Hello All,

First off let me say hello to everyone for this is my first post on this forum. I recently converted my 81 f-body to TBI. Basically a seamless conversion up until start up. First off I was facing a no spark, no pulse situation upon cranking. After several hours of internet research, I was convinced that the problem was my ignition control module. I verified the pickup coil resistance values to eliminate that as a problem so I went forth the with ICM. Well after I buttoned it all back together, she started up! Not exactly right away but I had fuel and I had spark. Well I let her run for a good 10 min before I realized I never reconnected the EST wire after setting my base timing. I shut the car down and reconnected the wire and went to restart. Now, with EST disconnected or not, I only have spark and no injector pulses. Fuel pressure is not the issue because I am not getting so much as a drizzle out of the injectors. I checked and verified that injectors have power but they still will not fire. I have been reading that this issue may be oil pressure switch related however, from what I understand; I would have no spark and no fuel if that were the case. I am mostly lost due to the fact that I had it running with no issues a good 10 min before all this. I also have been hearing a lot about heat soak issues but I assure you I completely and evenly coated the bottom of ICM with dielectric grease it was supplied with.

Basically any input is greatly appreciated!

Thank you,
Marcus

 

Check the resistance across the injectors.

 

Both injectors gave me a resistance value of 1.5 ohms

 

Hm, I think that could be low, I'll double check.

What about the voltage? It should be twelve volts.

 

Yeah voltage was just above 12 on both sides. I'm not exactly sure what this indicates but i finally was able to start the car using a remote switch to starter. When i crank with the key i get no injector pulse but when i start with switch it basically fires right up. I need to go back and search the wiring diagrams to see exactly what is happening but atleast she will run for now. Thank you for the help!

Marcus

 

Most likely you picked up accessory (ACC) power source to run the injectors. During cranking this power source goes dead. A quick check would be to put the DVM on an injector + feed and see if it stays during cranking.

Need to use an IGN+ feed for the injectors and the coil. This stays alive during cranking.

RBob.

 

Thats what I was thinking. There is no other reason why injectors should loose power during crank. I was certain i had spark upon crank however I must have not. Anyway, thank you guys for your solid input.

 

Well after some more diagnosis i discovered that my spark and fuel power is wired to the proper ign source and not acc. I checked voltage at injectors and have 12+v with key on. When cranking i have voltage but it drops to about 9.5v when cranking via ignition switch (will not start). If i use a bypass switch i get a reading of about 10v (will start). I was thinking that possibly the combination of some unwanted resistance in the ignition switch and my old tired out starter is dropping system voltage to just below its operational threshold. This was really the only thing i could think of being why it would start one way and not the other. Once started, it runs great! Does anyone know at what point the system stops functioning in a low power situation?