Keith Newman

Hello all,

i am am walking through troubleshooting a cranks but no start issue on my 1984 L-Jet, 5 spd

i have fuel to the rails
i have spark
i have cleaned all ground points
i have cleaned and verified resistance of 0.4k and 0.6 resistors
i have replaced the green wire
i have new plugs, wires, distributor, and rotor
i have new fuel filter and pump installed
i have new fuel pump relay installed
i have new L-Jet relay installed
i measure 12v at slot 30 on relay
i measure 12v at connectors 14, 15, 32, and 33 on 35 pin Ljet connector
i measure infinite resistance between connector 5 (ground) and locations 14 15 32 and 33
i measure 0v at injector connectors

From my research this indicates injector harness is bad.
Is there anything else to research or do you concur that the injector harness is bad?

thanks in advance for your help.

GT6ixer

First, congrats on owning the best model year of 928.

Has this car run before? You verified spark and fuel. Have you verified compression and timing? That's the next step before throwing more parts at it IMO.

Keith Newman

I agree with you on the nicest looking MY in my opinion.

to answer your questions.

Yes. It has sat for over 6 years though.

Compression and timing are good.

GT6ixer

Have you verified that fuel is being sprayed from the injectors? They could be clogged after sitting for that length of time. Does it start with starter fluid?

Keith Newman

Thanks for your reply.

there is no voltage to the injectors from the harness.

i did clean and verify injector function when I had them out to replace the hoses.

its does kick over on starter fluid.

The Forgotten On

^^ The computer switches ground to fire the injectors so it is worth figuring out why they do not have +12 volts at all times.

They get power from the fuse panel connector for the harness and it runs directly to the injectors with nothing else in between.

snoz

I went through this the other day. I was getting the ground pulse on one injector plug pin and was not getting 12V from the other. Use a noid light between the jump post each pin of an injector plug one at a time while cranking. You will be able to see if you are getting an injector ground pulse anyway. My issue was with the fuel pump relay. I had it out because I don't want my fuel pump running right now and didn't know it had to be there to get 12V to the injectors.

FredR

Ethan,

Different years have different wiring schemes but the basic principles of design should be similar and I would be somewhat puzzled [but not entirely surprised] if the design is such that voltage to the injectors is permanently live. The entire point of relay logic is to get live power when it is needed and not before.

The 84 model year has relays with pretty much the same functionality as the later models but there are differences. Unfortunately I could not fathom out the logic based on the MY84 wiring diagram convention shown in my WSM whereas for your model year [MY86] and mine [93 GTS] it is crystal clear.

Keith Newman

Thanks for the replies.

Blake, if I understand what you are saying then, if I am measuring 12 volts at the injector locations on the ECU connector plug for the but not at the injectors then the ECU is faulty?

Ethan, I did replace the relays with brand new ones before attempting to start and then diagnose. I suppose a new relay could be bad. I’ll try and do what you recommend.

thanks all

Stan

drscottsmith

Stan

you might try a jumper in the relay slot for testing. You can get really creative and put a toggle switch in the line so you can manually turn the relay on and off for testing as well.

check the ground point on the pass side valve cover as well.

Keith Newman

Thanks for the input Scott. I did clean those grounds as well.

i don’t have a Noid light, went to our local harbor freight to get one and they do not have in stock so I will have to order a kit and wait to test.

Shark2626

My local Auto Zone has a noid light set in its loaner tool inventory. If they are local to you your store should have one too.

FredR

You might consider removing the LH and fuel pump relays and then check for continuity between pin 87 of each relay socket and the voltage delivery terminal on the injector. If you get continuity you will know a bit more about why you not seeing voltage. Alternatively you can jumper pins 30 and 87 and test for voltage at the injector socket voltage terminal having tested for voltage on pin 30 with the relay out.

snoz

Noid lights are handy to have. A test light will probably work as well. As FredR said, the wiring for that circuit may be different for your MY.

cdk4219

On most efi engines the positive 12v side of the injector is fused and constantly on when the ignition is in the start/ run position. This circuit usually has nothing to do with the ecu. The ecu switches the ground side of the injector to turn on and off. The injector circuit is fused on the positive side, and should give power to the injectors when in start and run positions.