3.4L van intermittent no-start/fuel pump relay chattering
CANADA43
10-25-2004, 08:47 AM
(THIS IS FROM PONTIAC TRANS SPORT FORUM, BUT POSTED HERE DUE TO SAME VEHICLE PLATFORM AS LUMINA CAR /APV)
Anybody seen this problem before? I have a 1996 Pontiac Trans Sport 3.4L (without Passkey anti-theft) which on occasion won't start after being parked.
The fault only happens when the vehicle has been sitting, it does not die while being driven (so far). The engine cranks normally but does not fire. The peculiar and significant thing about the fault is that the fuel pump relay (driven directly by the PCM), chatters at about a 10 Hz rate. Instead of receiving a steady 12VDC to keep it energized, it is receiving a pulse stream from the PCM.
The first few times this occurred, I heard the relay "chattering " under the hood , and was able to start the van by unplugging the relay and shorting the connector terminals for the contacts together with a wire jumper ( these terminals connect "hot at all times" 12VDC from the PCM fuse (orange wire) through the relay contacts to the fuel pump +ve). The electric pump would run and I could start the engine easily. By the time I arrived at my destination, the fuel pump relay 'drive' would be normal and I could plug the relay back in and continue normal operation.
The last few times this problem occurred, the van would not fire -- no spark as long as the relay drive was pulsing. When it would finally go back to normal operation of the relay, the engine would start OK.
Last Spring in cold weather I had the van towed 2 blocks to a Pontiac dealer with this problem. The problem resolved before they inspected it; however after the Technician's inspection the Service Advisor said there was no fuel pressure at the test port and that the pump must be deffective (shorted), overloading the relay causing it to chatter. That diagnosis made no sense to me. They wanted to replace the fuel pump as the solution ($600). I got past the Service Advisor and asked the mechanic to show me the fuel pressure -- the car started on the hoist when I tried it, and fuel pressure was normal (>30psi). I left with just the relay changed, as they thought the original would have burned -up contacts from excessive current to the pump. (I cut it open later and found its' contacts to be perfect). While at the dealership I asked the S.A. if he knew of any TSBs, or other incidences of this problem; he said he had not heard of this problem before.
I had the same problem one week later, but not again all summer until this Fall's cold and wet weather. I had to have the vehicle towed home and it still wouldn't start the next day. As soon as I accessed the PCM behind the glove box and jiggled it the relay chattering stopped and car could start. However, later the same day, still in the driveway, the fault was back and no amount of jiggling or other manipulations had any effect. Later still I went out to test for spark, and the fault was gone. (Cold, wet weather all day).
Vibration seems to have little effect. I have checked the PCM connectors' security and pulled, twisted and wiggled the wires coming from it, as mentioned above, to no effect.
I suspect the fault is inside the PCM and is thermally induced by cold weather. The next time it happens I will try a warm hair dryer on the aluminum housing of the PCM.
I wonder if there is a fix for this (repair a cracked solder joint, etc.) other than replacing the PCM. I work in electronics, and will open the PCM up and inspect it before forking out for a new one.
Sorry for such a long dissertation, but does anyone know about this?
Anybody seen this problem before? I have a 1996 Pontiac Trans Sport 3.4L (without Passkey anti-theft) which on occasion won't start after being parked.
The fault only happens when the vehicle has been sitting, it does not die while being driven (so far). The engine cranks normally but does not fire. The peculiar and significant thing about the fault is that the fuel pump relay (driven directly by the PCM), chatters at about a 10 Hz rate. Instead of receiving a steady 12VDC to keep it energized, it is receiving a pulse stream from the PCM.
The first few times this occurred, I heard the relay "chattering " under the hood , and was able to start the van by unplugging the relay and shorting the connector terminals for the contacts together with a wire jumper ( these terminals connect "hot at all times" 12VDC from the PCM fuse (orange wire) through the relay contacts to the fuel pump +ve). The electric pump would run and I could start the engine easily. By the time I arrived at my destination, the fuel pump relay 'drive' would be normal and I could plug the relay back in and continue normal operation.
The last few times this problem occurred, the van would not fire -- no spark as long as the relay drive was pulsing. When it would finally go back to normal operation of the relay, the engine would start OK.
