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Noid test light brightness with L-Jet low impedance injectors
Since I have an '84 that means I have low impedance injectors. Would that also mean the Noid light pulse would be weak as a result of that even if everything is health?
My assumption was that the Noid light would be bright, mine is weak, but would the weak/dim glow be a result of the low impedance system even if everything is health?
Pulling this question out of my larger thread since I will be easier for people to find if they have the same question in the future.
The injectors need about 9 Volts min to actuate regardless of the impedance of the coils. The impedance determines the amount of current that's required to batch-fire them, but the voltage is the same. Your 'noid light sees the voltage difference between the two injector wires. Any resistance in the wiring will cause a larger voltage drop with low-impedance injectors than when using higher impedance units.
Remember too that the EFI brain actually switches the low side towards ground, while the high side is constant at battery voltage (via the injection relay). So if you use a meter (poor) or an oscilloscope (best) to supplement your 'noid light (good) for testing, it will need to be connected to the injector wiring only, completely isolated from the chassis and battery.
Hi Doc-thank you for the information and thoughtful response.
So if I understand this, which is 50-50 at best, that means that a weak Noid test light result is indicative of an unresolved problem. Either low volts to the harness or a weak ground, right?
Without the starter engaged I have 12+ at the injector harness, so for the Noid test light to be dim either the starter is sapping too much power or I've got with grounds?
I suppose that the Noid test light even illuminates is progress...
During cranking, the 9th injector under the airbox is the important one. If that one isn't functioning as intended, cold starts will be very difficult.
This injector is controlled by the Thermo Time Switch in the coolant bridge:
FYI for future reference: The other two temp sensors just to the right of the yellow circled are are the gauge sender and the Temp 2 sensor which goes to the L-Jet brain to adjust the injector pulse width as engine temps rise / fall.
Hi there - thanks for the additional details. I believe I have those connected properly but will double check. Pictures always help!
Could someone with an J-Let or knowledge of what a healthy result expressed by a Noid test light confirm that it should be a bright pulse, or is dim but pulse ok? If it is always going to be dim (like, perhaps, me) then I'll move along to other things to worry about.
The way that the L-Jetronic system survives low impedance fuel injectors is to limit the current to the fuel injectors. It doesn't pull the voltage down and hold it, it actually very quickly switches between low voltage and high voltage, thus providing the energy to keep the injector open but keeping current low enough so the system doesn't cook. This current limiting likely could limit the brightness of the noid light.
The LH-Jetronic injection systems use high impedance injectors to so the system can handle the current flow ... but they have a whopping big 3.3 uF 100V capacitor. If you look on an O-scope, there is a pretty hefty voltage spike when the transistor opens up!
The takeaway from my post #2 above is that the 'noid light can't tell the difference between low- and high-impedance injectors in the circuit.
Regardless of the injector impedance, you should see battery voltage on one lead of the injectors, supplied through the injection relay. The relay is one of the critical relay-relay-relays (others being ignition and fuel pump). The voltage on the other side of each injector is pulled towards ground to cause the injector to open. There's a switching transistor array acting like a switch, with a fixed voltage drop across the switched junction. It typically nets out to about a 1.2 - 1.5 Volt drop, plus the effects of any resistance in the wiring. The injectors ground to the intake manifold via the controller, so that ground point MUST be clean and solid if you expect the injectors to fire reliably. Any resistance there can cause some injectors not to open, and will also reduce the voltage difference that your 'noid light sees as you test. The total current in the circuit is high when the injectors fire, so it doesn't take a lot of resistance to cause the available voltage difference at the injector to fall below the minimum threshold needed. Your weak/dim 'noid light reading is telling you that you have resistance somewhere in the circuit. Replace the injection relay and carefully clean the ground connections at the rear right (passengers side on US cars) of the intake manifold to eliminate the most common spots for resistance in the injector circuit.
OK, so back to the dim light telling me I have a bad ground path.
And overnight-without changing anything else-I'm back to not getting a pulse on the noid light. Not even a weak one.
I've removed and cleaned the passenger side valve cover grounds 4+ times now. I get no resistance when I check the installled valve cover grounds and a known good ground spot
My pin out readings are fine. Battery connection nice and tight. Battery fully charged, and no change when I tried and the car was jumped to another running car.
I'm going to remove, clean, we install the new engine ground strap in the car to see if that makes a difference.
Check continuity between jlet pins for any continuity between the injectors and ground points, no shorts to be found via that test. More specially, no continuity between grounds 35,16, or 17 and 32,33 or 14,15.
08-28-2017, 03:10 PM
Noid test light brightness with L-Jet low impedance injectors
