Pre-engaged starter motor wiring.

#1
My first posting on this forum and I am wondering if anyone can help.
I have four Rover P6 manuals (genuine service manual) with differing wiring schematic.
I have a 1972 P6 which uses the lumenition MK12 (fitted in the late 70's) and the car was working perfectly (all gauges) so I know the wiring is OK, but since getting my car back from my mechanic who changed the (engine in situ) gearbox, the gauges stopped working. I discovered leads to the oil sender and water temperature transmitter were left off. Re-attached and smoke emitted from under the dashboard, so quickly isolated the battery. I removed the dashboard which revealed the burnt wiring. I unwrapped the electrical tape from within the inside bulkhead right through to inside the engine compartment where it splits off to the ignition coil and fitted the correct rated cable and decided to perform a static test which showed a dead short to ground on one of the cables which goes to the + side of the ignition coil. The manual shows a supply from the ignition sw pin 3 white cable to a junction (shares distribution to other circuits) then directly to the tachometer then out to the + side of the ignition coil. There is also a slightly thicker gauge cable using the same colour code which is white with a yellow tracer. Using my DVM with both leads disconnected from the ignition coil I have a dead short to ground. After some pondering and long extension leads, led me to the the starter motor which is where it takes a permanent 12 volts according to the service manual, but the white and yellow lead is connected to one side of the starter (cables look original) and when I disconnect this (joined via a bullet connector to the terminal of the solenoid) the short is removed, yet this looks like it's never been disturbed. Where should this join? Or can I just split using a double bullet connector and take a positive 12 volt supply from the main 12 volt supply to the starter motor.
Any advice, help would be greater appreciated.
Thanking anyone in advance for your reply.
I have tomorrow off and I want to make sure I am taking the right approach in resolving this anomaly.
Regards,
Dave
 

sdibbers

Well-Known Member
#2
Two things to help you get an answer:
(1) Original market the car was built for (UK, EU, US etc) and if it’s a 4 or 8 cylinder car.

(2) Try posting in the P6 Electrics forum

It seems strange that this all happened after the cad was worked on. Could a wire have been caught that caused a short? Is there any sort of visible damage on the loom?

When the wires get old they can sometimes be hard to identify, could the mechanic inadvertently disconnected a few wires and reconnected them incorrectly (this would be my first guess).

It could be the solenoid has suffered a full short which is causing the issue, but I haven’t seen such a fault before.

I would trace all the wires as per the wiring diagram for the age, model, market relative to what you have there. I’m almost sure that a wire has been misconnected.
 

roverp480

Active Member
#3
I was also wondering if some connection has been swapped over but was having difficulty determining what was possible without knowing exact spec. of car concerned.
 
#4
Two things to help you get an answer:
(1) Original market the car was built for (UK, EU, US etc) and if it’s a 4 or 8 cylinder car.

(2) Try posting in the P6 Electrics forum

It seems strange that this all happened after the cad was worked on. Could a wire have been caught that caused a short? Is there any sort of visible damage on the loom?

When the wires get old they can sometimes be hard to identify, could the mechanic inadvertently disconnected a few wires and reconnected them incorrectly (this would be my first guess).

It could be the solenoid has suffered a full short which is causing the issue, but I haven’t seen such a fault before.

I would trace all the wires as per the wiring diagram for the age, model, market relative to what you have there. I’m almost sure that a wire has been misconnected.
Thank you for your reply. In fact I have traced a dead short to the solenoid, so disconnected the leads which (bullet connectors) go to the solenoid and measured the cable which goes to the + side of the ignition coil and no short, so again, checked all the leads (HT lead) which again showed a dead short. Thought this very strange so disconnected main HT lead (thick lead) and measured dead short again and followed the thick cable behind the engine which enters the drivers footwell and the isolated post which has the main positive feed cable to this post and terminals feeding the rest of the car including the starter, only to discover this to be the problem. Next thing is; where do I get a replacement?
 
#5
I was also wondering if some connection has been swapped over but was having difficulty determining what was possible without knowing exact spec. of car concerned.
This was my first thought, but I guess with the placement of wires/cables, it would be difficult to misplace. I have discovered the problem to the main stud carrying the main positive feed cable from the battery to have a short straight to chassis. So next on my list is getting a replacement stud.
Thanks for you reply.
 
#6
I was also wondering if some connection has been swapped over but was having difficulty determining what was possible without knowing exact spec. of car concerned.
Thank you for your reply. Its standard except for the Lumenition MK 12.
I have done further investigations.
 
#7
Update:
I traced the short down to the main cable junction in the drivers foot well which carries the battery + cable which joins another cable to the starter motor. After some WD40 I managed to remove the nut and separated the cable expecting the fault to be the breakdown of the isolating nylon shoulder washer, only to find the brown lead which travels up under the dash onto another junction box/connector which has the dead short.
I removed the junction box as much as I could, but looks like more of the dash needs to be removed. Has anyone had any issues with this. See pictures.
I would appreciate any help and advice. Does this box breakdown? If so, where can I obtain one from? Or is there a suitable replacement?
Many thanks.
Best regards,
Dave
 

Attachments

quattro

Administrator
Staff member
#8
It's called an Ammeter shunt, why do you think it has a short? I've never had a problem with one, but I'm sure you can find one from one of the well known suppliers, Wins, MGBD, etc
 
