no electrical power
10-17-2017, 07:08 PM
#1
Track Day
Thread Starter
Join Date: Oct 2017
Posts: 15
Likes: 0
Received 0 Likes
on
0 Posts
no electrical power
I've had trouble getting my 1981 928 to start. It would start run a few seconds and die. Because the car sat over winter I assumed it was older gas. I drained the gas and added a stabilizer. Then I gludged together a fitting for an oil pressure gauge and mearured the fuel pressure at 30. The I replaced the "green" wire. The I got an oscilloscope and was going to measure the EFI pulse and I got a few clicks and it died. Put a charger on the battery but still it was dead. Took battery to Advanced Auto and it had 680 CCA out of 900 which is enough.
I want to get the voltmeter out am looking for easy to get to points first before jacking the car up on gravel. Any ideas? All help is appreciated
I want to get the voltmeter out am looking for easy to get to points first before jacking the car up on gravel. Any ideas? All help is appreciated
10-17-2017, 07:31 PM
#2
Burning Brakes
First of all, welcome to the forum.
A couple things;
- usually not best to make multiple changes at the same time with troubleshooting
-if it runs briefly it is probably running off the cold start system, classic behavior for that is it will cough to life and run for 2-4 seconds.
- good news is that if it runs off the cold start you have spark, which is always a plus
- probably time to make sure you have checked your fuel and FI relays
-make sure the battery is fully charged when doing your testing (and if you plan to drive it too)
Do you know where your fuel and FI relays are?
A couple things;
- usually not best to make multiple changes at the same time with troubleshooting
-if it runs briefly it is probably running off the cold start system, classic behavior for that is it will cough to life and run for 2-4 seconds.
- good news is that if it runs off the cold start you have spark, which is always a plus
- probably time to make sure you have checked your fuel and FI relays
-make sure the battery is fully charged when doing your testing (and if you plan to drive it too)
Do you know where your fuel and FI relays are?
10-17-2017, 07:44 PM
#3
Rennlist Member
I've had trouble getting my 1981 928 to start. It would start run a few seconds and die. Because the car sat over winter I assumed it was older gas. I drained the gas and added a stabilizer. Then I gludged together a fitting for an oil pressure gauge and mearured the fuel pressure at 30. The I replaced the "green" wire. The I got an oscilloscope and was going to measure the EFI pulse and I got a few clicks and it died. Put a charger on the battery but still it was dead. Took battery to Advanced Auto and it had 680 CCA out of 900 which is enough.
I want to get the voltmeter out am looking for easy to get to points first before jacking the car up on gravel. Any ideas? All help is appreciated
I want to get the voltmeter out am looking for easy to get to points first before jacking the car up on gravel. Any ideas? All help is appreciated
Try unplugging the MAF.
10-17-2017, 08:30 PM
#4
Chronic Tool Dropper
Lifetime Rennlist
Member
Lifetime Rennlist
Member
The '81 has a barn-door style airflow meter, a generation older than the hot-wire MAF systems.
Borrowing from 928 yoda Jim Bailey: Relay-Relay-Relay. These are ignition relay, injection relay, and Fuel Pump Relay. First two are inexpensive and common. the fuel pump relay is "smarter" and more expensive. Follow the instructions below for fuel pump relay jumper to diagnose without replacing. The first two -- just replace with new.
You are likely on the right track looking at fuel pressure first. Start with easy basic stuff first though:
-- Power for the fuel pump is supplied via a separate red wire that connects at the battery positive cable clamp. The connection must be clean, shiny and tight. These get disturbed when taking the battery in and out, and suffer from corrosion if the battery isn't properly vented outside the battery box while charging.
-- The ground strap that connects the negative battery terminal to the chassis is a known weak point. The copper braid inside the protective plastic sleeve will corrode over time, and that's accelerated by the presence of corrosive charging fumes if the battery isn't properly vented outside the well.
-- The fuel pump relay depends on a tach signal that's passed via the green wire from the distributor to the spark box.
-- The relay will close briefly if the car has been setting a bit (minutes not hours) as the key is turned to position 2 (engine run), then it drops out until it sees tach pulses from the engine cranking on the starter. Listen for the pump to run before cranking. Have an assistant listen for pump run during cranking. If you don't hear the pump run at those times, carefully remove the fuel pump relay and install a temporary jumper in the socket between 30 and 87. If you are making a jumper, build it with a switch in it so you can safely start and stop the pump for testing. Jumper with switch closed should cause the pump to run.
-- If the pump runs but fuel pressure is low, find out why.
-----
You can test for ignition function using a standard inductive-pickup timing light on a plug lead or a HT coil wire. Light should flash while crankin.
