How to replace Dynamo with Alternator

Ok there is nothing wrong with a properly maintained dynamo and control box on your imp, but only if they have been properly maintained! But either way fitting an alternator is a good idea as they are generally more reliable and charge even when the engine is idling - great for the times you get in traffic these days.

Tools Required
Soldering Iron, Crimp Tool (correct tool for the terminals you intend to use), wire cutter and wire stripper , 1/2AF sockets and spanners, 12" straight edge, general tools

Supplies Required
Lucas Alternator Plug, 2 metres of 6mm2 wire (Red or Brown), 2 metres of 1mm2 wire (Brown/Yellow or Brown), a proper rear alternator bracket but if you do not have one of those the get some steel tube with clearance for 5/16UNF bolts though the centre (one like the one between the water pump alloy and steel brackets will do, lots of 'heavy' and 'standard' 5/16 flat washers, some assorted 5/16UNF bolts and nuts, some wiring loom tape (NOT insulation tape!)/ flexi conduit and/or cable ties, electrical test lamp, new fan belt either an 1125 or an 1137 (HB1125 and HB1137 at Halfords) (buy both and return the one that does not fit). Electrical Terminals Pref an 5/16 / 8mm ring terminal to suit the 6mm2 cable or failing that a decent 10mm female blade (lucar) connector and a male and female blade connector with insulation for the 1mm2 wire (bullet connectors are also an option). You may also wish to have a flask of weak lemon drink to hand. (The hobbies of Simon Quinlank)

Parts
Most alternators from the scrap yard will work, but many these days are huge due to all the electrical loads on modern cars, the item that was factory fitted to the very last Imps is the Lucas LRA100, it was also fitted to a number of British cars of the same era, such as Mini's and Viva's. Many people have also sucessfully fitted the alternator fitted to 80's and 90's Mini and Metro's as they are still quite small in size. For this article we will be fitting the Lucas LRA100.


Section 1 - Mounting the Alternator

 

1. First job when doing this is to disconnect the battery negative terminal (white arrow) do not do any electrical work on you car without doing this! if you can't see which is the negative terminal if you car is a bit grubby, on a negative earth car (as most imps are) then is the wire connected to the chassis, on an imp this the point of attachment is a 7/16UNF thread hole at the end of the white arrow. The positive battery lead is on a negative earth car is the one that goes to the solenoid (red arrow) See Fig 1.
   
   Fig 1
   
    
2. Ok before we get involved with the electrickery we need to do some mechanical work. First remove the dynamo. Slacken of the bolts using your 1/2AF tools as indicated in Fig 2. You will also need to slacken of the bolt indicated in Fig 3.
   
   Fig 2
   
    
3. You will also need to slacken of the bolt indicated in Fig 2. Then rotate the dynamo to slacken the fan belt and then remove the belt.
   
   Fig 3
   
    
4. Next reach round to the back of the dynamo and disconnect the large wire and small wire, these should just pull off as they are lucar (blade) connectors (Don't be surprised id they literal fall of - they did here!)
   
   Now remove all the bolts indicated in Fig 2, support to dynamo while you do this to stop it dropping away and watch your fingers.
    
5. With the dynamo removed you can see rear dynamo support as indicated in Fig 3. Now either:
   
    
   1. If you have a proper rear alternator support bracket, then you will need to remove this bracket
      
      or
       
   2. If you do not have the proper rear alternator bracket leave this one in place as you can adapt it to suit the alternator.
6. Ok the next step is to stepback and have a look at the differences between the dymano and the alt. As you see in FIg 4, the alternator is quite a bit shorter than the dynamo. The relation ship between the dynamo pulley and the dynamo front mount is nearly but not quite the same as the alternator - the alternator is more compact generally apart from the diameter! You can also see in this picture the different rear support brackets. The one we are using is not a proper factory support (this was a pressed item) but a dynamo bracket that has has a steel tube welded/brazed to it to make the distance up.
   
   Fig 4
   
    
7. Right if you have a proper rear alternator bracket (or even a modfied one like ours) then fit this to the block - do not tighten the bolts up fully at this point as you might need to move it around a little.
   
   Fig 5
   
    
8. Ok I can already hear some of saying "Hey I don't have a proper bracket, only the old dynamo one!". Right well for you guys this is where the little piece of steel tube comes in. As stated above in the supplies listing a spare one of those spacer tubes that normally lives between the water pump alloy and steel brackets is just the job. Failing that you can buy short bits of tube at you local B&Q (other DIY stores are availbale!). Just buy a bit of steel tube (not iron or gas pipe!) that a 5/16UNF will fit into (take a bolt with you to check) and try and get some tube with thickish walls. Failing both those options you can (if you are a bodger at heart!) use loads of heavy 5/16 (8mm) washers to make the gap you, but unless you are called Steve best get some tube!
   