#9
It's called an Ammeter shunt, why do you think it has a short? I've never had a problem with one, but I'm sure you can find one from one of the well known suppliers, Wins, MGBD, etc
Well if I put my meter (ohms) between ground (door hinge) and the thick cable, one of which is the positive cable from the battery, I am reading 2.2 ohms which is what I read at the starter solenoid and the cable which from the same (smaller gauge) point, ends up at the + side of the ignition coil.
Back to the stud in the drivers foot well which is the common junction for the starter solenoid cable, along with the brown cable which goes up through the back of the speaker recess ending up at the junction box. (see picture) When I undo the nut which secures the three cables and then meter them individually, the short is removed, but still on the brown cable which goes to this junction box.
Top image below is the brown cable that reads 2.2 oms to chassis (ground) with the second images showing the cable going to a common junction to distribute the 12 volts DC to the rest of the car.
The brown lead can't be at the same potential as ground, which it is if you are reading resistance. (battery disconnected)
So are you saying the second image is the "Ammeter shunt" which is item number 29 in the schematic (Haynes Rover 2000 & 2200 owners workshop manual)
Thanks and look forward to your reply.
Regards, Dave
 

Attachments

quattro

Administrator
Staff member
#10
I don't have any wiring diagrams for the four cylinder cars so don't know which number the shunt is. If you have an ammeter, two of the wires from the shunt will go to that, so may be worth checking they're attached and intact.
 
#12
I find vehicle electrics a bit of a nightmare but if you disconnect the connections on the ammeter shunt one at a time and see if one of them removes the short on your brown wire this may move you on a bit further.
Best of luck
 

cobraboy

Well-Known Member
#13
Yes if you mean (ammeter) is the charging voltage indicator. I will check later. Many thanks.
Not being picky but he doesn't mean that.
Ammeters and voltmeters are different instruments and are wired differently.
An ammeter will tell you how much current is passing the instrument and in which direction, ie, as a charge or discharge measured in Amperes.
An ammeter does not indicate charging voltage.
A voltmeter will tell you battery voltage.
 

colnerov

Well-Known Member
#14
Hi, A safe and effective way to find dead shorts is to disconnect the earth lead from the battery and wire in a bulb between them. This gives a suitable load for the short without letting all the smoke out and the bulb will light. Now you can have the time to go round fault finding by disconnecting and checking, once you isolate the short the light will go out.

Colin
 
#15
Not being picky but he doesn't mean that.
Ammeters and voltmeters are different instruments and are wired differently.
An ammeter will tell you how much current is passing the instrument and in which direction, ie, as a charge or discharge measured in Amperes.
An ammeter does not indicate charging voltage.
A voltmeter will tell you battery voltage.
Ok, so I do have an ammeter which sits above my oil pressure gauge. I stand corrected and rightly so.
My problem is too much current is being drawn via the tachometer of which the output lead goes directly to the positive side of my ignition coil which I replaced on spec, but the primary coil resistance measures 1.4 ohms with the one I removed measuring 1.5 ohms. The cable is original, factory fitted. The manual shows a ballast wire (item 79 in various repair manuals) So with just over 11.5 volts on the coil with a 1.4 ohm resistance it's pulling just over 8 amps. Anyone know the rating of the cable as mine gets very hot and melts. Why? The car does run nice and smoothly, but I switch off immediately. The manual shows the tachometer going to the positive side of the coil in the service manuals. Now I have a Lumetion MK12 electronic ignition so the white / black coming from the negative side of the coil which use to go to the distributor is disconnected (correct) with this being replaced by a lead from the lumention pack. Everything worked with no change in wiring. So why is the wiring burning? Should the output side from the tachometer go to the negative side of the ignition coil, which would mean I would then need to source another 12 volt supply to feed the positive side of the coil. Two wires connect the positive side with one coming from the topside of the starter motor which is the cranking 12 volts supply and remains there after the car has started.

The lumention MK 12 was already fitted on the car (guess in the 70's) which ran perfectly, prior to my mechanic having the car to swap out the gearbox and since then, may not be connected, I get burning cable (from the tachometer)

Any help would be most appreciated. I don't see increasing the gauge of the wire to compensate for the current draw. Also conflicting reading on when to use a ballast resistor which depends in the primary coil resistance.
Looking forward to some help on this one. Many thanks in advance.
 
#16
I find vehicle electrics a bit of a nightmare but if you disconnect the connections on the ammeter shunt one at a time and see if one of them removes the short on your brown wire this may move you on a bit further.
Best of luck
I have replied to Cobraboy's thread.If this makes sense?
 

cobraboy

Well-Known Member
#17
I am sorry I cannot help. The only thing I do with Lumenition units is remove them, I don't like them.

Is the car an auto ? I cannot see why you have all this trouble from a gearbox swap. The only thing I can think of is a case of wiring up the gearbox reverse / inhibitor switch wrongly which has led to a short somewhere.
 

harveyp6

Well-Known Member
#18
The lumention MK 12 was already fitted on the car (guess in the 70's) which ran perfectly, prior to my mechanic having the car to swap out the gearbox and since then, may not be connected, I get burning cable (from the tachometer)
Go around all the joints between the engine and starter motor, engine and bellhousing, and (if it's a BW35) bellhousing and gearbox, and check for a trapped wire.
 
#19
Go around all the joints between the engine and starter motor, engine and bellhousing, and (if it's a BW35) bellhousing and gearbox, and check for a trapped wire.
That was my initial thought which one would have thought would be obvious. What keeps throwing me off this trail is that it does run perfectly aside from excess current draw through the cable from the tacho. I have looked under the engine especially around the starter motor and everything seems in order. Also what else would be that close to the gearbox to get trapped?
 
#20
I am sorry I cannot help. The only thing I do with Lumenition units is remove them, I don't like them.

Is the car an auto ? I cannot see why you have all this trouble from a gearbox swap. The only thing I can think of is a case of wiring up the gearbox reverse / inhibitor switch wrongly which has led to a short somewhere.
4 speed manual, so no inhibitor switch. I can see no other cables that would be that close to the gearbox to get trapped. The engine was not removed, but tilted in order to withdraw the gearbox. I am losing the will to live! lol.
 
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