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MG MGB GT V8 Factory Originals Technical - Wiring that engine bay
|I have just reached that stage of my v8 conversion were i should be firing the engine this weekend after completing the wiring of the engine bay. I have a brand new v8 wiring loom that i have fitted circa 1974 and a rover SD 1 engine, am i right in saying that i can do the wiring as per a factory v8 wiring chart, is it going to be that easy, do i have to use the coil that came with the SD 1 engine, i have a heavy brown and to smaller white wire on the rover starter were do they go ?. From where to where did the original engine earthing strap go as it looks like i dont have one, any help would be appreciated as it would be nice to hear the car fire this weekend a landmark for me.|
|Carbs? If so it should be exactly the same bar one possible exception. That concerns the coil and starter, but as the SD1s post-dated the factory V8 I would imagine they would be the same, 2300 and 2600 SD1s were. The looms for these SD1s and the V8 have a ballast resistor for the coil built into the loom, use a 6v coil, and a starter solenoid with an extra contact to provide a boost voltage to the coil during cranking. You must use a 6v coil if you use the loom ballast, or a 12v coil and bypass the ballast, you cannot mix and match or you will get problems. The extra solenoid contact is not required with a 12v coil, and if you do not have one for a 6v system you may get diffcult starting under adverse weather/poor state of tune conditions. This feature can be simulated with a relay if required.|
As for colours all the browns go on the same solenoid stud as the main battery cable. On my V8 and SD1 2300 and 2600 diagrams the solenoid operate wire is white with a brown stripe, and the boost wire is white/light-blue on the V8 and white/yellow on the SD1s. At the solenoid there were originally different sized spades for the operate and boost wires - standard for the operate and smaller than usual for the boost, but on some rebuilt starters I have seen they are both of the larger size. The other end of the boost wire goes to the coil +ve in all cases.
You must have an engine or transmission ground strap or the cranking current will damage choke, acellerator etc. cables and you will get slow cranking. On the V8 it bypasses the rubber gearbox mounts on the rear cross-member. On earlier MGBs it bypasses the front left engine mounting rubber, and on the in-line SD1s one end was on the lower starter mounting bolt and the other to a suitable chassis bolt somewhere nearby.
|Thanks for that paul just had a look in the garage and what you have said all makes perfect sense, the only thing that confuses me now is that in the wiring diagram for the factory V8 it shows only a single white and brown going to the distributor but on mine i have two wires a red and a blue ?|
|Just to add some further points i have found in a box given to me bye the person who had previously bought the rover engine a Lucas 2ce constant energy ignition unit which consists of a coil mounted on a squre backplate, would i be right in assuming that this coil is a 6 volt unit how would i intergate this system into the standard arrangement. Would i be better ditching this system and buying a different distributor, the engine was apparently seen running before purchase so i know the ignition unit works.|
|In reading the how to give your Mg v8 power book it looks as though the person that bought the engine not only should have retained the Sd 1 coil but also the ballast resistor unit with its plug in connections, oh what to do ?|
|i think im getting somewhere now i can see the red and blue wires plug into the ignition control box leaving two wires of it which obviously go to either side of the coil, my next question is, is this coil mounted on the control box a 6 volt coil requiring me to add the extra boost wire from the starter motor or would it be a 12 volt unit requiring me to bypass the resitor wire ?|
|Two wires from the distributor does indicate some kind of electronic trigger, but that didn't apply to the factory V8 and the colours don't tie up with the electronic triggers used on the rubber bumper North American MGBs.|
The two diagrams I have for the factory V8 show two white/light-greens on the coil +ve and two white/blacks on the coil -ve.
Measure the resistance of your coil primary with a digital ohmmeter. If it shows about 3 ohms it is a 12v coil, if it measures about 1.5 ohms it is a 6v coil. Note that Lucas Sport coils can measure about 2.4 ohms and these are 12v.
It sounds like you have the SD1 electronic ignition with a separate module under the coil, not the early OPUS (‘opless) system or the late distributor with the integral module.
The distributor is connected to the ignition module by a screened, twin cable and double spade connector. The red wire is connected to the small spade on the module and blue to the larger spade. The cable screen is attached to one of the module mounting bolt and the module is also connected to the engine by a braided strap.
The ignition module should have three wires ending in a three-pin multiplug. Connections to the multiplug are; white 12v supply from the ignition switch (no ballast resistor required), white/black is the tacho signal (and to the ECU if you have Efi), if there is a white/black with the black sleeve it is not used.
Incidentally, the sensor on the crank pulley has nothing to do with the ignition.
|Many thanks for that Geoff, just one thing puzzles me shouldndnt one of those wires also be connected to the minus side of the coil ?|
The white/black wire from the module is connected to the coil -ve and to the tacho and the ECU (if you have fuel injection). The white from the module goes to the coil +ve and the 12v ignition supply.
Where are you in Hampshire? Home for me was Ashurst before I moved to Scotland.
|Cheers Geoff, i live in Southampton not far from Ashurst as you are aware.|
This thread was discussed between 19/02/2004 and 24/02/2004
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