Coming in late....


If you don't mind me asking, how did you check them?



AC Delco D1906. The GM "long number" was 10482820 but it has been changed to something else....which I can't remember.

The a section in the ROM for testing the CEI which would identify a bad module. I'm not aware of a bench test for the module but there might be something out there on the www.

The module (and coils) often fail after absorbing engine heat.





If it's really old, or original, it'll say 1875990

Cheers
DD

 

Common culprit on V12s as well....but the Lucas ignition cars didn't have 'em !

Cheers
DD

 

Doug supplied the GM part numbers, it would probably be one of his posts on another forum that I would check for an answer anyway so Thanks to Doug for the info. How did you check coils - primary resistance cold vs. hot? Any oilly goo under the coil? I seem to remember that they crack sometimes and make a mess as they fail.

Ron

 

When checking coil resistance short the meter leads together and note the reading before measuring the coil. Then subtract this from the coil reading to get the actual coil resistance. You need to short the leads because the lead resistance could be as much as 0.5Z (Ohms) this is significant and needs to be corrected for. (Unless of course you have a meter with a NULL function.)

The coil should be about 0.8-1.0Z at 20°C the resistance of the coil will change as it gets hotter. So measure it at ambient.

 

As Warren said.

Mine tested within accepted spec when at ambient, and the car started fine. Was a mongrel to start when a hot start was required.

That is when I found the "front" coil was 2.4 ohms, and NOT hot to the touch, BUT, the "prime" coil was over 3 ohms at engine operating temp.

Too hard, single T coil installed all good.

My a/c fuel cooler has been gone since the Sanden upgrade in 1997, and as with Warren in our "friendly" outback 52c++, and NO isssues with fuel of any kind. I still got a/c however, toooooo old for NO a/c, getting soft.

The ignition module I always use is an Echlin TP45, should be readily available in the USA, getting harder to find down here now, and my amp is mounted out the front of the radiator, where that 2nd coil used to live and NEVER gets hot or even warm, so all is good.

 

I checked mine at ambient temperature and made sure my meter was zeroed (infinitied?) before testing. I got a reading that fluctuated between .68 and .80 ohms (expected a stable reading). The HT lead and the small wires except for those connecting the coils were disconnected during the test.

Waiting for a new distributor cap and rotor right now. I'll check it again at operating temperature once I get it running. Thank for the help fellas!

 

Were you using a digital or analog meter?

I would expect the reading to reasonably stable - using a DVM I would expect stability to ±1 count for a good meter.

Was the reading stable when you shorted the leads?

Bare in mind temperature can effect the reading as temperature increases the resistance will also rise, this can be as much as 0.5Z/10°C

The 0.8Z is measured at 20°C so if your ambient temp was 10°C coil resistance will be lower.

Were both coils roughly the same?

 

I've inserted answers in the quote above.

On a slightly separate subject, I received my new distributor cap, rotor and spark shield last night. I knew my rotor was ugly and needed to be replaced, but I didn't realize how bad my cap was until I had the new one to compare it to. The new one has a spring loaded contact so the HT lead from the coil actually touches the rotor. This was missing from my old one! It looked like it had rusted and fell out. I'm surprised the car even ran. I thought it idled well before, but it's super smooth now (awesome!!)!

The new vacuum advance is also installed, so as soon as the weather clears up, I'll take it for a spin and see if the new parts make as much of an improvement as I expect.

 

You need to remove the coil from the circuit - this is most likely why your readings were unstable, especially with solid state components with a capacitor across the coil. With the coil still in situ you could have could have something else effecting the readings.

When you parallel resistance you will get 1/R=(1/R1+1/R2)/Rn

Lets break this down and assume you have 0.8Z for the coil and 5.0Z somewhere else.

1/0.8+1/5.0 = 0.689Z
...... 2

If the 5.0Z is 0.8Z you will halve your reading to 0.4Z so the reading will be inaccurate.

I hope I did not too technical, but it is important to understand what can effect readings so you don't go replacing coils that are OK.

 

To make sure we're clear, I disconnected everything (HT lead inlcuded) from the coil in th V except the two wires that connect it with the coil in front of the radiator. Are there other components in that line that I wasn't aware of?

 

FYI, my Marelli tends to have a lumpy idle if the fuel cooler is not working.

 

SO if I understand correctly the 2 coils were still connected together - am I correct?

If so disconnect them the 0.8 to 1.0Z is the primary winding resistance, measured across the 2 screw terminals.

Secondary coil resistance measured between the "-" terminal and HT lead should be 2.5Z

The ROM I have does not show the configuration of the secondary coil so I will assume the resistance is for each coil....

If this is ok then get ready for some voltage measurements.

Disconnect the "-" from the coil and connect voltmeter between coil "-" and battery earth, with the ignition ON you should read battery voltage.

