Bleeding Electricks

[RobinTR6]

Having sorted my brakes (I think) now seem to have an electric gremlin. Ive noted that if I switch on all the lights the fuel pressure drops to about 80/85PSI. The Lucas pump is refurb'd as is the PRV and is running from a relay powered directly from the starter, even tried off the alternator no difference. Switch lights off , all working OK. Im assuming that either the lights are drawing too much current or the pump connections are sub optimal.The alternator is new and 45amp rated, the loomis also new.

 

Thoughts please (before I take a match to it)

 

How can I check the current and volt draw at the pump. Please note the problem isnt apparent till I hit 3000RPM , I have a glycerine gauge plumb'd into the MU to monitor pressure.

[Peter Cobbold]

The 3000rpm may be because the engine takes more fuel. Below that the pump is delivering enough to keep MU fed and enough to open the prv. But once the fuel take by the MU exceeds the pump's max flow rate the psi will drop off.

The lights may still drop the voltage, even if you take the pump directly off the alternator.

I'd try a fresh battery. But wait for an expert to come along!

Peter

[Steves_TR6]

Dodgy earth? Just as important as the positive side !

[RobinTR6]

New battery CCA 680 so will try earth, thanks guys.

[BlueTR3A-5EKT]

Run a single fat wire in parallel (and additional) to the existing wire from the power source direct to the pump. You can run the wire out the bonnet over the car and into the boot and tape in place. Test drive again. Your existing pump supply wire could also be a problem like the existing pump to chassis earth may be. If no change it is not the cable size.

 

Cheers

Peter W

[AlanT]

There are two quite distinct situations which may account for your problem and you must decide which it is.

Or indeed if you have both problems.

 

1. the alternator and its voltage control mechanism, is failing to keep the required voltage, when faced with the combined load of lights and pump

 

2. the alternator ragulates voltage correctly but you dont get this voltage at your pump.

 

In the first case you must decide if the alternator/regulator is misconnected/faulty, or simply not by design, able to meet the requirement,

 

In the second case there must be a section of wire or a joint somewhere that carries both the current for the light and the pump.

The resistance of this wire/joint is too high and voltage drop across it increases when the lights are on and this drop applies also your pump.

 

You will see that the basic method of solving electrical problems is to divide the problem into two and find which applies by a simple test. This is call "halving the region of uncertainty".

 

All electrical fault finding is based on this but be aware that you can be confused if there are actually two faults to find.

The "weird" problems usually result from a mixture of unrelated causes.

[jerrytr5]

But does the car run poorly?

I tried 3 different fuel pressure gauges on my experimental efi - all were wildly inaccurate and not to be trusted.

Maybe it it's just the gauge?

Jerry

[RobinTR6]

Jerry, runs fine till you turn the lights on and get the revs to 3000rpm, then stutters and won't accelerate cleanly, ie its sounds like its missing. Notably I can see the fuel pressure drop from 105 psi to about 80. Given the pump is running off the starter connection via a relay then from comments problem is bad earth, dodgy alternator (mind you its new but given my recent experience with a new brake master cylinder proves nothing ). The circuit to pump shares nothing with the lights apart from the power from alternator (I think).

BTW I have asked before. The new loom has three wires to alternator, fat brown/white which is main feed, thin brown yellow, whicH think feeds charging light in dash and a thin brown, currently not connected which has 12v even when ignition is off. I haven't connected this to anything as I can't get an answer as to what its for, have asked here and the supplier, and looked at wiring diagrams but can't fathom.

Edited August 10, 2014 by RobinTR6

[AlanT]

I suggest to do nothing until you have a clear knowledge of how and why this is connected up.

 

Electrics does not yield to guess work. There are too many possiblities even with a simple cicuit.

It only yields to analysis, understanding and tests.

 

Sounds to me as if you may have not connected the sense wire.

 

Try this:

http://www.vitessesteve.co.uk/PDF/LucasFaultDiagnosisServiceManual.pdf

Edited August 10, 2014 by AlanT

[RobinTR6]

Alan, thanks, I dont have an ammeter capable of reading more than a few amps so cant do some of this. I cant see a 'sense' wire that you refer to in the documentation.

 

From the Auto sparks and other wiring diagrams the third wire that exits the loom with the main brown/white and brown yellow indicator wires is connected to the solenoid terminal (ie the +_ve battery terminal) I still dont know where this should go (I assume NOT the alternator as itsalways got 12V , ignition of/on as its connected to battery +ve). Is this the sense wire to which you refer

[Peter Cobbold]

If the pump is trying to pull fuel through a clogged filter a drop in volts caused by turning on the lights might tip it into flow deficit.

Most PI pumps have a more tortuous live supply than yours yet work OK , lights on or not.

Peter

[AlanT]

I dont know much about alternators. You have got to look this up.

