1996 Lumina APV fuel relay

Hello all My first post...an electrical challenge, I hope someone can help figure this one out. 3400 Engine. This problem is intermittent. Sometimes (once every dozen
or so starts) the engine turns over but doesn't start. When this happens, I have found out the following so far; - When I turn the ignition key to the 'run ' position (where the Powertrain Control Module is normally supposed to provide voltage to the fuel pump relay to cause the pump to run for about 2 seconds), the relay chatters very loudly and DOES NOT shut off. The pump keeps running even without turning the ignition swith all the way to start the engine. I did replace the relay with a new one and NO CHANGE in symptoms. I still have no idea why the relay would be getting this jittery signal voltage. It must be a 'jittery' supply from the computer because as the relay is chattering away, the fuel pump is chattering due to the fluctuating voltage as the relay goes off/on/off/on rapidly. I tried supplying a nice smooth 12 volts straight from the battery to the grey wire if the relay and voilla, the fuel pump immediately runs smooth. BUT, when I then turn the ignition key to try to start the engine, it doesn't start. Becuse the engine is actually starting for the time being, I cannot test fuel pressure, spark under no-start condition. I just read another post that mentions the oil pressure switch on some GM's supplies power to the fuel pump (relay?). Not sure what THAT'S all about, but maybe something wrong with this switch causing interfernce voltage? The other head-scratcher is, why wouldn't the engine start at these times? The pump is running smooth. Thing is, at the moment the engine is starting/ running fine. Actually, don't know if it's rlated, but when I hooked up a fuel pressure gage to check pressure in the rail, the problem ..magically went away again...coincidence? Maybe the computer is having problems and sends weird voltage to the fuel relay and NO VOLTAGE to actually run injectors/spark etc??
ANy ideas are greatly appreciated? PS; sorry for the long-windedness Ted
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When you applied B+ to the gray FP relay wire you bypassed the relay, PCM, and wiring inbetween. That doesn't prove much as you already know that *two* relays have exhibited the same buzzing problem. You need to look at B+ and ground to the relay coil. You may have to wait for the intermittent problem to occur again to test with a DVOM, but for now you could start wiggling wires one at a time around the relay, then check for adequate terminal tension between the harness female and relay male terminals.
Even if the problem is not currently present, you can check the relay coil for potential wiring problems. Unhook the relay from it's mount but leave it's wire plug connected. Start the engine, then flip the relay over so you can see the back side of the wire terminals. Ground your DVOM and probe the Black/White wire. Anything over about 0.2 volts is excessive. Probe the other three wires. They should all read very close to battery voltage at the positive battery terminal. If any are noticeably lower then suspect that circuit. The Green/ White wire is from the PCM Blue connector (C1) terminal 8. The Orange wire is from the FP 10 amp fuse. The Gray wire is B+ to the FP (this circuit and the FP ground seem to work by your test results).
I would disregard all of the other little details that you chalk up as strange at the moment. The buzzing relay and accompanying no-start is the biggest problem for now.
Toyota MDT in MO
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Thankyou Toyota MDT
Everything you say is sound testing/ideas, thankyou. However, I'm also afraid you're right about the need to wait for the symptoms to happen again as the van ran fine all day today (actually it's my daughter's van so I basically just wait for her call at any time of the day/night that she's stranded somewhere cause ..the car won't start). With the engine running, I did wiggle just about every wire/harness etc I could get at short of getting my hands caught in the serpentine belt. But no amount of wiggling would duplicate the chattering relay or cause even a 'blip' in engine operation. When it comes down to it, there is still that one unusual symptom of there being voltage applied to the relay even though it SHOULD have stopped after the requisite 2 seconds or so upon first turning the key to 'run' position. The reason I had applied clean +12v to the grey terminal of the relay was actually not to proove/disprove a defective relay (I had already replaced the old relay with a new one and because there was no change, I eliminated a bad relay as the cause), but rather it was to see if the car would actually start if the relay was receiving clean +12v rather than a 'chattering' voltage. If I had my fuel pressure gage available at that moment, I would have installed IT to see if maybe the chattering relay and chattering fuel pump was causing low fuel pressure and that maybe THAT was the cause of no-start condition. Upon applying the clean 12 volts, the engine STILL didn't start (ya, I should have at least checked for spark, but I had no helper). So, then I thought it seems like the computer is baffed because it seems to be doing 2 weird things 1) it WAS supplying a chattery voltage to the fuel pump relay ALL THE TIME, even AFTER the 2 second run time has elapsed after turning ign key to 'run' and 2) when I DO crank the engine over while the fuel pump is chattering (and even upon connecting 'clean' +12v power) it STILL didn't start. So, to me sure seems like the computer itself is sending bad voltage, but also is NOT sending the required voltages/signals etc to cause the motor to 'fire up'...oh oh..I now have a headache..
Re "disregarding other 'strange' details", I don't like to "disregard" any symptom just yet because they may proove valuable as pieces of the puzzle. I think maybe having a diagnostic meter connected at the time of no start may be beneficial to see if that shows something or maybe a trouble code has been set in the computer indicating some OTHER component is/was at fault? Maybe that oil pressure switch interconnection that other members have mentioned may exist and may be causing problems? Tomorrow I'll head to the library to get a detailed wiring diagram (Chiltons) of this van and study it to see if that helps understanding the possibilities...
For now, I'll have to cross my fingers and either hope the problem never happens any more (right, like THAT'S gunna happen) or hope it does happen in the driveway at home when I have lots of time to troubleshoot. Toyota ...Thanks again ! The next time there's a 'no-start' condition, I'll be checking those other relay wires for ground etc.
Ted

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Correct, applying B+ to the gray wire directly operates the pump. The pump has a chassis ground that is independent of any other ground points in the circuit you are checking. Three things I can think of are 1) you didn't crank/release/crank the key enough times to prime the injectors during your direct wiring test; 2) the temporary wire lost connection during the test 3) the fuel pump or wiring from that point is bad also.
I'm not ruling out a defective PCM, but rather trying to get you to prove all circuits involved before replacing it. If you really do think that the PCM is failing even when you are positive the fuel pressure is adequate (next time bring your fuel gauge) then prove it! What is it failing to do besides close the relay? Is there spark? Is timing anywhere near normal during cranking? That's why I think it is a fuel only problem, and unless you do this for a living (and don't suck at it), then you need to be as systematic as possible. Again, I'm not ruling out a PCM problem. In fact, I would start testing at the PCM driver as hinted at last time. You can read the B+ PCM output right at the relay, Green/White wire.
If I missed something you wrote, yes, I scanned over the text really fast.
Toyota MDT in MO
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Hello Just thought I'd provide closing remark about this problem. Actually there's nothing new to report as the problem still has not recurred. I'm pretty happy about this becuase up here it has been way cold and I don't relish the thought of working on the car outside. If there's further action with this problem, I'll post updates. I'd like to think that if there's anyone else out there experiencing a similar problem, they could learn somthing usefull from this post. Even though there was no absolute resolution, due to non-recurrence, hopefully some info exchanged here was of some help. Special thanks to 'comboverfish' who provided valuable tips etc. to help me 'think outside of the box' Cheers Ted
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