Ricardo Vega II

Ok, so the noid light flashes ever slightly when I crank it but when I put the voltmeter I only see at the most 1.5-2.0 volts. I checked the volts on fuel pump when cranking and is at 10 volts or so when turning over.
I'm assuming voltage to injectors should be 12 V, right? At this point it's safe to assume its the ECU that has gone out?
Ricardo Vega II

Landseer

Did you clean the valley grounds?

Afterwards, charge the battery, change out the ground strap with a $7 advance auto version and jumper all three relays. What happens?

Not sure you can narrow it down to ECU with that type of troubleshooting.
You can pull both computers and send to one of the 928 computer specialists as well.
Just haven't seen many successful troubleshooting attempts using a voltmeter here due to the multiple failure points.

I had one that defied logic, till I found a defroster power wire on back of panel had melted into the LH relay socket feed wires. Inspection and empirical checks are the high percentage win.

Ricardo Vega II

I don't know what valley ground is.
But my strap is good and when grounded I do get the 12.67 volts when I ground one end of my voltmeter to any metal item and the + to battery post. I swapped out all three relays in place of the horn n all three honk, if that's what you're referring to.

RV II

rgs944

Yes make sure the battery is at full charge. Sometimes a persistant problem will only show up at lower voltage and magically disappear when you get a little more juice flowing. Did you look at spark using an old spark plug grounded to metal? Also I do not see anywhere that you indicated the tach was jumping while cranking the engine? Also can someone point out where the CPS wiring plug is for my future reference. I know it well on the 944 but have never needed to screw with it on the 928.

John Speake

You should get a 12v feed on the injectors, the LH ECU grounds the free end to fire them....they are all wired in parallel.

Check the two smaller leads on the battery +ve terminal, they are the dedicated battery feeds to LH and EZK ECUs. The ground point in the valley is under the engine airfilter housing, hidden somewhat by the throttle pulley wheel. But first check the ground reistance to chassis from the LH and EZK ground pins.
If less than 1 ohm the ground point is OK.

You do have a WSM ?

Ricardo Vega II

Every time I've cranked it I have made sure I have the jumper cables on it so that the right voltage is cranking it over.
The tach only jumps once to 500 and then nothing after cranking it for several seconds.
RV II

Ricardo Vega II

And yes, I do have spark coming out when grounded to metal chassis.
RV II

Ricardo Vega II

Guys, thanks for your help. I'm new to all of this so bare with me.
So I do get 11.4 volts on the injector harness with I put one of the voltmeter leads to one the pins and then the other lead to a metal part on the chassis. The same voltage shows up when I do the other pin n have the key turned on to the point before I can crank engine. But tach doesn't move at all once it starts at 500 it then stays at 0 but bouncing just lightly.
John, I'm kinda of confused on where the pins would be for the LH n EZK.
RV II

Leon Speed

See here: https://rennlist.com/forums/928-foru...nt-thread.html

Landseer

What you need is the test plan that explains it for the 1987 car only.
Its a bit easier to follow and doesn't confuse with the later model year changes

dr bob

I'll ask a late question-- Did you or anyone spend any time in the battery well prior to the problem? The power to the injectors and the fuel pump routes through accessory wiring that connects to the positive battery terminal, piggybacked on the bolt that clamps the terminal. These are the several smaller red wires connected there. There are also two for the cooling fan motors, BTW. Anyway, loose connections there at the battery will cause some of the symptoms you are seeing, like the low voltage in the injector loop when they fire. You can't reliably see the actual voltage there using a digital meter, since the settling time for the digital voltmeter is longer than the duty cycle for the injectors. For grins, lift the battery ground strap and then take apart all those terminals at the positive clamp. A little sandpaper or a mini wire brush will make the eye terminals shiny again. Put them back together carefully and snug the small nut on the minor end of the clamp bolt. Not a bad idea to clean the main connection where the clamp attaches to the post while you are in there. Same with the ground strap at both ends.

A few years ago, a few of us 'experts' spent a Sharktoberfest afternoon wrestling with electrical issues on a guest's car. We ignored the battery end of the systems since the owner assured us that the battery connections had just been cleaned. Finally dug into that area to find that they were in fact clean but they were also still loose. Most of the time, gangs of problems that show up at the same time are tied to some single point of weakness or failure. Loose or dirty connections there at the battery aux leads can display a wide array of symptoms depending on what is touching what in the stack and where there's real resistance. These "minor" conductors and connection will cripple the car, so cleaning them is part of the annual 928 electrical system PM program.

rgs944

Interesting Dr. Bob when you mention that there are two leads that go to the cooling fans. I am assuming one lead for each fan? I wonder if a bad connection there is sometimes mis-diognosed to a bad final stage control module.

Ricardo Vega II

Landseer,

Is that a manual that is online? Does anyone know how to maybe "test" a module?

Ricardo Vega II
Broken Black 1987 928 S4

Landseer

What Dr Bob said above is really important to me. Establishing that solid electrical baseline condition makes so much sense.

I'd like to add that the negative ground straps on these cars have emerged as key failure points. Before troubleshooting any car I replace the strap with a generic, at least temporarily. (As I recall, both Dr Bob and Alan both expressed some surprise at the effect of corroding battery groundstraps recently in separate situations).

Also that my layman's experience, corroborated by a close friend who runs a German-only independent shop, is that the CPS condition cannot be determined from the tachometer twitch.

PM sent with 87 test plan and tech specs, along with advice to buy the CD set.


To test a module you can put them in another car.
Or assume they need work and send them to one of the two rebuilders known to this board.
While you are at it, send the MAF too, spend the money and have all three evaluated and improved.

Ricardo Vega II

Guys, so the issue was the ECU. I sent it in to have it Che led and sure enough, they replaced the hybrid chip sent it back to me and I installed it and after a good amount of battery charging turned it over a few times and boom! "Houston, we have lift off!"
I have learned so much and wanna thank all you guys for your input! These cars are amazing and fun to tinker with.
Thanks again and your support!
Ricardo Vega II
1987 928 S4 Black