Tails

John,

Excellent information, it looks as though my electrical maintenance has taken on a new turn to check continuity of circuits and tab connections, but in not trying to stealing your post I put forward a problem that has existed since I have had the car, now over 10 years, where I will use the existing EMF within the circuits 'live' to check for faults via voltage drop in the circuit/s across resistors, connectors etc., to try and locate the fault and solve my problem.

I'm currently trying to trace a fault in the information circuit of my digital dash. When the ignition is switched on or the engine is started the odometer I resets to zero and then starts recording when I'm underway after travelling .2 to .3 kms it continuously resets to zero. If I bring up odometer II into the display, it indicates that it has been recording in the background correctly, but once in the display it will also reset to zero and then go through the reset to zero routine as described above ever .2 to .3 kms. When I bring up the stop watch display, I cannot get it to function when I use the reset button to start, stop or reset as designed.

From my deductions it would appear that the reset button circuit has either an intermittant earth or has lost contact via terminal 2 of plug 2 in the digital dash. I have checked the continuity of the wiring by taking an ohm reading in the circuits that are in circuit when the reset button is pressed, however the problem still exists. To make matters more difficult, it is an intermittant fault as the system come back online now and again, works correctly and hang around for a few months and then it takes a holiday again.

I have cleaned up the earth connections to MPIV and I dropped the resistance by 3 ohms in the reset button circuit when pressed, so my next move is to check the circuits live and measure the voltage drop in the circuits and across connector within the circuits where I can get at them. The reset button circuit go through terminal T5, pin 30 (visually checked, OK).

If this does not indicate the fault, I will remove the digital dash and check the connector fingers for propper contact, corrosion etc to ensure that connector 2 on plug 2 is making the correct contact and again check the resistance and the voltage drop "live" in each section of the circuit to try and pinpoint the fault whilst bring into to the digital display the 4 circuits circuits controlled by the information switch via connector 1, 7, 8 & 9 of plug 2 in the digital dash that can be reset by the reset switch.

In this instance I will not inject an external 12 volt EMF into the circuits as it may cause damage to the ECU in the digital dash.

I have checked the reset switch function externally of the dash and it functions as designed, I have checked the information selector switch, cleaned contacts etc and it is also functions correctly in that it cycles the display throught the dash, however it won't reset to zero the average speed or the average fuel consumption when pulled back for 3 seconds which connects it to the earth MPIV.

In addition the reset switch does not appear to have any effect when pushed to activate the reset function. as I push it before odometer I or II resets themself to zero with the fault.

I believe that the switching mechanism of the reset switch is to take the system to earth via MPIV to get its activation function.

As usual the description of the function of the reset switch and the information switch is not available outside the description in the Owner Manual and there is no modus operanti available that I could find in the WSMs or Technical Articles currently available, as to the design or function of the digital dash's CPU.

Would you or any other members or users of this forum like to comment on this fault finding procedure or rectification other thanreplace the digital dash at great expense.

Thanks,
Tails 1990 928S4 Auto

P.S. Who said the electrics and electronics on our 928s are easy?

jon928se

Bob

I also read the OP that way, initially, until I realised that it made no sense. What he should have said was that the wire in the harness was isolated by disconnecting at both ends (EZK plug and T2 sensor) then the wire was connected in series with a bulb and 12V applied across it.

I share your sentiments regarding applying volts across a sensor randomly - need to know the voltage normally applied when in use and the internal resistance of the circuit in the brain that is using the sensor info to stand a chance at doing it safely (not blowing up the sensor)

dr bob

Jon, reading the OP again, it does say "with all three connectors removed", EZK, LH and Temp-2. My bad for missing that first time around. Hopefully a future searcher will scroll all the way down here to post 18 and suddenly gain the clarity it took me 17 posts to realize.

DanielD

I've been troubleshooting some wiring issues in my car. The connections don't appear to be the only problem in the engine bay. Most of the insulator on the wires has dried out to the point of cracking to pieces when you move the harness. I am seriously considering rewiring the whole engine bay because of this issue.

John Speake

I wonder if there is a periodic glitch on the reset line. Do you have a data logger you could use ?

John Speake

Measure the ressistance from sensor body to chassis, just to be sure. Should be less than 1 ohm.

Tails

John,
Thanks for your time and your reply.
Unfortunately I don't have a data logger, so will have to battle on with what I got. I will do a bit of investigation on the data logger and see what I come up with.
I will let you know what I find when I can get time to get back to the problem. My major problem at the moment is that I never have enought time to fit everything in being retired. Maybe I will have to go back the corporate culture and relearn to sleeping quicker.

Tails 1990 928S4 Auto.

John Speake

Yes, when you're retired you wonder how you ever had enough time to go to work :-)

John (semi-retired)

AO

Ahh yes, this was the clue that told me my TempII was not grounding properly.

Water bridge bolt to crossbrace bolt < 1 ohm

Temp II body to cross brace bolt > 40 ohm

Temp II body to water bridge bolt > 40 ohm