   *Tip, you will need to cut the tube to the correct length you need to get a nice square cut, for this you can use a pipe cutter (plumbing one will do steel if you go slow). If you don't have a pipe cutter, then if you put a pipe clip (jubilee etc) over the tube and tighten it up it will give you a nice square edge to cut againts with your hacksaw, then just neaten up/deburr with a file.
   
    
9. Right time to mount the alternator. It basically fits in the same way as the dynamo, fit the bolts to the main mounting points first (yellow arrows Fig 6), you will need a longer 5/16UNF bolt for the rear mount to take into account the steel tube/rear alternator bracket Because the distance between the alternator and the front mount is shorter that the dynamo, you will need to add some washers between the front alternator mounting the cast alloy water pump bracket in order to move the alternator in the direction of the green arrow. Nip the bolts up (yellow arrows) so the alternator is not wobbling around. Then stand above the above the alternator pulley and visually site to see if it is in line with the water pump/crank pulley's. Get it close enough using this method, you can fine tune the alignment later. Put the bolt in as per the white arrow in Fig 6, you may need to bend the adjustment bracket a little to meet the alternator or use washers.
   
   Fig 6
   
   
    
10. So now we have the basic mechanical work done with apart from fine tuning the alignment, belt fitting and tightening up all the bolts which we will do later....it is time to move onto the electrics! But first if you have your flask of weak lemon drink, DRINK IT NOW! YES ALL OF IT!

1. Ok just in case you have got this far without dissconnecting the battery DO IT NOW! refer to point of section one above. You should never do any electrical work on your imp without doing this first, especially as they are normally not fitted with any fuses, your imp could end up a burnt out wreck if you create a short with the battery still connected!
    
2. First job is to make up the new alternator cable, for this you need to the two lengths of wire (6mm2 and 1mm2), the alternator plug kit, the loom tape/cable ties/flexi conduit, the 8mm ring terminal and the terminals you have chosen for the thin 1mm2 wire). Now normally I perfer to crimp connectors onto wire (as long as you good quality crimps and tool that is!) but in this case I prefer to solder the alternator terminals on the wire, manily due to the fact that the Lucar terminals are 'flags' (that is the wire comes in at 90degs to the terminal) and my crimp tool does not like, and soldering is quick, plus more people are likely to have a soldering iron than a proper crimp tool! I don't intend to give a full lesson on soldering here, or teach people to suck eggs, but just a quick run down and some tips.
   
    
   • 'Tin' each part. Strip insulation from one end of each of the two wires and tin the conductor, then tin the terminals.
   • Clean and tin the solderin iron tip, this helps with the heat transfer.
   • Always heat the part to be tinned up with the soldering iron.
   • Always feed the solder into the item to be tinned when it is hot enough, never melt the solder directly with the soldering iron..
   • The solder should look bright and shiny, if it is dull and grey, it was not hot enough.
   • Place tinned parts together, you can fold the crimp ears around the wire using long nose pliers, then heat assembly, feed extra solder into the assembly, when solder flows remove heat and keep assembly still until cooled, joint should be shiny and smooth, not grey and dull.
      
   • the big terminal is for the 6mm2 wire and the little one for the 1mm2 wire!
3. once the solder has cooled, you can assemble the terminal into the alternator plug. You will notice that there is space for two large flags and one small, push the small Lucar terminal into the alternator pluh housing until it clicks. Then insert the large Lucar flag into the alternator plug (Fig 7), it does not matter which you use. Then also insert the spare large Lucar flag in the empty terminal (green arrow), nothing electrical about this last step but will help stop the alternator plug coming loose if you no not have a alternator plug clip (blue arrow). Be sure to push all the terminal into the housing until they click. Then clip the back of the plug on.
   
   Fig 7
   
    
4. Now before you fit the new loom you need to get to the right length, so loosely plug the alternator plug into the alternator, then run the cables as per Fig 8, just temp hold the wires to the car etc with string/tape/cables ties. Make sure you put a bit of slack as marked in Fig 8 by the arrows.
   
   Fig 8
   
    
5. Disconnect the large Lucar terminal arrowed in Fig 9, just tuck the wire and terminal out of the way for now.
   
   Fig 9
   
    
6. Ok trim the 6mm2 cable to length and crimp on the 8mm ring terminal so that it will line up with the post (the one that the battery lead is attached to, and NOT the one the starter cable is attached to) on the solenoid as per Fig 10. Ignore the thin in Fig 10 for the time being.
   
   Fig 10
   
    
7. Now onto the control box, Fig 11 show the item with clearer terminal markings, you should be able to see the markings moulded into the black cover. It looks confusing but is quite simple.
   
