MBMB

Relay #IV ("Relay X") is clicking randomly (after car is warmed up, something is vibrating loose). Trying to diagnose the problem. What is the "X" to which terminal 86 is connected?

Thanks,
Mark

Mongo

Isn't that the EZK relay?

https://www.928gt.com/t-89fuse.aspx

hacker-pschorr

GlenL

It's #10

Tails

Relay IV is the ignition relay X

The X is circuit X where the relay terminal 86 is connected to and is the X-Bus (+12 v when ignition is on, stays on until ignition off & door open).

Circuit 31where terminal 85 is connected to is the ground - battery negative (most connections will be via Ground Points - MP designator points)

If you have clicking in the relay then the hold in selonid coil is probably not working correctly and the clicking is the opening and closing of the contacts. I would suggest a replacement relay to see whether this stops the problem.

The part number is 141.951.253.8 OEM is listed as $8.32 at 928 Specalists.

Tails 1990 928S4 Auto

MBMB

I switched to another relay; the problem persisted.

dr bob

The "X" bus relay is used for loads that need to be on with ignition on but not when the starter is engaged. The coil of the relay is linked to the 15 bus, hot when ignition switch is in 'run' or 'start' position, and the 50 bus,which is hot only when the ignition switch is in 'start' position. The relatively high impedance of the relay coil means it will energize whenit's in series with the low-impedance starter solenoid.

If the X relay is "chattering", look for a poor connection to the starter solenoid. You can diagnose the relay coil electrics rather easily with a voltmeter. Make a couple short jumpers to connect the relay to the socket, so you can use your meter to test for voltage at the two wires with the relay connected. You should see battery voltage on one wire, continuously, while key is in run position. Relay is chattering, right? On the other wire, you should see very low voltage continuously. If the voltage goes up and down as the relay is chattering, look at the circuit to the starter solenoid, particularly at the 14-pin connector by the jumpstart terminal under the hood.

Unfortunately I don't have wiring diagrams with me so I can't call out specific test points by name/number.

;(

dr bob

Sorry--

Jus reread the OP. Missed that you are diagnosing a relay that's --connected to-- the X-bus, not the X-bus relay itself. Solving that is even easier, since the power to the X bus is traceable and the ground connection in the CE panel isn't hard to find-- right above the panel.

WallyP

You had me pretty bewildered there for a few minutes, Bob.

;-)

Alan

This is incorrect.

X (X-Bus) is simply an output of the ignition switch - the accessory position. It is active in accessory and ignition positions and off in the starter position. The X relay (often in positions IV) is a repeater for the X-Bus (X) - feeds circuits that have high loading on X functionality (e.g. fog lights etc).

Alan

MBMB

Okay, so the ground connection is solid and the connection to the X-bus is solid and the relay doesn't act up till the car has been running a while, when it starts chattering, apparently with vibration, and swapping it for the horn relay doesn't make things any better.

Your thoughts?

dr bob

Pigtail the relay out from the CE panel, and add a couple light bulbs to the mix. One from ground to the 85 terminal on the relay, the second from ground to the 86 terminal. Use small bulbs, low current draw, and watch them as your problem shows up.

If the bulb to terminal 85 and ground flashes in concert with the relay chattering, you have a poor ground connection between the relay socket and chassis ground. The light will be off normally and flash dimly when the relay chatters if the ground is intermittent. Use an analog meter set on volts for this if you want instead of the bulb, but the chattering will likely be too fast for a digital meter to follow.

The bulb from 86 to ground should come on with key in run, go off during cranking/starting, back to full brightness when engine is running. If it starts to flash in concert with the relay chattering, the problem is an intermittent connection between the 86 terminal in the relay socket and the X bus itself. If the X-bus relay is doing the chattering, you'd likely see the results in other X-bus clients at the same time, but since many are lights you may not see them dimming much as the X-relay chatters. New 53B relays are cheaper than the troubleshooting effort, so to me it makes sense to replace them both with NEW to eliminate them as possibilities.

MBMB

Thanks, Bob. I'll try replacing the relay with one that isn't 21 years old tomorrow, and if that doesn't work I'll try your first suggestion.

MBMB

Further study of the wiring diagrams suggests that, instead of the X Bus (per H36) Relay IV is connected to the 15 Bus (per J94).

I've produced Dr. Bob's suggested test rig, and will test drive with it tomorrow. MP V is freshly shiny, so I'm betting on a supply problem rather than a current problem.

Other things connected to the 15 bus: Fuses 7-13, rear window defog relay, ABS relay, fuel pump relay. I'm not noticing problems with fuel delivery.

MBMB

So. Test rig in place (Red LED from IV 86 to ground and green LED from IV 85 to ground). Once the car warms up and vibration / acceleration is just right, the relay chatters. Red LED stays lit, green LED flashes, which suggests that something is interrupting the circuit from IV 85 to ground.

Suggested diagnosis? I've been studying the wiring diagrams till my eyes cross, and I don't see anything to do but run a jumper from IV 85 to MP V and call it fixed.

According to the wiring diagram, IV 85 (the negative side of the relay IV coil) is connected to:
I-II 31
Q13
XIV 87

Q13 runs to terminal 50 on the starter solenoid, which, as I understand it, energizes the starter solenoid (and engages the starter). Which makes no sense to me—if this is how it's wired, whenever the 15 bus is hot the starter should engage.

Clearly, I'm missing something.

Your thoughts?

Thanks,
Mark