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MG MGB Technical - Bad Earthing
| I am in the process of refreshing the connections on a 1980 roadster.The wiring diagrams per the repair operation manual show two earthing connections in the cockpit,one of which in the vicinity of the windscreen wiper & one on the passenger side. I am having difficulty finding these. Can anyone help? I think the black wire attached to the windscreen wiper motor is to earth the motor itself. I am also trying to slacken the earthing screw under the bonnet which is tucked behind the brake master cylinder with no success. Is it a pozzidrive head by any chance? I have been meaning to do the above for sometime as indicators slow down etc.when fan or wires on & more recently ignition light stays on dimly for a few miles after starting but then goes out. (I think after a jolt/ bumpy road.Alt / water belt changed a couple of years ago & I only do approx 1500 miles a year - tension OK). Thanks Charles |
| Charles9 |
| I have looked at the manual again & it looks like there is only one earth connection in the cockpit somewhere near the windscreen wiper motor. I also made a typo towards the end when ment to say "when fan or wipers on" not "wires". Apologies. Cheers, Charles |
| Charles9 |
| Windscreen wiper motor securing bolt, large eye with several earth (black) wires. All screws on MG's are pozidrive. (Should be!!) |
| Allan Reeling |
| Thanks Allan.The earthing point which appears to be above the windscreen wiper motor is impossible to get to without removing the motor. (It is most uncomfortable to access upside down whilst being trapped below the steering wheel). I will assume that earthing point is satisfactory! Cheers, Charles |
| Charles9 |
| On both mine there is an earthing point to the body, not the motor bolt, top right-hand corner of the bulkhead immediately above and to the right of the motor. As Charles says probably not easy to get at unless you remove the motor. But the symptom described i.e. indicators slowing down when the wipers are on is not an earthing problem but a supply problem, and to some extent is 'normal'. Original MGB indicator flashers are very sensitive to changes in supply voltage as well as current, they use that to 'indicate' bulb failure at a corner by not flashing, leaving the working bulb lit permanently. As such some slowing is inevitable, but should be minimal with the engine running and the alternator charging. As more electrical loads are turned on the will slow slightly, but should always be within the MOT limits. If they slow beyond that then you need to do some voltage tests on the 12v supply side, more than the earth side. Modern electronic flashers are not affected by voltage changes, they indicate bulb failure in a completely different way by flashing the remaining bulb at double-speed. The only earth for the indicator on CB cars is where the light units mount to the body. RB cars do have an earth wire, which is shared with the headlights and the heater fan, and is in the engine compartment. The wipers use the other earthing point behind the motor. So whilst a bad earth for the front indicators might affect an RB car when the heater fan is on, turning the headlights on would have a far greater effect. It's more likely to be a poor connection in the green circuit that feeds 12v to ignition switched and fused circuits. This would probably be in the fusebox of pre-77 cars, but 77 and later have an ignition relay and separately fused ignition circuits. A 1980 has three separately fused ignition circuits - one off the white from the ignition switch which feeds the ignition system and via an in-line fuse the indicators, heater fan and GT HRW; one off the ignition relay which feeds the fuel pump, OD, ignition warning light and via another in-line fuse the cooling fan; and a third circuit which feeds all the other ignition fused stuff via the fusebox. |
| paulh4 |
| Thanks Paul for your detailed response. I will report back my success or otherwise once I have completed the exercise. Cheers, Charles |
| Charles9 |
| I have added relays to several circuits (headlights, overdrive, A/C, reverse lights) and have earthed some accessories to the chassis where practical. I have also upgraded the alternator to 65 AMPS and added an extra 12 v battery in parallel. All of these things have helped improve the performance of electrics on the B in general. |
| Glenn Mallory |
| Since my last post i have done the following:-
Cleaned battery posts ;removed battery earthing strap & after cleaning up grounding point reattached strap which is in good condition; done the same for the engine to chassis earthing strap;cleaned up all contacts on ignition & starter relays & replaced front relay with a new spare;replaced 4 way fuse box with new lucas box ensuring all fuses clean at each end;cleaned 3 in line fuses & wire contact buttons within fuse holders; replaced all lighting female sleeves behind grill (cleaning all bullets in the process); cleaned bullet wire connectors to front indicator housings; at the rear of the engine compartment replaced (so far) 2 female double sleeves (WN WN to WN WN & G G to G G ) and 2 single female sleeves (GR to GR & R to R); disconnected the main positive feed & 3 positives (brown) from back of starter solanoid & overdrive wires / reverse light (?) wire contacts - cleaned & reattached; cleaned low tension coil contacts + & - ;checked voltage reading of battery at posts (12.6 volts); same reading recorded at brown connection at fuse box; same reading on others at other fuse terminal points. I thought i had cured the problem as the next time i started the car the red ignition light went out straight away & the indicators worked. I checked,using a gunson's voltmeter, the alternator output. Their notes indicated that the light load voltage at minimum 2500rpm should be between 13.6 & 14.6 (for most apps) i recorded 14.0. The heavy load voltage should be above 12.6 which it was (i recorded approx 13.0). Thinking my problem was resolved , i washed & polished the car the following day & moved it out of the garage with no problems . When i started the car again a few hours later the old problem occurred again!!! I have since driven it again and as in the past, after 2 - 3 miles the ignition light faded out & everything worked perfectly.The only thing i have done in the last couple of months is put some vaseline (not oil) on the entrance to the key slot on the ignition switch (as advised by my usual garage man who is experienced with MGBs) as having difficulty in getting the key in to disengage the steering lock. I am beginning to think i may have an intermittent fault with the alternator. I would welcome any further help! Cheers Charles . |
| Charles9 |
| Charles, I chased a similar, dim ignition light, problem on a friends late R/B roadster. After going through the obvious candidates and fitting a good replacement alternator it turned out to be the ignition relay (Black, cylindrical plastic). |
| Allan Reeling |
| UK cars are wired differently to North American, the ignition warning light on the former coming off the ignition relay hence a bad relay can cause the problem, but on North American it comes direct off the ignition switch.
