JobeJoe

Is there a write up around here that focuses on identifying if you have a failed L-jetronic? I'm not super crazy about going too far down the rabbit hole without ruling it out first. Instead of boiling down to it!

If you've seen my other posts, I have an 80 US l-jet AT that was left for dead. Cranks then dies. Been going down the list of the usual suspects.

But what caught my attention was the noid light test, no pulse, no flash. Since I put in a new green cable, new afc relay, and fp relay, I figured I would try voltage at the fuel injectors. I've read somewhere; maybe pelican, that even with the key off, the l-jet has the circuit open at the injectors and you should see 12V on either term. With this in mind...

Just some quick findings, checked voltage on a random fuel injector:

Injector to passenger cam cover, key off, 290 mV, basically nada

Injector to cam cover ground (above C&H), key off, 290 mV

Injector to cam cover ground (above O), key off, 390 mV

Injector to cam cover, key on, 12V

Injector to cam cover ground (above C&H) key on, 390 mV

Injector to cam cover ground (above 0) key on, 12V.

Based on this, something is way off. I should get 12V to any injector prong with the key off, correct?

Can I jump the ground from C&H to the O temporarily to see if I can get a noid light to flash?

Kiln_Red

Follow these diagnostics step-by-step.

Rich9928p

The L-Jetronic injection system requires many signals to work. There MUST be a connection to the temperature sensor and airflow sensor ... if those circuits aren't connected it won't work.

The RPM signal comes from the spark box pin 15 and connects to L-Jet relay pin 1. Double check that that circuit is good. The same RPM signal connect to the tachometer, a failed tachometer could ground out this signal.

This is how the fuel injectors are powered. The AFC [Air Fuel Control] relay XVI pin 30 always has 12 Volts regardless of ignition switch position. The AFC relay switches between pins 30 and 87/87a, and the 12V at pin 87/87a then goes to the fuel injectors. [Note, even though it is not shown on the schematic, there are two AFC relay socket position 87s, so a jumper needs to have two leads out from the 30 position to connect to the two 87 [87 and 87a] positions]

a. Measure voltage at pin 30

-- If 12V present, next (with ignition turned on) is there 12V at pin 87/87a?

If yes go to (b)

-- If no, check the connection at the battery. A direct cable connects the battery to the AFC relay pin 30. Is there 12V at pin 15? If there is no voltage, the ignition switch could be bad or the path between the ignition switch and AFC relay has an open or short circuit.

b) is there 12V at the fuel injectors (pick any one to check)?

-- If yes, this says that your problem is with the fuel injection "ground" path. Go to (c)

-- if no, there is a wiring problem between the AFC relay and fuel injectors.

c) is there 12V at L-Jet connector pin 10 and 29?

-- if no, there is a wiring problem between AFC relay pin 87 and L-Jet connector pin 10 and 29.

-- If yes, check grounds for the L-jet controller. L-Jet connector pins 5, 16, 17 and 35 should have a very low resistance (via ohm check) to ground. For this check, set your VOM to Ohm mode and measure the resistance between the L-Jet connectors and a ground point on the car. If the Ohm reading is high to infinite, there is a wiring or connector problem with the ground points.

Rich
'93 GTS Cover Girl
'87 S4
'79 5-speed rescue
'79 Euro 5-liter track beast

joehelendetroit

I had thought the Ljet box was bad on my 80 and have since purchased two extra units, both of which I have had professionally tested. What I have since learned is the Ljet itself rarely goes bad, problem is usually the imputs detailed above.

If you want an extra one, you can have one of the ones I have for $150.

James Bailey

If you wish to test the brain......send it to me and I will plug it in the very brown 1980...they seldom fail however.

JobeJoe

I agree, everywhere I check, people state they rarely fail. I'm going to follow the steps above and hope that sheds some light.

James Bailey

Rare and NEVER are two different things just find a car to test it in or get a loaner brain...that once and for all tells you that the brain is NOT the problem. For just the cost of shipping it I will make sure yours works... !!

JobeJoe

Not a bad idea at all, anyone got a loaner? I would be happy to out down a deposit and pay the shipping on a known good brain. PM if interested in working something out fellas.

JobeJoe

Found a broken vacuum line as well, time to dig up a diagram on the vacuum system as well.

JobeJoe

b) is there 12V at the fuel injectors (pick any one to check)?

-- If yes, this says that your problem is with the fuel injection "ground" path.

Question on this - for FI voltage test, where should the key position be? On, off? Because with the key off, I get nothing, but with the key on, I get 12V at the ground above the ch on the pass side cover. And no voltage at the ground above the O. This is with the hot lead to the injector plug and black lead to ground.

Rich9928p

The ignition switch "on" should turn on the AFC relay. If there is not 12V with ignition switch on, remove the AFC relay. Check the relay socket 30 position for 12V, there should always be battery voltage at the 30 position. If no, fix that. If yes, run a jumper wire from 30 to 87 and 87A .... check for 12V at the fuel injectors now. If there is 12V the relay has failed. If there isn't 12V, there is an open circuit between the relay socket and the fuel injectors.

Rich
'93 GTS Cover Girl
'87 S4
'79 5-speed rescue
'79 Euro 5-liter track beast

JobeJoe

That was insightful. While I had 12V at pin 30, with the key on, I had nothing at 87a or 87. So now I'm ready to check pin 15, but are you referring to the l-jet connector? Because I don't believe there is a 15 on the AFC relay.

Kiln_Red

If I understand correctly, then your MY should have the AFC relay at XVI and it should be a simple relay. So no 87a. You don't need a 3-way jumper. Just bridge from terminal 30 to 87 to bypass the AFC relay.

Replace the relay and find plug W on the bottom of your CE board. Turn the key to the RUN position and check for voltage at W2, W3, W4, and W5. No voltage? Now jumper the AFC relay and check again.

Report back.

JobeJoe

I sure will. Lame question, what is MY mean?

Is it worthy to note I bought this afc relay from Roger about two weeks ago and is basically unused?

But I will jumper the connection to rule out a bad relay.

Kiln_Red

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