You can test for injector function using a "noid light", an LED indicator that fits between an injector and its electrical plug. (local parts store, Amazon, etc.) It should flash while cranking. The injectors are batch-fired so if one flashes it's reasonable to think that all are getting a signal, at least enough to start.
after that...
The barn doors do sometimes stick after extended storage. The doors are also known to butt up against mouse nests and other debris that might be dragged into the intake by similar rodentia. Same varmints also like to chew on wiring in that area.
Start with the terminal cleaning, the ground strap replacement, and the critical relays. Move on to deeper testing if those don't get you going. From that point you will have an idea whether the problem is fuel supply, injection, or ignition. Troubleshooting steps vary with what you find. Post back your results these first steps.
Borrowing from 928 yoda Jim Bailey: Relay-Relay-Relay. These are ignition relay, injection relay, and Fuel Pump Relay. First two are inexpensive and common. the fuel pump relay is "smarter" and more expensive. Follow the instructions below for fuel pump relay jumper to diagnose without replacing. The first two -- just replace with new.
You are likely on the right track looking at fuel pressure first. Start with easy basic stuff first though:
-- Power for the fuel pump is supplied via a separate red wire that connects at the battery positive cable clamp. The connection must be clean, shiny and tight. These get disturbed when taking the battery in and out, and suffer from corrosion if the battery isn't properly vented outside the battery box while charging.
-- The ground strap that connects the negative battery terminal to the chassis is a known weak point. The copper braid inside the protective plastic sleeve will corrode over time, and that's accelerated by the presence of corrosive charging fumes if the battery isn't properly vented outside the well.
-- The fuel pump relay depends on a tach signal that's passed via the green wire from the distributor to the spark box.
-- The relay will close briefly if the car has been setting a bit (minutes not hours) as the key is turned to position 2 (engine run), then it drops out until it sees tach pulses from the engine cranking on the starter. Listen for the pump to run before cranking. Have an assistant listen for pump run during cranking. If you don't hear the pump run at those times, carefully remove the fuel pump relay and install a temporary jumper in the socket between 30 and 87. If you are making a jumper, build it with a switch in it so you can safely start and stop the pump for testing. Jumper with switch closed should cause the pump to run.
-- If the pump runs but fuel pressure is low, find out why.
-----
You can test for ignition function using a standard inductive-pickup timing light on a plug lead or a HT coil wire. Light should flash while crankin.
You can test for injector function using a "noid light", an LED indicator that fits between an injector and its electrical plug. (local parts store, Amazon, etc.) It should flash while cranking. The injectors are batch-fired so if one flashes it's reasonable to think that all are getting a signal, at least enough to start.
after that...
The barn doors do sometimes stick after extended storage. The doors are also known to butt up against mouse nests and other debris that might be dragged into the intake by similar rodentia. Same varmints also like to chew on wiring in that area.
Start with the terminal cleaning, the ground strap replacement, and the critical relays. Move on to deeper testing if those don't get you going. From that point you will have an idea whether the problem is fuel supply, injection, or ignition. Troubleshooting steps vary with what you find. Post back your results these first steps.
10-17-2017, 08:32 PM
#5
Rennlist Member
The car could run without the AFM hooked up (accidentally did this in my 81), but AFMs never fail in this fashion, first for everything I guess.
Most likely it is the fuel injection relay as it seems like the fuel pump works but the injectors aren't firing.
The fuel injection relay is #8 on the top row.
The computers themselves rarely fail but with the aging caps inside of them they are starting to.
The L jet ECU has no input on the cold start system really. It doesn't even have control over the fuel pump like in the later LH jet, the ignition system controls it.
Hope this helps.
10-17-2017, 08:35 PM
#6
Rennlist Member
Model year 81 cars have either K jet or L jet. The original poster hasn't said if his car is NA or ROW so it's just guessing at this point.
The car could run without the AFM hooked up (accidentally did this in my 81), but AFMs never fail in this fashion, first for everything I guess.
Most likely it is the fuel injection relay as it seems like the fuel pump works but the injectors aren't firing.
The fuel injection relay is #8 on the top row.
The computers themselves rarely fail but with the aging caps inside of them they are starting to.
The L jet ECU has no input on the cold start system really. It doesn't even have control over the fuel pump like in the later LH jet, the ignition system controls it.
Hope this helps.
The car could run without the AFM hooked up (accidentally did this in my 81), but AFMs never fail in this fashion, first for everything I guess.
Most likely it is the fuel injection relay as it seems like the fuel pump works but the injectors aren't firing.
The fuel injection relay is #8 on the top row.
The computers themselves rarely fail but with the aging caps inside of them they are starting to.
The L jet ECU has no input on the cold start system really. It doesn't even have control over the fuel pump like in the later LH jet, the ignition system controls it.
Hope this helps.
Dang, was sure I read that twice, as 91.
