3.9L Coil Question - Defender Source
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  #1  
Old December 15th, 2017, 12:08 PM
gwmckee
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George
1994 NAS D90 #1919
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3.9L Coil Question

I've been trying to troubleshoot an "engine-stumble-under-load" issue. One of the threads here suggested the coil may be the problem. I want to bench test the two coils I have to see if they are within spec. The first is a Bosch that was on the truck when I bought it. The second is a Lucas replacement I bought from AB (RTC5628L). Google searches and a call to AB got me nowhere. Does anybody here know what the resistance specs are for these coils? Primary and secondary winding values would be appreciated.

Other suggestions for possible solutions to the problem are welcome. The truck starts and idles fine. Mileage is a little on the low side. Occasional very slight back-fire when shifting. I noticed the stumbling between 2,000 and 2,500 RPM with the Lucas coil. I then swapped the Bosch coil back in and now the stumbling occurs between 2,500 and 3,000 RPM.

Other Info:

- Timing is set to 5 degrees BTDC
- New Cap, Rotor and Plug Wires from Rovers North
- No CEL or codes thrown
- Resistance on the TPS seems good across the spectrum

Other possible sources... MAF?

As always, thanks in advance for the help!
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  #2  
Old December 15th, 2017, 01:28 PM
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Davis
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Chris Davis
94 NAS D90 6.2LS
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What distributor are you running? I have had this happen when running my Mallory with the use of the ballast resistor and it was the ballast resistor.

My hardest to trouble shoot (the first time) was when one of the diodes failed in my alternator and at 2400 RPM it was leaking enough A/C into the wiring that it would affect the firing of the fuel injectors. I had the alternator checked, they found the A/C leakage, I rebuilt it and all problems went away.

It could be the coil, but two coils doing that is strange.
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  #3  
Old December 15th, 2017, 02:30 PM
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Rocky
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Chris B
72 + D1 drivetrain =?
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3.9L Coil Question

There is something weird going on with all these engines now. An occasional or chronic misfire or no start scenario. With no obvious cause or cure.
Itís as if the engineís eating coils and other components.
One suggestion was that the ground from the engine to battery was weak or limiting. Iíve not been able to prove that.
The electrical energy has to be going somewhere.....
Bizarrely three weeks ago in wet weather it went back to the no start mode. Two weeks ago I drove 500 miles in it. No problems at all. No parts swapped. Though plenty of parts travelled with me just in case...



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  #4  
Old December 15th, 2017, 03:22 PM
AlexSWB
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Aleksander
1995 Range Rover Classic
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I had a similar problem

Turns out the Bosch original coil i bought was made in Brazil.
Primary and secondary resistance was both fine.
But that does not help if they use the cheapest of the cheap materials to make it.
In the end the tower was sparking all over the place.

I work in one of the biggest car part wholesalers in Scandinavia and we sell Bosch.
But i cant recommend the retro Brazil made Bosch coils anymore.

First i threw away the Bosch coil and bough a German Beru coil
I cant remember the number but i can find out.

I Set the reluctor gap to a tight 0.10"
Checked for rotor play, my dizzy was fine though.
Changed: Genuine Lucas Cap, rotor, NGK plugs, wires (threw away the ebay blue silicones) and bought an NGK set. Changed ingition amp to a Lucas one. Turned out it was dead out of the box, changed to another ignition amp made by Mobiletron that worked.

However, my MAF, idle air valve and oxygen sensors were brand new
I dont know in what shape they are in your Truck.

If you can unplug the MAF sensor and the engine does not change rpm its time to get a new MAF.
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  #5  
Old December 15th, 2017, 08:21 PM
gwmckee
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George
1994 NAS D90 #1919
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Quote:
Originally Posted by Davis View Post
What distributor are you running? I have had this happen when running my Mallory with the use of the ballast resistor and it was the ballast resistor.

My hardest to trouble shoot (the first time) was when one of the diodes failed in my alternator and at 2400 RPM it was leaking enough A/C into the wiring that it would affect the firing of the fuel injectors. I had the alternator checked, they found the A/C leakage, I rebuilt it and all problems went away.

It could be the coil, but two coils doing that is strange.
Hey Chris - the dizzy was a replacement I bought from Rovers North soon after I bought the truck. I might have 25k miles on it. The setup is basically stock. I had the alternator rebuilt last summer as well (forgot to mention that in my original post). One thing of note... the amplifier module on the side of the distributor is a bit loose but Iím not sure that matters. The weird thing to me about the two coils is that, although the stuttering happens with both, it happens at different RPM ranges.

