I have swapped ignitors , motronic ecus, coil packs, and relays. I added a ground between the motor and frame tonight.The 5-8 cel was working up until it started running on 4 cylinders. I need to check the bulb yet. To top it off my house got blasted with lightning at 5 am and fried a bunch of ****.Not a good electrical spell here.
If you have your battery in the rear corner of the engine bay, the bullet connector will be right under the air filter box, and, there is another one coming off the negative on the battery. Remove the air box and you'll see it. While you are near the battery there will be a connection on the positive terminal for power to the cab of the car. Check that too. As for to fuses, actually check the condition of the fuse box itself. It has happend on occasion that the box itself will go bad. Have a look good look at it to make sure it hasn't burned something up on the board. I hate electrical problems. It's usually something so stupid and small that creates the problem, and can be a BEEOTCH to find. Oh and, sorry to hear about your house getting popped by lightning, hope it wasn't too bad.
I found the grounds under the airbox,valve cover and twist connection on left shock tower.They werent dirty but i took a little file and wire brush to them anyway. I have a new connection to the positive battery post for the additional feeds. It was a little corroded so I cut back 3 inches to fresh wire and borrowed a crimper and new lug from my electrician buddy. No changes. So...I decided to go pull the fuse box. I pulled the relay plug for 5-8 clear out and inspected the prongs along with its fuse.I found that i have a Viper alarm that kinda scares me but the wiring going to my bad 5-8 bank relay and fuse is in good shape. I unhooked the alarm anyway for the hell of it but it still runs on 1-4 with no tach. I put a voltmeter on the input to the 5-8 ECU while running and I'm getting 12.5 volts to it which leads me to think its not in the fuse box with a relay or fuse problem? I very definitely have no spark on 5-8. If i am getting good power to the ecu and either ecu will make 1-4 run I gather its got to be tps or maf or crank sensors?
Well crap. And you swapped the coils from side to side? How about the ignition control modules? Did you swap those from side to side? How about the condition of the wires and female pins on the connector to the Motronic? Remove the Motronic to give you more room and then closely inspect the pins and wires under the rubber boot. Oh and completely remove that alarm just to make sure. Other than that, I'm clean out of ideas. I'm sorry I couldn't give you the solution. Maybe someone else has the answer?
I think I am going to snoop around the crank sensors again. I assume I should be getting a voltage from a pair of the 3 wires going into each sensor with the key turned to "On"?
Maybe there really is a bad cat. That's what the warning light is supposed to be there for, I think. How does it run at 7k rpm in 3rd gear? Still pulling?
It was running like a streak of ****. Pulled hard and ran fine. I can feel huge flow of cold air coming out of 5-8 bank when its running on 1-4, so I dont think with that kind of flow that the converter is collapsing. I have the cat ecu's unplugged right now too. I guess anything is possible though.Thanks for the tip.
I have a vague memory of someone here once stating that he had found a fuse (apparently original) wired into the harness directly adjacent to the 5/8 Motronic ECU. If I remember this correctly, no one else had ever seen or heard of such a thing, but darned if he wasn't right. Might be worth doing a little poking around to see if you have a "hidden fuse" tucked back there somewhere. I don't see it on the factory wiring diagram, or in the parts book.
Wow-that would certainly do it. Thinking back to last night though-I put a meter to the harness coming into the 5-8 ecu and had 12.5 volts in several spots. I dont have any idea what those are feeding in the ecu though and there might be one more thats fused 007 style. Thanks for the tip-I'll check it out.
Been working 10 hour days clear through weekends and just getting back to my car. I downloaded the Motronic 2.7 diagram from the service manual website. I imagine I have had my paws on it already,but can anyone tell me where Engine Power Ground (cyl 5-8) is located? It is listed as 116b on the schematic.
Just crawled out from under the 348. With the key on I get 5 VDC to my 1-4 crank sensor and nothing to my 5-8 crank sensor which is my dead side. I pulled back the rubber boot and the wiring is in good shape. The wiring diagram gets a little funky for me but I think the crank sensor input and output goes to the Motronic on that side? I had a voltmeter under there and when I found no power going in I made a trial ground from the sensor ground to the engine but it made no difference. Anybody know what kills the crank sensor input other than an outright cut wire or bad ground?
Check the wiring loom running through the firewall. Maybe the cover over the loom got cut and is causing it to ground out????? There is a rubber boot that should be covering the metal. You are going to need to remove the ecu completely out to see the loom running through the hole. Also have a look at the connection under air box between the two looms, it joins the looms together. It's right around the same area for the plugs to the O2 sensors, right above the gearbox. It's a roundish plug.
I had 3 problems going on at the same time. Lost a ground to the ECU 5-8 that went to the valve cover. I restored that and still had nothing on that side. I had continuity down to the fusebox to the ECU but not between the relay and corresponding connection-same for my 5-8 fuel pump. I havent had much time until tonight to work on it but I isolated the different circuits and got fused power back to the ECU and then wired a fused relay where my 5-8 pump gets relay signal power from the 1-4 pump hot wire and turns on. It makes my old 1971 Porsche 911 look pretty nice with its old points and condenser Fired up on 8 tonight and sounds mean. Now I am going to install my other new crank sensor and button everything up. I want to give a big thanks to everyone who helped me out like Ernie,Miltonian,Saint Bastage and Plugzit.
That is excellent news, congratulations on finally figuring this one out and repairing it. Way to hang in there and not give up.
Thanks AceMaster. It definitely sucked to trace down everything but I love the car and it actually allowed me to see most of it electrically for the first time. Now I am going to probably sell my 911 and look for another with a larger motor or a Lotus Esprit Turbo to add to the 348.. God help me
...and that's what makes the repair job even more gratifying Consider it a bonding adventure between yourself and your 348, I'll bet you love it more now than you did before this job
