car at all times. Having a known-good spare, a relay chart and a removal tool pretty much guarantees that you'll never need to use it on your own car. Your car is aware of the spares you carry, trust me... But it's there for a friend when he/she calls to report that they are stuck someplace remote, maybe an owe-me-a-beer distance away.

Seriously, the road-warrior toolkit should have at least a spare, a removal tool that you have tried and demonstrated in your garage/driveway, and a relay chart. My travelling toolkit is quite meager-- a set of mini-jumper cables to boost a low battery, a relay and the tool, a simple jumper with a switch, a few spare fuses, a few spare bulbs for the front, a buch of old towels and a spray-can of window cleaner. Important stuff!

There was a thread on making labels for the relays a few months ago too, eliminating confusion as to which relay does what. Find the thread and take advantage if you can.

Oh yeah-- disconnect the battery ground before yiou start prying a relay from its socket with a conductive tool.

greg928GTS

Are you suggesting that I should buy replacement relays for all of the different relays in my car? Serious question.

When you say "one or two new ones", how do you determine which one or two to have?

SteveG

I think Bob means a spare of the more common ones to go (Fuel Pump, Ignition). The "53" covers about half of what is in there. IIRC there is one that is $65, I don't think I have that one.

Kevin in Atlanta

Greg, did you ever resolve this? My ABS , differential lock warning lit up tonight. Did not notice if the speedo was dead.

Tails

Greg,
You vehicle has an on board diagnostic function built into the Instrument Cluster and the description of how to operate this is included on JM's Technical CDs, titled "Diagnosis of the Instrument Cluster for 928S4 '89 Information Technik".

Porsche state that you need a special tool No 9293, however, all that is required is to insert a jumper lead between pin 5 and 13 in the 19 pin Diagnostic Socket in your car. It is located under the cover of the passenger's side sill. The designation is Terminal T46, number of pins 19, location 11eK. Pin 5 has a green wire with a vilot stripe and Pin 13 has a red wire.

I you also read the 1987 through to 1993 Service information Booklets, also contained in JM Technical CD's, you will get an excellent description of the ABS, PSD and Instrument Cluster interconnections. You will need to flick through each year to gather the upgrades as they occasioned with the introduction of each new item of equipment installed into each MY. If this upgrade is carried over to successive years it is not repeated in your MY GTS, hence the need to flick through each year from the introduction of the 87S4 through the GT and into the GTSs.

I have also read fault diagnostics on the ABS, but at the moment I cannot put my hand on it, but I sure you will find it in JM CDs.

Tails 1990 928 S4 Auto.



This on board diagnosis can chech thte correct functioning of the instrument cluster as well as a diverse switches and sensors in the vehicle can be checked by means of this on board diagnosis. During diagnosis th instrument cluster, the instruments's three display boxes are used to indiacte the faults.

MGW-Fla

A common known reason to get both the Diff.Lock Off & ABS Off warnings is from a dirty wheel sensor. Not having the Speedo go out too indicates its not the sensor that operates for the speedo.

Kevin in Atlanta

Speedo is dead too. Swapped relay n/c.

Kevin in Atlanta

Brought the GTS by Hennesey Porsche to get a reading. $$$

They could not pull any codes because there's no power to the unit. Should be a fuse. Checked fuse 28 and it's good. Swapped out relay XV with a spare I have.

No change.

worf928

AFAIK, Greg's issue was resolved with the application of a new ABS relay (his was dead) and a new fuse.

Kevin in Atlanta

I checked the fuse and the relay. They are ok. I am puzzled why there is power to both sides of the fuse and 30 on the relay with the car off and the key out of the ignition.

Is there any possibility that the shop that jacked up the left rear of my car to fix a flat could have moved the speed sensor out of alignment?

Rob Edwards

Driving the GTS to Greg's shop on Tuesday, hit a bump on the freeway, lost the speedo, the ABS light lit up, and got a PSD and Diff control warning on the digidash. Greg first discovered that I'd forgotten to tighten the bolt holding in the front left ABS sensor, but tragically that didn't cure it. But a new left front ABS sensor did.

James Bailey

Glad that took care of it !!!!