If the dim glow if the ignition warning light with the engine running and the alternator charging occurs frequently enough and stays on for long enough you need to measure the voltage on each side of the bulb, and the alternator output terminal. When running this should be the same, i.e. at about 14v with minimal electrical load, but both sides should drop as the load is increased. It's this voltage being the same both sides that keeps the map out, so for it to glow one side is lower/higher than the other, and the actual voltages will give a clue as to what is happening. The brown/yellow and the brown can be measured on the alternator plug. If the brown/yellow is either lower or higher than the alternator output terminal the alternator is faulty. If they are the same at about 14v, but the white to the warning light bulb is lower, then you have a bad connection somewhere from the alternator output terminal, via the solenoid connection point, up to the brown wires in the engine bay and cabin, and through the ignition switch. Late model cars had a double-brown wire coming up from the solenoid, with a rectangular connector to the brown wires going to the fusebox and the ignition switch. This connection has caused problems in the past, and get get warm when lots of things are switched on. The white comes off the ignition switch, through the steering-column multi-plug, to the fuel pump inertia switch, to another multi-plug behind the dash. So check the white both sides of the steering column multi-plug, and if they are the same and lower than the brown at the alternator, check the both sides of the brown in the same multi-plug. That should narrow down where the problem lies, i.e. in the multi-plug (which can explain it going out after a jolt), or inside the ignition switch, or in the brown connector referenced above. |
| paulh4 |
| Paul Where is the rectangular connector located? |
| Dave O'Neill 2 |
I have the attached picture from somewhere (unfortunately I don't have a note of who or where) that shows it (arrowed) by the fusebox. Out of interest it also shows (immediately below the fusebox) the thermal circuit-breaker that these cars had for the second green circuit supplying the dual cooling fans, instead of an in-line fuse.
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| paulh4 |
| Hi all,
Thanks for your feedback. Allan, I replaced the ignition relay with a new one (per my previous post) & have just been to check that I hadn't replaced the starter relay by mistake which I hadn't! Paul, your picture is not something I recall ever seeing under the bonnet of a 1980 MGB UK spec. I have attached a picture of mine with the only change being an additional line fuse for a permanent connection of a radio / cd player to maintain memory function - hence my one off red wire. (This has been in place for 3 years). A wiring diagram of late UK MGB from Haynes clearly shows a 4 brown wire connection a way from the starter solenoid on the way to the starter relay with the other two browns going to the live side of the fuse box & the headlight switch.I did not find this in the engine bay at the rear when replacing the female sleeves (as per my previous post).Could this be behind the dashboard / by the steering column? Just in case the starter relay is causing the problem I have ordered 2 more relays as always useful to have spares for the future if not actually the solution to the current problem. (No pun intended )! Barrie 's Notes only referred to a 4 way connector with 3 whites on the bulkhead under the bonnet ( my car had 4) with a further bullet connector with 4 whites under the dashboard near the staring column. Cheers, Charles 
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| Charles9 |
| You appear to have a bundle of wires taped to the loom, just above the coil. What colour are they? 
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| Dave O'Neill 2 |
| Well spotted Dave. I had not really noticed these before. There appears to be 3 wires that end,presumably bonded together in some way as it looks like a bound stump leg. Colours appear to be one white,one white with a trace of brown & one pink(possibly faded purple). They are as the car left the factory & untouched since then. (I have had the car from new)! I can't find these on a wiring diagram. My car was built sometime in May-July 1980. Any ideas? Cheers, Charles |
| Charles9 |
| My mistake - the double brown and rectangular connector is on North American spec only.