    
   • 'D' - the fat wire connected to terminal D is the other end of the fat wire that used to go to the back of the dynamo
   • 'W/L' - Simple this one, W/L is the wire that goes to the little red light on the dashboard or in the binnicle
   • 'F' - the thin wire connected to terminal F is the other end of the thin wire that used to go to the back of the dynamo
   • 'B' - One of these fat wires goes to the ignition switch, the other goes to the solenoid and is the one you disconnected in Fig 9
   
   Fig 11
   
   
   
   Terminal 'B' is a common terminal so it does not matter which way round the two fat wires are conected, but you need to know which is which! First disconnect the two wires from the control box. Method one is to connect a powered test lamp or continuity tester (on every digital multimeter) to the terminal you disconnected in Fig 9 above then buzz out the two wires you have just disconnected from the control box one at a time - the one that lights the lamp or sounds the buzzer is the wire YOU DO NOT NEED, mark both ends of his wire with tape/marker pen so you know which it is (we will call this wire the solenoid wire). If you don't have a test lamp or a continuity tester you can ID the wires physically by unwraping the loom and seeing which is which, you want the fat brown wire that goes between the 'B' terminal and that dissappears with the rest of the loom into the car (we will call this the wire the power wire) and NOT the fat brown wire that goes to the solenoid (solenoid wire).
    
8. Now unwrap the loom and move the 'power'fat wire and the thinner brown/yellow wire that was connected to W/L from their current positions marked with dotted lines (Fig 12) to their new positions marked with solid lines (Fig 12). You will not now the fat brown wire goes to large (10mm) male lucar terminal, you may need to trim the length of the wire a little and also check the condition of the terminal on the end of the fat brown wire, if in doubt replace with a new 10mm female lucar terminal. You will also note that the thin brown wire with the yellow strip or 'tracer' needs connecting to the thin wire in you newly made alternator loom. You can make this connection with a male and female lucar terminals of bullet terminals, now you can trim the thin wire in yoru alternator loom to length. Ok that is the difficult electrical bit done, now you just need to make good all the connections and tidy the wiring up.
   
   Fig 12
   
    
9. Remove the alternator loom and wrap with loom tape (remember DO NOT use insulation tape it is rubbish for this job), or bundle the alternator loom together with cable ties at regular intervals, or run the wires through some small diameter flexi conduit. Then once you have finished the loom you need to clip and secure the loom to the car, you can do this by cable ties to the existing loom bundle. Make sure you leave some slack at the alternator end to allow for engine movement, plus the alternator plug firmly into the alternator and then if you have one pop the retaining clip on. Now at the other end undo and remove the nut and spring washer on the solenoid post (remember one the battery lead is attached to) and fix the 8mm ring terminal to the stud, refit the washer and nut and tighten. Reconnect up the thin wires and bundle up with the rest of the cars loom. You can now remove the control box, you can leave the old dynamo wire and the 'solenoid' wires in place just tuck the ends out of the way and wrap in loom tape (you could of course remove all the redundant wires if you wished and then rewrap all the engine bay wires for a neat look)

1. You might remember that the alternator is roughly mounted in place, but it needs to be aligned as accurately as possible to ensure the water pump and alternator bearings are no overloaded and the belt does not wear out too quickly.
    
2. As per Fig 6 above the main mounting bolts should be fairly tight, you now need to check the alignment of the alterntor pulley in relation to the water and crank pulley's - getting this wrong with at best wear the fan belt rapidly or at worst loose it and also damage the alt/water pump bearings. Make sure the main mounting bolts (so the alternator is not tipping) are tight fit a fan belt then swing the alternator up to make the belt tight.use a straight edge such as a steel rule to check alignment. (fig 13)
   
   Fig 13
   
    
3. insert/remove washers in the area indicated to get the correct alignment - a little tip is get a selection of 'heavy' and 'standard' 5/16 washers as they are different thicknesses; useful for gettingthe right thickness ie two thick, one thick one thin, two thick one this etc.
   
   It is also worth using the steel rule in other planes plus sighting along the fan belt runs to check the alignment.

1. Ok adjust up the alternator using the adjuster bracket to get the correct fan belt tension (about 1" slack in the centre of the longest run water pump to crank pulley)
    
2. Make sure all the mountings are tight.
    
3. Make sure the alternator plug is fitted in to the back of the alt
    
4. Make sure the other electrial conections are correct, spinning the alt up with out it being connected up WILL DAMAGE IT
    
5. Make sure the noting is going to dangle / get tangles up in the engine (such as the old wiring if you have left it in place).
    
6. Reconnect the battery - Nothing should happen at this point!
    
7. Ok ready for a test. First check that the ignition warninglight is not blown - this is now an essential part of the new charging circuit - if it is blown the alternator will not work properly (see Aldrins comment - a resistor is a good idea).
    
8. Turn the ignition key one click - the ignition (red) warning light should light up.
    
9. Start the car, the red light should now go out! Ifit does not there is a problem -switch off and investigate. If it does got out you can also check the output by using multi meter across the battery even at a fast idle voltage should be 13volts +
    

(c) Andy Smith 2025