Replacing the Spark Plug Wire Set on the V12 (M70) 

Story by the backyard hack mechanic: Sean750
Document history
:
# 08 Nov 2003: added some comments from Sam
# 03 Jan 2003: initial version

 


Problem :  The production of the 750 series started in September 1987. That means that the oldest 750's are now about 15 years old. Most cars of that age (or perhaps a few years younger) will definitely need a new spark plug set sooner or later.

The OEM (original equipment manufacturer) of the complete orginal spark plug wire set is the German company BERU. This company still exists, and the set listed at the local BMW dealer is identical at the set at your local parts seller (if its selling BERU ignition wires). Unfortunately, that doesn't make much of a difference in price. These are simply outrageously priced wire sets. Some comments from folks who did made that trip to the dealer and survived it:

'I wonder if anyone has ever rebuilt their spark plug wires.  I'm having a hard time spending $400 or more for a set a plug wires.'

'I recently asked my dealer for the price for a set of spark plug wires for my 12-cylinder engine. $525. I nearly threw up!  If that thing ever breaks down because of bad spark plug wires, I'm leaving it where it is and putting a FOR SALE sign on it!!'


A complete original wire set (for both banks) will be something like 550 Euro/USdollar incl. taxes. A wire set from another supplier (for instance, BOSCH or AD) will be the same or a bit lower priced (~400 Euro/USdollar) but don't expect great differences. When you inform at your local auto parts seller for prices make sure that you are talking about a complete wire set or just a wire set for 1 bank only. When a price of about 200 Euro/USdollar is mentioned and you'll think you've got yourself a bargain, most of the time people talking about just 1 bank only.

Another problem is the presence of the CYLINDER IDENTIFICATION SENSOR, the donut-shaped sensor wrapped around spark plug wire #6 and #12. Some aftermarket wire sets don't have that sensor, leaving you with a big problem. This V12 won't run without the sensors, although some local parts seller will tell you something else.

If you purchase a 735 wire set including the sensor, you can use this sensor and there is no need to swap the 750 sensor. The 735 will fit, connector is the same.

A reader of this site (Sam) comments about this:

"The 735 pulse cable worked fine , the cable is longer than the original but I don't think it made any difference."

Models : 750i(L) with the M70 V12 engine

Used technique : First of all, the spark plug wires of the M70 aren't fancy, special high-end stuff. Even the ignition wire end plug (distributor side) and the spark plug connector are standard connectors, also used on other BMW's and even other brands.

The problem is the cylinder identification sensor installed onto 2 plug wires. What does this sensor actually do? Instead of what many people thinking, this sensor isn't for the ignition section of the DME (that's the crankshaft position sensor) but it's for the fuel injection section.

The M70 uses dual Bosch Motronice v1.2 semi-sequential fuel injection systems. The 12 injectors are running in 4 sets of 3 injectors, meaning that a set of 3 injectors is opened and closed simultaneously. So one bank one 1-3-5 are triggered, and after that 2-4-6. On bank two 7-9-11 and 8-10-12. For this system, the computer must know when to fire which group of injectors. Simply said: when the DME notices that #6 is firing, the injector group 1-3-5 is triggered to inject fuel.

On the second bank a identical system (including sensor) is present, allowing the engine to run on 1 bank only (limp mode) in case of a malfunction of the other bank.

Therefore you need switching of the cylinder identification sensor (unless you purchase a wire set including the sensor, see Sam's comment above).

When purchasing a 735 wire set, keep in mind if your existing spark plugs have a large plug end or a small plug end. If you get a wire set that only fits to a small plug end, and the end of your spark plugs cannot be screwed off, you'll be ending up buying other spark plugs. Sam about this:

"I got the wires, but the connectors for the spark plugs are smaller than stock, they will not fit around the plug. These connectors are made for without the large spark plug ends.The only spark plugs I found that fit the 750 and the end screwed off were the split fire platinums. I went ahead and got them. Plugs were 6.00 a piece.Ouch...The cable ends were made by BERMI , I wish they had the wide ends on them .I have 12 brand new Bosch platinums that I need to find a home for.."


Lets start unbolting things : The following procedure was performed on the right bank. If you are planning to replace the spark plugs wires of the left bank (seen from inside of the car), please check out the 'removing spark plug #12' section prior for removing the spark plug wire #12 of the left bank.

As said, after lots of searching and investigation I've found an alternative for the expensive wire sets. The solution isn't very far away though. You can use 2 aftermarket spark plug wire sets of the E32 735i 6-cylinder without any problems (2x6=12 but I'll bet you already found that out yourself). But, and here comes the tricky part, its essential that you can remove the wire end plug (preferably distributor side). At some sets, you can remove the wire end plug, at some sets you can't. If you can't remove the wire end plug, you can't use that particular wire set.

I've bought a fairly decent priced set of 85 Euro/USdollar from Eurocable (no affiliation), and they assured me that the wire end plug can be removed:

Here you can see that this set originally is intended for other bimmers:

Below you see the difference between the old leads and new leads. Notice that the original wire end connector has a 90-degree angle, and the new one doesn't have that. Some other aftermarket sets I investigated did had that 90 degree angle connector, but that particular connector couldn't be removed, meaning that it is unusable for my purposes.

First of all I wanted to separate the wire end connector from the new lead. I inserted a small flat-blade screwdriver between the boot and the wire, and sprayed some silicon-oil between them, allowing the boot to remove easily from the wire:

The copper-colored connector had a small black plastic 'thingy' around it, which I removed first. After that, the boot was easily removed:

Cutting the old lead to remove the cylinder identification sensor (takes a bit of courage, I know..):

Putting the new lead through the donut-hole. My excuses for the not_so_sharp_photo, but you'll get the point. If you can't put the copper-colored connector of the new lead through the hole, you must remove also this connector and after mounting the cylinder identification sensor re-install this connector with some pliers (or something similar) to ensure a proper connection with the spark plug wire:

Put the boot back on (use some silicon oil to easy up the process) and reinstall the black plastic thingy:

I just love that photo above. Install the remaining new leads to the distributor cap, keep position in mind (distribution cap is removed to make it more visible. If you are removing the distributor cap, write the original position of the leads down on some paper to ensure they are reinstalled exactly the same way):

Notice that the old protective shield/boot of the distribution cap will not fit anymore, due to the straight angle of the new wire ends. Perhaps you can cut a hole in it, allowing the leads to run freely. I discarded it:

And here you'll see the installed distributor cap and leads in my engine bay, everything fits smoothly (although it seems not possible, in fact there is plenty enough space to reinstall the air filter housing, I put the wires somewhere underneath it):

Total amount of parts and cost :
With some discount from the local parts seller, the total amount for the 2 x 735i spark plug wiring set was about 150 Euro/USdollar incl. taxes. Now where talking.

Total amount of time : about 2 hours for the right side (1 bank only). Check out the 'removing spark plug #12' section for removing the spark plug wire #12 of the left bank.

Skills needed/difficulty level : some technical skills are convenient.

Satisfactory level after the job done : I just saved myself about 350 Euro/USdollar. Very satisfactory indeed.

Satisfactory level after several thousand miles/km: almost a year later, done about 40.000 kilometer (25.000 miles), with no problem whatsoever.

If you would like to add any comments, remarks and/or corrections to this procedure, feel free to email Mike Oswald and we'll put it on our site. Share your experiences with us so others may benefit from it.