Last Spring in cold weather I had the van towed 2 blocks to a Pontiac dealer with this problem. The problem resolved before they inspected it; however after the Technician's inspection the Service Advisor said there was no fuel pressure at the test port and that the pump must be deffective (shorted), overloading the relay causing it to chatter. That diagnosis made no sense to me. They wanted to replace the fuel pump as the solution ($600). I got past the Service Advisor and asked the mechanic to show me the fuel pressure -- the car started on the hoist when I tried it, and fuel pressure was normal (>30psi). I left with just the relay changed, as they thought the original would have burned -up contacts from excessive current to the pump. (I cut it open later and found its' contacts to be perfect). While at the dealership I asked the S.A. if he knew of any TSBs, or other incidences of this problem; he said he had not heard of this problem before.
I had the same problem one week later, but not again all summer until this Fall's cold and wet weather. I had to have the vehicle towed home and it still wouldn't start the next day. As soon as I accessed the PCM behind the glove box and jiggled it the relay chattering stopped and car could start. However, later the same day, still in the driveway, the fault was back and no amount of jiggling or other manipulations had any effect. Later still I went out to test for spark, and the fault was gone. (Cold, wet weather all day).
Vibration seems to have little effect. I have checked the PCM connectors' security and pulled, twisted and wiggled the wires coming from it, as mentioned above, to no effect.
I suspect the fault is inside the PCM and is thermally induced by cold weather. The next time it happens I will try a warm hair dryer on the aluminum housing of the PCM.
I wonder if there is a fix for this (repair a cracked solder joint, etc.) other than replacing the PCM. I work in electronics, and will open the PCM up and inspect it before forking out for a new one.
Sorry for such a long dissertation, but does anyone know about this?
jeffcoslacker
10-25-2004, 10:22 AM
Never run across that one....but it sounds to me like you are approaching the problem logically and should find the problem. I guess all I can offer is my stock answer, verify the ground before you chase electrical faults. Sounds like you probably already know that.
richtazz
10-25-2004, 11:20 AM
If you want to pull the PCM open , I would bet it's a loose connection or corrosion in it. IF it's just a loose connection, you can re-solder it, if it's corroded, it replace time.
CANADA43
10-26-2004, 08:04 AM
Well, the hair dryer worked this cool wet morning, after 4 minutes of heat the fuel pump relay stopped chattering and car could start.
Regarding the PCM, can it be operated on? Someone suggested that it would be hermetically sealed in potting compound, silicone, epoxy or similar material, making it impossible to work on.
Re: the ground connection; it's good on both the pump and relay; they won't energize at all without it. But hey-- I wasn't thinking about the ground for the PCM. A poor ground would certainly screw that up...
Regarding the PCM, can it be operated on? Someone suggested that it would be hermetically sealed in potting compound, silicone, epoxy or similar material, making it impossible to work on.
Re: the ground connection; it's good on both the pump and relay; they won't energize at all without it. But hey-- I wasn't thinking about the ground for the PCM. A poor ground would certainly screw that up...
putz65
02-08-2005, 04:44 PM
I also have this problem with my 96 lumina apv. I have replaced the relay and the fuel pump and it would still do it. The only difference with mine was that it only did it during hot summer days and it was always in the morning. The way I got it to run was either to pull the relay out and wait 10 15 minutes then put it in and it would start or just wait 10 15 minutes with the relay in the car and it would start it was a very weird problem but I haven't had it happen all winter.
CANADA43
10-25-2011, 01:47 PM
Sorry for not posting the solution here in more timely fashion. I have not been on forum for some time.
I resolved this problem by replacing the PCM computer. I had opened the original one from the car but even though I work in the electronics field, could not find any faults on the circuit boards, connectors, etc. All components are sealed within a coating for vibration & moisture resistance.
I bought a used PCM and the vehicle runs perfectly with the new unit. No problem with it in the intervening years.
I resolved this problem by replacing the PCM computer. I had opened the original one from the car but even though I work in the electronics field, could not find any faults on the circuit boards, connectors, etc. All components are sealed within a coating for vibration & moisture resistance.
I bought a used PCM and the vehicle runs perfectly with the new unit. No problem with it in the intervening years.
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