Jag was running well until the other day when started stalling. While attempting to diagnose, I noticed the ign amplifier is getting hot, faster than the motor. After less than a minute of the motor running the amplifier is too hot to touch. I replaced it, but it's still doing the same thing. So I'm concluding that the amplifier gets too hot and shorts out causing the car to stall. When it cools off, the car will start again and run another minute. I checked the wires around the amp to make sure none were shorting out but didn't see anything. other ideas? the coils?
Thanks for the help.

 

Maybe I should add that the 5.3 has the Lucas ignition and the plugs, wires, & distributor parts were changed a few years ago. It had been running fine... Thanks!

 

More info for you specialists out there.... at a cold idle, the amp was registering 110 degrees F. After opening the throttle to 2k rpm for about 30 seconds the amp was reading 130F. After letting idle another 5 min, the temp was 150F. I have not been able to find an operating temperature for the ign amp. The primary ign coil is reading 1.6 ohms while the secondary coil is reading 1.8 - 2.0 ohms. The Accel universal high output coils on my shelf read 1.6 ohms. I still don't see any shorting wires or other signs of what the problem may be. Should the amplifier really be running as hot as it is? I still think there is something making it run hot, I just don't know what. Thanks for the help!

 

The amps DO get hot. I've never measured the temp...but yours sounds higher than I would expect.

The factory coils are about 1.0 ohm primary resistance. Not sure if too much coil resistance would cause your problem. I'll have to mull it over.


Cheers
DD

 

Yes I too would be curious on that... I am still having issues with the 1986 xjs starting and staying running.. I have not changed any relays or parts.. not sure that's the problem... the car runs real nice I will drive it around for awhile.. some times it will stall out and then take a few minutes to restart.. other times it will not restart for hours... the pump runs like it should upon start up, but sometimes will shut off after engine is running for a few seconds.. I have tried to pinpoint problem by messing with the wires while engine is running, assuming a short, but to no avail..... I have purchased a relay and will get an ignition module (GM) but now I'm just throwing money at it... Are both the relays in the trunk fuel pump relays and should they be the same?

 

I'm presuming you're referring to the "fuel pump" and "main" relays in the trunk. Both are a bit unique in that they have 30-85-86-87-87 terminal configuration (as opposed to the common 30-85-86-87-87a). The main relay is further unique in that it has a diode on terminal 86.

Either is a possible culprit in engine stalling and/or an inoperative fuel pump.

The ECU is also suspect if the fuel pump doesn't work, as it provides the ground for the f/p relay. It's somewhat common for that aspect of ECU function to give trouble. Grounding the orange wire at the f/p relay bypasses the ECU function...but also allows the pump to run whenever the key is "on"...which carries some safety issues.

The ignition modules usually fail when hot...but that's not carved in granite.

Cheers
DD

 

Thanks for the reply. Do you think the ignition module in the distributor could cause this?

 

What amp did you buy?
A few years ago after replacing plugs, wires, distributor cap and coil, I blew the amp(within2 blocks of home). Bought an aftermarket amp and car was running fine but the amp was getting superhot. Replaced the amp again with a GM model and while it did naturally get good and warm it was nothing like the aftermarket.

 

I didn't know there is a GM replacement part available... that would be great... do you have the part number or where I can find it? I replaced it with a used OEM off ebay. I dug into distributor to check it out and it all looks good. I don't see anything that could be shorting. The amp only goes to a couple places, right? The ECU for speed and tach, then the fuel injection, the distributor and coils. So it has to be one of those if there's a problem.... I'll change coils next.

 

A bit of clarification: There s no GM replacement for the *amplifier*.

However one of the parts * inside * the amplifier is a GM ignition module and they're known to fail.

Delco# D1906 GM#10482820

The GM number has been superceded but I can't remember the new number. Just Google it

Cheers
DD

 

Yes that's an important point. I just replaced the module inside the amp unit..thanks Doug.. Could a bad alternator affect the fuel delivery... The kitty wouldn't start again and the battery was low... I plugged it into a charger overnight and it started and ran the next morning as if nothing was wrong....
My Kitty wants out of the garage......
chip chip cheerio.
Doug

 

Yes.

No.

:-)


The fuel injection system, like the other systems, wants and relies on full voltage to operate properly. If the battery was fully charged you could unplug the alternator and engine would run fine....for a short period of time until the battery drained.

But the F.I. system doesn't actually "know" if the alternator is faulty. There's no direct tie-in.

Cheers
DD

 

Ok, according to Jag-lovers.org, inside the amp is the GM ignition module, 2 resistors of 10K ohm, 6.5 ohm, a zener diode, and a condenser. The free XJS book (by Kirby Palm with tips by many Jag mechanics) from the website lists step by step diagnostics for most systems also and they have a large parts interchange list. According to the book, either my zener diode in the amp is bad or I have an overvoltage problem most often caused by a bad plug wire, making the diode run too hot. You can make your own amp with a trip to autozone and radio shack, and save a lot of money in the process....

 

To test the diode use a meter most have a diode function these days. The most common failure is to go leaky so will have a reading one way and be open VCR the otherway. If your meter does not have a diode function use ohms on about 10kZ range

 

Oops a normal diode will read one way and be open cct the other a faulty one will either be open cct both ways (usually will have cracked as well) or give reading both ways. One note here that the diode should be tested out of the cct as other components can cause incorrect readings

 

sorry for confusing the issue..... I meant the GM ignition module
broke my own rule...VVVVVV