Hkp7

It seems obvious that the ignition computer uses the green wire and the pickup to determine where number one cylinder is, but what does the injection computer use for injection timing, or better yet, how does the injection computer know the rpm of the engine. Is it through the same line? I am working on an 84 US. I see that engine has no crankshaft position sensor. But I do see it has an extra leg on the fuel pump relay which I am guessing is for the tach signal?? Any experts on these circuits? Thanks.

Landseer

Injection computer gets fed that signal from the ignition computer, which gets it from the distributor as you noted.
There is no other pickup that is converted to a tach signal, that is in-turn driving something else.
All from that distributor signal you noted already.

There is a document that describes this system pretty extensively for the 84 and earlier electronically fuel injected 928 USA cars.

Its located in the Morehouse CD compilation. Do you have?

According to it, fuel pump starts when it begins seeing that pulse.
No expert for sure, but know where the docs are.

Hkp7

I dont have that cd. Where do I get a copy? Also I noticed on a previous post I read today some of the tach readings at different driving speeds seem to be different from mine. I do although have an automatic. I am at 2000rpm at 80mph. Im trying to get back into diagnosing a problem where the car runs rough under load. I have recently seperated from the wife and am now getting my tools set back up. I have had numerous posts about this car. My interest in digging in has sparked again. I am wondering if the extra leg on the fuel pump relay is for the tach signal for the injection.

Landseer

Fuel pump runs single speed all the time and simply recirculates back to the tank.
Pressure regulator maintains the injectors at approximately steady 39 psi.
So, the pump is on or off, if that helps.

Rough running at load can be different things I guess.
For me its been cruddy distrib cap or arcing wires due to a nicked place.

CD is about $75 from Roger.

Good therapy, so long as you can pace yourself and always be satisfied with making slight improvements to the car, not necessarily solving the issue. I've got one giving me fits and sometimes I just stare it down and walk away, but I'll be back, it knows it.

Hkp7

39 psi is ringing some bell for me, I will have to look up my old posts and see what mine was running. One question, I have been some time under the impression that fuel pressure regulators get manifold vacuum to reduce the amount of pressure until the vacuum is released by opening the throttle to increase pressure. Am I backwards on this?

jpitman2

The injectors are not 'timed' in any way, they just get fired (opened) for enough millisec to provide correct mixture for the amount of air being ingested, without any regard to where any cylinder is in its cycle. Diesels, and maybe petrol 'direct' injection , are timed to deliver at some point BTDC, I believe.
jp 83 Euro S AT 53k, Continuous Injection System

IcemanG17

Yes the green wire takes a signal from the dist to the ignition "brain" box which then gets sent back to the main CE panel.... Whats odd is the L jet cars fire in batches of 2 cylinders...vs 1 batch fire for all 8 on the LH cars? I don't know why....

I think the fuel pressure spec was 36 to 39....since my 84 was around 36 at idle, but it did increase with load....

Landseer

Document indicates that the diaphram of the regulator is indeed manipulated by intake manifold vac somewhat, although we have discussed here that the introduction of vacuum is for safety reasons only to suck-up fuel if a break occurs. Apparently that is wrong and it does have some impact.

Injectors fire once per crank rotation, dumping 1/2 fuel for each combustion cycle. Volume of fuel is controlled by computer (width of pulse).

I've had a bad O2 sensor that caused the 84 to have some missing under load (not much, just a few odd blips during accel).

Moved that whole exhaust system to another great running 84 car, complete with O2 sensor ---- and the 2nd car started the same missing behavior (a few blips / flat spots). And the first car stopped doing it.

Unplugged the bad sensor, 2nd car resumed good behavior
New sensor --- ran better still.

Hkp7

My pressure regulators have continuous manifold vacuum at idle as they are connected now. The manifold vacuum right now is 17 inches