 

I guessed that alternators use remote voltage sensing.

If they don't, then you are likely to get voltage variation with increased load.

[jamesStag]

Hi Robin,

that's a slightly odd problem (If I remember correctly you are running Lucas pump and PRV? )

 

Whilst I generally agree with the comments made to date, could I just let you know what I've experienced on my car and also just a couple of further things to try/check...?

 

I'm am running Lucas too - I checked out my pump elec connections given the tips on this forum. Fortunately my loom had been upgraded with a 'fat' supply wire to the pump (not via a relay incidentally), but I noticed that with say typically 13.6V at the battery I was only getting around 11.6V across the pump itself.

I noticed that the earth wire to the pump looked a 'bit weedy' in terms of wire gauge compared to the fat supply cable. So I doubled it up with a local earth from the fuel filter mounting point and T'd into the fuel pump motor earth --- Wow an extra volt (nearly 12.6V) and noticeable increased pitch in pump speed.

Definitely a happier Lucas pump, and I was able to measure the running current which was about 6.7amps.

 

In terms of checking your alternator have you checked battery voltages with it running and with/ without load?

You should see a noticeable rise in battery voltage with engine running, compared to stopped, and it should be maintained with extra load like the headlights.

as a guide typical battery voltages:

Engine stopped - around 12.6V

Engine running (above say 1500rpm) - around 13.8V.

the volts will drop a little at idle, say around 13 to 13.2 ish. (as not enough revs to charge significantly)

 

Key guide to me is that with load, like your beam headlights on, at above 1500rpm, you should still have the high end of 13 volts (above say 13.6V)

I generally find that if you look at your headlights, there should a noticeable slight increase in brightness from idle as you raise the revs to around 1500. This gives a general indication that the alternator is giving a reasonable output.

 

Hope this is of some help.

Cheers,

J.

[Guest ntc]

The extra wire is for fitting the early alternator and is not needed.

[RobinTR6]

Ok I'm on a series of checks will report progress, thanks all

[RogerH]

Hi Robin,

as Alan/Neil have suggested, the thin wire is a sensor. If your Alt has a third small male spade fitting then it goes there otherwise leave disconnected (but insulate it).

 

You could use it as a 12V low power supply take off.

 

Roger

[AlanT]

So Roger, some alternators run remote-sensing and some don't, is that it?

No wonder people get confused.

 

Running with local-sensing is just asking for voltage to vary with load.

[RobinTR6]

OK progress, no small male in alternator only the one for the warning light. The earth bullet looked good but when pulled the bullet came away from wire, new bullet installed. Ran better still not perfect.Added another earth cable this time to chassis. Better still not perfect. Tested voltage at pump vs voltage from relay....Ohio nearly 1 volt drop. The wire from relay to pump is new, new loom. I'll try a direct feed from battery with another cable, tried this already but had not found dodgy earth at that stage. Anyway real progress and has hinted at work through logically and you'll get there. Thanks all and I'll update on final fix.

[AlanT]

Two, or even three, thin wires do not do as well as one thick one.

This is because a lot ot the thin wires is actually insulation.

[RobinTR6]

OK will put one thicker earth wire. Currently with ignition on but not engine on

 

12.5v at battery

11.9v at relay input

11.9 v at relay output

11.3v at pump terminal

 

Should I expect more volts at pump ?

[AlanT]

0.6V drop between battery and relay.

0.3V between the relay and the pump.

 

So what are the cable lengths and types involved in this?

 

I'd be looking at the battery to relay path first. Got a fuse in there?

Got a poor contact somewhere?

[RobinTR6]

The cables are new and 25 amp rated or 33 amp rated. One from the starter +be terminal is where the battery live connects to the relay which is next to the wiper motor. The other is to the loom connection that would normallybe the connection to the cut out switch that would be on a later TR6. This goes through bulkhead to the connector to the rear loom, so standard.New loom 2.7mm cable so should be 20 amp+

[jamesStag]

Evening.

Yes I agree with Alan's comment re extra earth. Sorry I should of mentioned I didn't just 'double up' - the additional earth I installed was also a 'fat' cable (sorry not sure of sq mm value) but similar heavy cable to the supply.

 

Also agree that the volt drop you have from the battery to the relay input seems to be significant for what I assume is a fairly short cable run. So yes what else is in the circuit there that could give you a poor connection?

 

Glad you've at least got some improvement...

Cheers,

J.

[Peter Cobbold]

I would be looking for a short to earth in the lights' circuit.

[RobinTR6]

Peter, thanks. I'll try and eliminate the voltage drops that I've highlighted as these exist without the lights on, then thru the light circuit. As I've said loom is new and I was very careful when installing including the light switch ad I've had grief there before. All the connectors are new too. Ok ill take a look after I've improved volts without lights to pump circuit.