1987 928S4, no start.
I've changed the CPS, but it still refuses to run.

The fuel pump will not run unless jumpered, but even then it will not fire. Spark is confirmed.

Clearly the injectors are not firing, but I cannot tell why. I have a jumper on the FP relay and it whirs away and we put a fuel pressure gauge on and it has pressure. I want to jumper the LH and EZK relays. Do I just connect 30 to 87, right? Been way too long...

The tach needle didn't move, so I will investigate W11 (I think).
I had another thought. What about the factory alarm? Is there a way to make sure that is disabled?

I'll be diving into this again tomorrow. So any help today would be greatly appreciated.
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AO
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It's alive!

Wow! What a journey though.

We checked to make sure the block was grounded. We jumpered the LH relay and installed a noid light. Cranked... nothing.

We checked for a square wave on pin 1 of the LH, we saw some activity, so we decided it was probably good. When I went to plug the LH back in, one of the guys saw the noid light flash. I disconnected the LH, noid light flashed again. So, we now knew the harness was good.

Some where along the way, DaveK9 found a write up on how a bad tach can disrupt/corrupt the signal to the LH. This signal goes through pin W11. We didn't have a pin extractor, and tried ever combination of tools to try and get that pin extracted. Finally, after about two hours of fudging around, we just cut the wire.

VROOM!

Had this been an auto, we would have also inspected the kickdown relay, as I believe this same signal travel through or is in the same circuit as the kickdown relay.

So for now, at least the car runs (first time in three years). The tach doesn't work, but it runs and runs strong.

We will have to figure out what the exact cause is, but that can wait.
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AO
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Assume you cut this green tach wire near the dashboard or near CE pin J11 - not the wire at CE pin W11?

The LH wants to see that the engine is rotating before turning on the fuel pump(s) and activating the injectors, if the line is shorted to ground it won't see that.

Note - The kick down relay is connected to this also.

I'd check the pod connections for possible shorts.

Alan
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No... we cut W11. The LH clearly had a signal on Pin 1. Everything else seemed fine - except the injectors would not fire. After we cut w11, everything works, except the tach.

We will check the pod and see if we can find the issue.
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AO
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I should have said that looking at the 12v square wave on pin 1 with a meter on a.c. volts would give a reading of 3-4v (depending on the signal processing of the meter).

I don't know what reading you obtained before cutting the wire ? I guess it must have been low.

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John
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I just looked at the diagrams too. The mechanism for the failure is apparent: Ground always wins. What follows is what Alan wrote with more words:


The speed signal from the EZK is wired in parallel with the LH and the tach and the kickdown relay (XV if auto.) Disconnecting/cutting WW11 breaks the circuit as it goes to the tach and XV (J11 is the lead that goes directly to the tach from the fuse panel. That lead could have been disconnected to isolate only the tach.)

Ground always wins. On the fuse-panel side of W11 the line is shorted to ground. Ground is sucking down to zero the otherwise periodic speed signal on its way out of the EZK. The components after the EZK - wired in parallel - don't see pulses from the speed sensor, they see ground because of the short. The EZK is in series with the speed sensor and W11: the EZK sees the pulses first and isn't affected by the short after W11.

Three possible culprits:
1) wiring behind fuse panel from W11 to J11. That's easy to test with W11 and J11 disconnected.
2) Wiring between cluster and J11. I would look carefully at the first 6 inches of the loom at J11. I have seen shorts between wires close to the plugs on several occasions.
3) cluster. Disconnecting cluster and J11 will enable ruling out wiring: if, with cluster and J11 disconnected, J11 - loom-side - has continuity to ground then the dash harness has a short, if not, short is in the cluster.

A related but different question for Alan: what is XV doing with the periodic speed signal? Diagrams show a transistor in relay. Is W11 providing the gate?

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-- Dave C

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This is remarkable... I helped a guy the other day who had a very similar problem at W11 caused by J11 on the CEB. Also the Tach signal was pulled to gnd. Coincidence?

@Dave C: maybe more correct wording: the tacho signal goes from EZK #13 in parallel to the diag socket pin #3, and to LH #1, W11, and from there connected over the CEB to J11 and to the kickdown relay socket. It is not in series or so.
 

Here is a picture that shows the signal path of the tacho signal:

relay XV, the kickdown relay, uses the engine RPM signal to decide when the RPM reaches the level when it should force the transmission to up shift when in kickdown mode.

regards
Theo
1992 928GTS midnight blue