The multiple brown connection is behind the dash, but is crimped and soldered not part-able connections as in the attached, and is very unlikely to be the cause of the problem. Can't possibly be the starter relay - with factory wiring anyway. That bundle of white, white/brown and pink are part of the ignition wiring. White comes from the ignition switch daisy-chaining on the ignition relay, white/brown goes to the in-line fuse for the GT HRW, indicators and heater fan, pink is the ballast wire going to the coil. The two whites daisy-chaining at the ignition relay (terminal 85 or 86) should be spot-welded into a single spade connector, so normally there wouldn't be a problem there. The ignition warning light comes off the ignition relay contact, which also feeds the fusebox, in-line fuse for the cooling fan, overdrive and fuel pump. Any bad connection in that circuit is likely to be in the brown feeding the relay, the relay contact, the white/brown wiring to the fusebox, or the connection point on the fusebox. There are three white/browns there, but only two spade terminals, so two of the wires are in one spade connector on one spade, and the third wire is on the other spade. If the single wire is the supply then a bad connection there will affect both of the other wires. But if the supply wire is bonded into the wiring connector with one of the others, then only the third wire will be affected. Apart from the supply wire one of the others feeds the cooling fan, and the other the fuel pump and overdrive. If the warning light only glows when either the fan or the overdrive is on, that is a clue that the problem is in that brown - relay - white/brown - fusebox circuit. But if it glows at other times then it is something else, which is why really you need to compare the voltages on the ignition warning light wires, in order to see in which area the problem lies. |
| paulh4 |
Forgot picture
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| paulh4 |
| Paul, Thanks for your clarification on the wiring. I will follow up your suggested voltage tests. Cheers, Charles |
| Charles9 |
| I had hoped to report that my problem with the ignition light had gone away after my last post on 19 July as after one more incident of the light staying on for a few miles I had no further occurrences, using the car regularly & attending the MGB register picnic on 6 August! The only unrelated issue then was with a poor idle which I cured (as previously experienced) by removing the carburettor pistons, cleaning & refitting).
Mid August the problem reoccurred of the ignition light staying on for a few miles,together with no indicator flashing being available until the light went out.I wondered if it was a duff battery cell (the battery was 7 years old although I have used a battery conditioner on it since new, the last 3 years being a C-Tek which I believe has done it's job well). I replaced the battery this week - on starting up (having already ensured the new battery was fully charged) the ignition light stayed on,albeit not quite as bright,for about a mile & then went out & the indicators started working. The following day before starting up,I noticed the clock (this is a 1980 Roadster)was not working - went round tapping any wires under the bonnet to see if a loose connection there would make the clock work but no joy. I turned on the radio which is directly wired (with an additional fuse) to the purple contact on the fuse box) & then turned it off again & the clock started working again!(The cockpit & boot lights work). Sometime later(yesterday), the clock had stopped again,although I noticed it was pulsating (like a battery clock with a flat battery). I started the car,although for the first time ever, there was no ignition light before & after the car started which it did as per normal. Once again the indicator lights didn't work until I deliberately went over a pothole at which stage normal service was resumed (including the clock working). I put the car straight in the garage at which stage I turned off with the red ignition light coming on brightly (as it should do). This morning, being careful not to disturb anything, I noted that the clock had stopped working(in pulsating mode again)& when I turned the ignition on there was no red light at all. I have been wondering if there is a fault with the alternator (which might be in some way be affecting the clock if the rectifier is defective). I pulled the plug off the back of the alternator & as per the Repair Operation Manual checked the IND & Main + plug socket connectors with a voltmeter (with the ignition switched on they all show the correct voltage when using a meter to earth) which suggests there is not a fault in the related wiring. I had hoped the clock might start working with the alternator disconnected,but it didn't! I replaced the alternator plug' turned on the ignition again and as previously the ignition light didn't come on again , but am confident the car would start as per yesterday! I am therefore at a loss as to what the problem is (and whether the clock issue is a separate problem altogether)! I note from the archives (I think 2013) that the ignition light circuit in some way triggers the alternator to charge, which otherwise needs revs of 3000 rpm to start working (beyond my understanding). Given the country lanes I use from home & therefore the relative slow levels of engine speed for the first few miles, this might give a clue as to what the problem is !!! I apologise for the long ramble,but any help as ever would be appreciated! Cheers, Charles |
| Charles9 |
| Charles, with the problem you have described, I would definitely be taking a close look at the alternator and charging system. Have a look at this link from Paul Hunt's website
