Mark--

I replied to this thread on-the-fly yesterday (literally between planes) from my PDA but it didn't make it I guess.

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Are we diagnosing the X-bus relay or the one in the original post diagram?

The 31 terminal on the X-bus relay follows the terminal path you describe to the XIV (starter relay) trigger to the starter solenoid, so you have a chance to put your test light on that to help isolate the failing connection. You'll also find that the cicuit to the starter solenoid passes through the 14-pin connection (T-18, pin 14) by the jump post, and runs through the front-of-engine harness to the starter solenoid. A poor or loose connection at the 14-pin would be the next focus I think, since that's relatively accessible and easy to clean/inspect. A loose connection there will 'arc over' with the starter solenoid's high current load but my be loose enough to go intermittently open when just the X-bus relay coil current is passing through.

That connector is not quite as bulletproof as those used on most other mass-market cars. More modern connectors have better tensioning on the individual pins, a locking mechanism of some sort, o-rings on the housings if not the individual stabs, and more waterproof pass-throughs where individual conductors enter the housings. Many 928 harness connectors look like they are hand-assembled, and intended just for use where there is no chance of exposure to the elements, vibration, or other mechanical strain. After relay-relay-relay and ground-ground-ground, connector-connector-connector is the next logical weakness to investigate. IMHO anyway.

 

Bob,

We are diagnosing relay IV on an '89 GT, which the wiring diagram (on page 97-331 in J93) refers to as "Relay X," but which appears to be connected via terminal 86 to the 15 bus rather than the X bus.

The wiring diagram is proving fairly unhelpful.

Page 97-331 (the CE wiring diagram) shows terminal 85 on relay IV going to Q13 (and thence to the starter solenoid), but Page 313 (the lights wiring diagram) shows that terminal going to MP V.

The 1988 wiring diagram, on page 97-303, shows terminal 86 on relay IV going to X and VI 86, which explains why it was called "Relay X"; the name must have carried over from '88 to '89 while the unction changed.

The 1990 wiring diagram, on page 97-361, is entirely different.

All of this is just by way of delaying the inevitable: removal of the CE panel and tracing the wire paths myself.

(Where is the X-bus relay, by the way?)

 

Relay IV is the X-Bus Relay, supplies power to bus X when there is a difference in potential between the 15(directly from ignition switch in 'run' or 'start' positions) and 50 (indirectly from ignition switch 'start' position, through Starter Relay XIV terminal 87). When the ignition is in 'run' but the starter is not engaged, Relay IV is pulled in, and power passes from Bus 30 (Battery direct always positive) to bus X.

To function, the relay needs to see voltage from 15 -and- a low-impedance path to ground through the starter solenoid. You were checking the 85 terminal on relay IV, and found that there's an erratic connection between 85 on the relay and the starter solenoid by watching for loss of that low-impedance path through the solenoid.

The path of current flow is circuit 15 (true when ignition switch is in 'run' .or. 'start' positions) -> Terminal 86 on relay IV -> coil in Relay IV -> Terminal 85 on Relay IV -> Joins the circuit between Terminal 87 on Relay XIV (the starter relay) and CE panel external connection O13 (connection at drawing coordinates E89, inside the CE panel). O13 has the fat Yellow wire as part of the harness that runs on the right side of the engine bay, under he edge of the fender, to connector T18, pin 14. On the engine side of that connector, the wire is also Yellow through the Front of Engine harness. It finally terminates on the starter solenoid at Terminal 50.

You are searching for the erratic connection between Relay IV pin 85 and that yellow wire connection at the starter solenoid. While it is possible that the internal connection in the CE panel is bad, it's much more likely that the connection at T-18, pin 14, is erratic. That connector is often exposed to water when it rains or the car gets washed, gets removed and reassembled when the TB is done, moves around when the engine moves around. When it gets hot in the engine bay, the connection becomes erratic. All your symptoms point to that connector, the connection of the yellow wire to terminal 50 at the starter solenoid, or a harness problem around it in one of the yellow wires that attach to pin 14. Try cleaning that connector first because it's easy. Do the connector at the starter too. If the problem persists, you'll have your test light wired beween the starter solenoid connection and CE panel connector O13 where the big yellow wire connects there, looking for that erratic fash as the relay chatters.

On the drawing, O13 could easily be read Q13. It's hard to tell even on the paper copy. The terminal number looks just as fuzzy on sheet 10 as it does on sheet 11. Look for the 4.0 Yellow wire in the connector; there will only be one.

 

Outstanding. Thank you for the generous and thorough answer. I'll post results tomorrow.

(I still don't understand why, when current is flowing through the coil of relay IV, the starter solenoid is not energized, but I love a puzzle.)

 

The relay has a very high impedance coil compared to the low impedance starter solenoid. I don't have the actual circuit resistances handy, but for grins we'll say that the relay coil is 1100 ohms while the starter solenoid is 100. With 12 volts in the circuit, the relay coil would see 11 of them in series with the solenoid. That's enough to pull the relay closed. The solenoid sees only 1 volt, not enough to move the solenoid and starter drive. The relay coil is likely higher resistance, and the solenoid is way lower, clued by the 4.0 (~10ga) yellow wire use between the starter relay and the staretr solenoid.

 

Well, Bob, and Alan, and others, thank you very much. Cleaning the 14-pin connector (wire-brushing the pins and using Deoxit in the sockets) seems to have done the trick.

 

Great news! Armchair diagnostics from halfway across the country, and it actually worked!

 

I'm pretty sure it's Q13. O13 runs to the left lowbeam.

 

Mark,

Good to hear that you have successfully solve the problem with the help from dr bob, Alan and others.

Unfortunately not all circuits drawings are backwards or forwards compatable even for the digital dash model cars.

Unfortunately a lot of my 928 files and drawings are stored away at the moment, so it was a little like working with one hand tied behind ones back as I have concentrated on my 1990 model car.

Tails 1990 928S4 Auto

 

Thanks, Tails. I had the opportunity to look at the 1990 wiring diagram in the course of trying to figure out what the 1989 diagram meant. No help at all—lots changed between '89 and '90.

 

The X relay function is identical between '89 and '90 models. There are a few more connections from the starter relay to starter wiring between the starter relay and the X relay, but they are just distractions that don't affect the function of the X relay in either car.