Hi all,
Yesterday, out of the blue, after months of reliability my xjs stranded me in a parking lot. I cranked and cranked it but It wouldn't even try to catch once. It had been running fine and I shut it off then this. Now I have several clues.

1. The Temp gauge was not reading last night and I know the car can't cool down all the way in 2 hrs. In 70 degree weather. I checked the sensor lead. Ok. Can this inhibit the car from starting?

2. I noticed before it got towed away, that one of the bolts securing the ignition amp to the bracket had fallen away somewhere. Insufficient ground causing non- start?

3. This seems to me like an ecu issue. I'll pull the one from the parts jag and see if I have any luck with a fresh battery.


Any other ideas would be greatly appreciated..

Thanks!

 

A flakely temp guage in and of itself will not prevent the car from starting. If ALL the gauges are inoperative it might suggest a voltage/ignition supply problem....but we're stretching a bit at this point.







I really don't recall a bracket for the amp. I thought it bolts directly to the left intake manifold. But, anyway, I think one of the amp wires IS supposed to be grounded under a bolt head....so that might be a good clue. If I'm remembering correctly, that is







ECU failure is possible but there are other much more likely suspects. First task is to determine "has spark" or "does not have spark" and work from there


Cheers
DD

 

Thanks for the pointers Doug! And I meant the ignition coil, not amp sorry. And now I have another problem. Overnight the car decided to heal itself so now I can't know if it will strand me again or what! I don't have much to go on now other than that I messed around a little with the fuel pump relay before the car's miraculous revival. I guess the tow bill was enough to satisfy its money thirsty personality for now! :-)

 

Intermittant no-starts are often caused by poor connections, corroded terminals, poor grounds, etc.

If you had the time and energy you could go around the car and carefully inspect-clean-tighten all the connections, clean terminals, etc. Sometimes the fault is very subtle such as a terminal inside a relay socket that doesn't contact the relay pin firmly enough.

This is a useful chore on general princples even if you don't happen across the source of the actual problem. Most of the infamous Lucas problems can be traced back to faulty connections.

Cheers
DD

 

I'll clean fuel pump relay terminals as well as any other relays nearby and hope for the best :-)

 

It's back! It won't start again. At least this time it's in my garage. What I noticed though was that both times now, it has been cool outside like mid to high 50's F. Coincidence?

 

Ok so I have been doing some tests and there indeed is no spark. I have been trying to trace it backwards and I highly suspect the ignition amp. I should have a new one by tomorrow. One question though.. Is/where is, there a relay that sends power to the ignition amplifier?

 

I don't recall an amp relay....at least not a relay for the amp only. If it IS powered from a relay it would likely be the "ignition load relay" which powers up other items. Is anything else not working?

Cheers
DD

 

No but here is a big chunk of information: so I started diagnosing the no spark. With key "on" I had 12v to both sides of both coils. (they are linked in parallel) SO.. there are 5 wires going to the main coil. 2 come from the ignition amp (solid white, white w/ black stripe). 2 link the aux coil in parallel with the main coil (pink, pink w/ black stripe). The one left is apparently an aux 12v (pink). I swapped the amp and coil out from my dad's car and still no start. Like I said I got 12v off both sides of the coil. I was very confused. "I have voltage to the coils so why no spark?" then it hit me. "If the coils are only getting 12v, what is the purpose of the amp?" I unplugged the aux 12v and sure enough, The coil was dead on both sides! So it seems the amp was not providing any voltage. I refered to the schematic. The amp gets pulse from the ecu. The previous is all very complicating so here is the basics: THE AMP IS GETTING NO PULSE FROM THE ECU. So is there any interface box between the amp and the ecu? My problem apparently is the ecu pulse failing to get to the ignition amp. Where do the ignition amp wires go to?

 

The amp doesn't GET a pulse FROM the ECU. It SENDS a pulse TO the ECU

Cheers
DD

 

Right. And if a test light is put to the neg post it should flicker when cranking





The amp doesn't amplify voltage to the coils. It amplifies the voltage signal from the pick-up inside the distributor





Not sure what you mean by "aux 12v"





See my previous post





Can't remember the amp wire colors off hand. But...there are two wires coming out of the distributor that plug into the side of the amp. Unplug them from the amp.....they should be on a one-way plug. Check the resistance bewteen the terminals in the plug. Should be 2.2k to 4.8k ohms. If not your pick-up coil in the distributor is faulty

Cheers
DD

 

Wow thanks so much Doug! I was waaaayyyy off. I checked the resistance between the two wires and open circuit. So I guess I have a bad pick up coil?

 

The early schematics aren't always clear on ECU inputs vs. ECU outputs. Later diagrams clearly differentiate between the two





That would be my conclusion, yes.

Cheers
DD

 

Ok so guess what? we parked another car in the garage. the ambient temp rose to about 95 degrees. I tried to crank it and it started. so I shut it off and took the resistance of those same leads and it went from open circuit to 3.28k ohms. so when it will run is is 3.28k ohms, and when it won't start it is open circuit. So in other words it looks like you were dead on Doug! Plus heat seems to bring it to life which explains why it went dead out in the cold. Time to oder the pick-up coil!

thanks!

 

Sorry for resurrecting this post but I thought I should add closure. I had the pick up replaced along with the distributor cap, plug wires, coils, and amp. It runs better than it ever has with snappy throttle response and even an increase in power!