http://www.mgb-stuff.org.uk/electricstext1.htm#theory With the operation of the ignition light being so intermittent, there would seem to be a problem with at least the Ind circuit from the alternator. Also, have you checked the Red ignition warning lamp and its connections? Looking at the wiring diagram for your car (page34): http://www.advanceautowire.com/mgb.pdf The most likely common point of failure is the ignition relay (assuming the alternator and charging circuit are ok) Have you checked the new relay you have fitted to see if it has an intermittent fault? Other things to check are that all the fuses are tight in the fuse holders and of course the ignition switch, which I don't think you have mentioned. There have been a lot of threads with strange electrical faults being caused by faulty ignition switches. Andy |
| Andy Robinson |
| Hi Andy,
Thanks for your most informative response. The wiring diagram you refer to is the same as I am using so it confirms we are on the same page. In my last post I said I had checked the P.D. of the 3 sockets within the alternator plug(with the ignition on) which all showed 12.5 Volts when earthed using my voltmeter.In the case of the IND socket this proved (to my mind) that the cabling back through the ign light, via the spade on the fusebox , via the ign relay as controlled by the ignition switch is Ok. (It is impossible to get to the back of the ignition light itself). When I last stopped the car after the intermittent problem had cured itself (on that last trip the light never came on at all on start up or during running) the light came on as I turned off the ignition switch. This suggests to me that the alternator/ rectifier was working at the point of switch off. Once, however, the residual energy has dissipated from the alternator/rectifier circuit, the effective earthing / completion of the ignition light circuit through the alternator seems to be broken again.(For the light to work it must have a complete circuit for current to flow in either direction & goes out if the voltage both sides matches when the alternator / battery & wiring all OK). I have just checked that when I turn on the ignition the light doesn't come on , but I can hear the fuel pump clicking that is on the same circuit. I am minded to remove & check / replace the alternator & for good measure swap the newly replaced ignition relay for yet another new spare that I have to hand. I have a new ignition switch but am loathe to change that until I have eliminated any other potential faults. Do you think my logic is sound? Cheers Charles |
| Charles9 |
| Hi Charles, If you are hearing the fuel pump when you turn the ignition on, but getting no red ignition light, then I would be looking back towards the alternator and the diode pack and regulator. As it would seem that your ignition warning light does work albeit haphazardly.
You will find this useful from Paul Hunt's website http://www.mgb-stuff.org.uk/electricstext1.htm#faults "The ignition warning light is lit when the ignition is switched on but the engine is stopped by 12v coming from the ignition switch, and an earth from the IND terminal on the alternator. This earth comes through the field winding and its slip-rings and brushes, from the field terminal of the voltage regulator. During normal running the voltage regulator varies the resistance of this earth, to vary the current through the field windings, which controls the output voltage of the alternator. If there is a problem with the field winding or its slip rings and brushes, or with the voltage regulator itself, then that is almost certainly going to affect the output voltage of the alternator, especially as more electrical loads are switched on. Check the brushes for wear, and try cleaning the slip-rings." Intermittent faults that you are chasing are always hard to find and it could be that you have a fault with the alternator. Alternator parts are readily available and not too expensive if you need them. http://www.jcrsupplies.co.uk/products/ALTERNATOR_REBUILD_PARTS Andy |
| Andy Robinson |
| Hi Andy, Thanks for your speedy response & the useful links.I shall remove the alternator & see if I can spot anything obviously amiss , if beyond my abilities, then get it checked out by some local auto electricians who have been recommended to me. Cheers, Charles |
| Charles9 |
| Hi Andy,
I pulled the alternator today & using my old battery on the work bench tested if there was a P.D. using the IND contact to prove an earthing route when not spinning & there was none. I removed the plastic backing which showed the wiring at the top to be OK.This suggested the problem was inside! I took the alternator to the auto electricians who confirmed it was defective internally. They replaced,while I waited,the regulator,rectifier & the brushes & then confirmed it was working satisfactorily.This was all done within 1/2 hour which was most impressive (assuming everything functions normally once I have completed refitting it tomorrow )! When I returned home I repeated the P.D. test using the IND contact which confirmed there was now a satisfactory circuit made! If there are any members of this site who live near Redditch I can recommend "Auto Electrics Redditch" who were most helpful - the technician said he frequently worked on my type of alternator (an 18 ACR Lucas). Thanks again Andy & indeed Paul & others for your help. Cheers Charles |
| Charles9 |
| Hi Charles, great news and well done in finding the cause of your problem. Just goes to show the benefit of logical fault finding and testing. Certainly great service from your auto electrician! Cheers Andy |
| Andy Robinson |
This thread was discussed between 22/06/2017 and 06/09/2017
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