QUOTE(ClayPerrine @ Dec 29 2014, 04:15 AM) 
I suspect it is a blown diode causing a reverse voltage spike.
You've got an O-scope, don't you? Time to check the voltage with it!
--DD
Full Version: Intermittent Problem with Betty's car
You've got an O-scope, don't you? Time to check the voltage with it!
--DD
QUOTE(Dave_Darling @ Dec 29 2014, 03:26 PM)
QUOTE(ClayPerrine @ Dec 29 2014, 04:15 AM)
I suspect it is a blown diode causing a reverse voltage spike.
You've got an O-scope, don't you? Time to check the voltage with it!
--DD
I also have a freshly rebuilt alternator that was already scheduled to be installed on her car tonight.
QUOTE(ClayPerrine @ Dec 29 2014, 04:30 PM)
QUOTE(Dave_Darling @ Dec 29 2014, 03:26 PM)
QUOTE(ClayPerrine @ Dec 29 2014, 04:15 AM)
I suspect it is a blown diode causing a reverse voltage spike.
You've got an O-scope, don't you? Time to check the voltage with it!
--DD
I also have a freshly rebuilt alternator that was already scheduled to be installed on her car tonight.
the one real man repair on these cars is the alternator in place...
rich
QUOTE(ClayPerrine @ Dec 22 2014, 05:17 PM)
About 2 weeks ago, Betty came home and said her 74, 1.8L Ljet 914 was intermittently cutting out. I took it out to test drive it, and sure enough, it will randomly cut out. Sometimes mildly, sometimes it feels like the engine shuts off completely, then turns back on.
So I started troubleshooting.
Nope, grounds for the injectors are fine.
Put the LM-1 in the tailpipe and had her drive it. Every time it cuts out, the LM-1 shows it going lean. I put an L-Jet tester I have on it and ran through all the tests and it shows as working fine. I had it running in the garage, and I went through a "wiggle test", wiggling every wire and connection on the engine. Nothing I do can replicate the issue in the garage.
So I started throwing parts at it.
To date I have replaced:
The air flow meter.
The fuel injection wiring harness.
The double relay.
The dropping resistors.
The fuel pump.
The ECU.
The CHT.
The whole distributor with a different pertronix, and tried points too.
Nothing I have done to date helps. So I am hoping the collective knowledge of this place will help. I have checked all the simple stuff already. This is going to be hard to fix, whatever it is.
I am now to the
stage, and looking for suggestions.
Sort of late to this discussion but I had a cut out problem similar to what you describe. It was the temperature head sensor. It was not seated properly causing it to cut out. I tried similar things but once I replaced that, the car ran fine. If you still have problems, you might want to look at that.
Will we be seeing a FS post from you soon? You have lots of spare parts now, that's for sure.
Hope you found it.
As I remember, it is a good feeling.
Hope you found it.
As I remember, it is a good feeling.
QUOTE(ClayPerrine @ Dec 29 2014, 01:30 PM)
I also have a freshly rebuilt alternator that was already scheduled to be installed on her car tonight.
I'd still suggest checking the voltage with an O-scope. It will give you more confidence that the alternator was the problem--and will tell you what kind of problem it was.
--DD
Relay board failing.
QUOTE(malcolm2 @ Dec 29 2014, 10:38 PM)
Will we be seeing a FS post from you soon? You have lots of spare parts now, that's for sure.
After almost 30 years of owning and working on 914s, I have LOTS of spare parts. I buy up the L-Jet fuel injection parts as soon as I can, because they are getting rarer. I also have lots of heater parts, engine tin, and windage trays too.
I even have bus alternator harnesses that were mis-labeled as for a 914. They have a pigtail off the 3 pin connector to hook up the charge light circuit. They would be perfect for someone with carbs who wants to eliminate the relay board.
A little late and a bit off topic but.....
Disconnect the alternator and run the car ?
Doesn't that damage the alternator ?
Disconnect the alternator and run the car ?
Doesn't that damage the alternator ?
QUOTE(rgalla9146 @ Dec 30 2014, 09:24 AM)
A little late and a bit off topic but.....
Disconnect the alternator and run the car ?
Doesn't that damage the alternator ?
Yes it is a possibility if the engine is running. It is called a Load Dump and is normally associated with disconnecting the battery since batteries receive most of the maintenance and battery cables/terminals can become corroded/oxidized.
In this case, I think he only disabled the alternator by removing the control voltage to the armature. No issues here.
I agree with Dave to look at the battery/alternator voltage with an oscilloscope. Voltmeters are great but can't show any voltages that are moving or spiking quickly.
QUOTE(rgalla9146 @ Dec 30 2014, 08:24 AM)
A little late and a bit off topic but.....
Disconnect the alternator and run the car ?
Doesn't that damage the alternator ?
Nope. Just turns it off. It will just run on the battery at that point.
QUOTE(ClayPerrine @ Dec 30 2014, 04:22 PM)
It will just run on the battery at that point.
And you've found that it runs correctly with out the alternator providing energy but stumbles with the alternator powering the car, correct?
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