Re-connect the "-" to the coil and then connect your voltmeter between coil "-" and battery earth with ignition ON you should read no more than 2 volts - if the reading is higher than 2 volts check all of your earthing points.

 

Hi Guys. May I jump onto this thread? I'm a newbie, so go easy
Have an XJ-SC (1986) V12. When the engine is hot, under any
acceleration (and in particular going uphill, it's
embarrassing), she coughs and splutters and occasionally
backfires.
I've changed the fuel pump and checked the filter.
Seems to only happen when the engine is hot.
It was suggested that the ignition amplifier module was the culprit so I purchased a new module from the States. It lasted
one 30min trip. Went to start the engine again and no spark
whatsoever. In pouring rain (naturally) I put the module
that was playing up back in (worked ok when cold). Drove for
about 15mins. Engine dead cut again. Tilt-tray brought the
car home. On looking at the whole module now, LOTS of burnt
wires mostly going back to the rear coil!!
I will have to replace the whole module housing, not just
the part so I can get all the new wires.
Damn!
These amplifiers are quite expensive and I'm interested in the possibility of changing to a single coil, which I presume has a different ignition amplifier. Therefore before I spend a lot of money I would like to gather your thoughts. Thanks. Rod.

 

Hi Rod,
Best thing to do is start another thread you will get more help, also put the car details in your signature V12 I6 etc.

 

Warren,
My results are in your write-up below. Thanks yet again

 

I wanted to toss one other idea out there. As far as I know, I'm still running the original fuel pump. It's not making any unusual noises, but if it's old and weak, maybe it's struggling against the higher fuel rail pressure I'm seeing when the engine compartment is hot?

 

Thanks for your advice and have done so. Rod.

 

According to another forum member, the HE engines with Lucas ignition (not Marelli) have two coils in parallel. Primary resistance is 1.2 ohms each, and when wired in parallel, the result is 0.6 ohms. When I test my coils individually, I get about 1.1 ohms on each and roughly 0.7 ohms when connected. I think this is close enough to spec to rule primary resistance out as the problem. BTW, this is at ambient temperature. I still haven't had a chance to test with a hot engine.

Yep this would be right


I got nothing (infinity) when I tested this. What the heck does that mean? Would it even run if my secondary winding is totally shot?

The coil is basically a voltage stepup transformer - usually with a 1-100 turns ratio. Meaning with 1volt on the primary winding (between the + and - terminals) you will have 100volts on the secondary ( - and HT to distributor).

The car will not run if the secondary is open circuit did you get this measurement for both coils?


Disconnect the "-" from the coil and connect voltmeter between coil "-" and battery earth, with the ignition ON you should read battery voltage. Cool. That's what I got.

Great the coil primary is good.

Bummer. Battery voltage again. Could this be related to the secondary winding resistance noted above? What is involved in checking all the earthing points? That could be a dozen different places, right? Maybe these are shown on a wiring diagram somewhere?

You should effectively have a short circuit between the coil "-" and battery earth (this would be a good earth). If you have a bad earth it will create resistance between coil minus and earth, this become a voltage divider wich will cause a voltage drop meaning the coil is not getting enough voltage on the input.

You could try running a separate wire from coil minus to battery earth. You should get minimal voltage if you re-perform the test.

To find the bad eatrh (or broken wire) use your Ohm meter to trace each wire back. Measure each wire from end to end you should have ZERO resistance (no more than 0.05Z).

If you have not found the problem when I get home I will do you a diagram with more explanation.

 

I have attached a diagram of how the coil connects (it is very basic).

The red lines are what you measured coil minus (CM) to battery earth.

With CM disconnected you are in fact measuring directly across the battery. Because the coil has such small resistance there is negligable voltage drop so you should measure battery voltage. The Volt meter establishes the connection you removed (CM)

With CM re-connected the circuit for the coil is made and there should be no voltage drop (this is the voltage you measure with CM re-connected), although a small 2volt drop is acceptable. What is happening here is the voltage (actually current but lets not confuse the issue) is flowing through the meter not the wiring back to battery earth, meaning you most likely have a bad connection or broken wire somewhere.

 

I finally got tired of throwing time and money at this problem and took it to a local shop that specializes in old Jags. This is actually my wife's car, and she was starting to complain so I had to take drastic measures.

At any rate, the mechanic (Jesse at J&S - Big Thanks!!) used a diagnostic machine to determine that the problem was both the air temperature sensor and the coolant temperature sensor. He replaced those, adjusted the throttle butterflies and rods and set the ECU. The total bill was just south of $400 (ouch!).

I haven't had a chance to put a lot of miles on it yet, but so far the idle is smoother and the hot start problem seems to be completely cleared up. I usually prefer to do my own work, but in this case I was chasing my tail. If this turns out to be the permanent cure, it was worth the expense.