------ Follow up post added December 15th, 2017 09:25 PM ------

Quote:
Originally Posted by Davis View Post
What distributor are you running? I have had this happen when running my Mallory with the use of the ballast resistor and it was the ballast resistor.

My hardest to trouble shoot (the first time) was when one of the diodes failed in my alternator and at 2400 RPM it was leaking enough A/C into the wiring that it would affect the firing of the fuel injectors. I had the alternator checked, they found the A/C leakage, I rebuilt it and all problems went away.

It could be the coil, but two coils doing that is strange.
Quote:
Originally Posted by Rocky View Post
There is something weird going on with all these engines now. An occasional or chronic misfire or no start scenario. With no obvious cause or cure.
Itís as if the engineís eating coils and other components.
One suggestion was that the ground from the engine to battery was weak or limiting. Iíve not been able to prove that.
The electrical energy has to be going somewhere.....
Bizarrely three weeks ago in wet weather it went back to the no start mode. Two weeks ago I drove 500 miles in it. No problems at all. No parts swapped. Though plenty of parts travelled with me just in case...



Sent from my iPhone using Tapatalk
Hey Chris - the wet weather seems to exacerbate the issue for me too. Iíve checked the wires for leakage by misting with water while the engine is running but havenít been able to detect anything. I ran a new ground from the battery to the frame and then to the engine. I think Iíll clean the connections and put some dielectric grease on the fittings.

Thanks for the input!

------ Follow up post added December 15th, 2017 09:29 PM ------

Quote:
Originally Posted by AlexSWB View Post
I had a similar problem

Turns out the Bosch original coil i bought was made in Brazil.
Primary and secondary resistance was both fine.
But that does not help if they use the cheapest of the cheap materials to make it.
In the end the tower was sparking all over the place.

I work in one of the biggest car part wholesalers in Scandinavia and we sell Bosch.
But i cant recommend the retro Brazil made Bosch coils anymore.

First i threw away the Bosch coil and bough a German Beru coil
I cant remember the number but i can find out.

I Set the reluctor gap to a tight 0.10"
Checked for rotor play, my dizzy was fine though.
Changed: Genuine Lucas Cap, rotor, NGK plugs, wires (threw away the ebay blue silicones) and bought an NGK set. Changed ingition amp to a Lucas one. Turned out it was dead out of the box, changed to another ignition amp made by Mobiletron that worked.

However, my MAF, idle air valve and oxygen sensors were brand new
I dont know in what shape they are in your Truck.

If you can unplug the MAF sensor and the engine does not change rpm its time to get a new MAF.
Hi Aleksander - Thatís great input. Now I have my to-do list for tomorrow. Iíll let you know what I find out.
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  #6  
Old December 15th, 2017, 09:12 PM
cowboy
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steven
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Quote:
Originally Posted by AlexSWB View Post
First i threw away the Bosch coil and bough a German Beru coil
I cant remember the number but i can find out.
Yes, please.


Quote:
Originally Posted by AlexSWB View Post
changed to another ignition amp made by Mobiletron that worked.
Part number?
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  #7  
Old December 15th, 2017, 09:24 PM
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Mike
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Triple check the electrical connections to the ignition module (and the coil)
My Defender started to buck under load a few time on me and also stalled. I thought it was a fuel issue, it wasnít. The connections to the ign mod were loose. I buttoned them up with dielectric grease and all is good.

Cheers...
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  #8  
Old December 16th, 2017, 08:31 AM
AlexSWB
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Aleksander
1995 Range Rover Classic
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Quote:
Originally Posted by cowboy View Post
Yes, please.

Beru ZS120

Part number?
Bought the mobiletron from Powerspark (simonbbc) ebay.
IG-D1909H / STC1184

I would rank those STC1184 like this in terms of build quality:

Beru ZM 020
Lucas (old stock)
Mobiletron
Lucas (new)
Intermotor
Chinese made / Tong yang and the like that Britpart uses.
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  #9  
Old December 16th, 2017, 08:46 AM
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o2batsea
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Bill Adams
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Stumble can def be your ignition amp module.
Also eliminate any possible vacuum leaks.
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  #10  
Old December 16th, 2017, 09:26 AM
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Davis
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Chris Davis
94 NAS D90 6.2LS
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If your ignition module is loose, and if it get's a ground through it's case, then that is suspect as well. Regardless, tighten that up. I don't know about that particular ignition module...
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