Sunday, June 13, 2010

Now I know why it's called an alternator....

...because you alternate between thinking you've fixed and not!

This is getting very silly indeed, I though maybe I had a dodgy alternator so I started checking that out today, Oh hang on, let's get back to the beginning

The symptoms are that there is a persistent misfire, it's not on a specific cylinder but it's more like she's not fully firing all the time. It's through out the rev range and doesn't seem to respond to any fuel or ignition settings (choke in/out, advanced/retarded) you can't "drive through" it.

Engine is fresh and has excellent (190psi) and equal compression. Cam timing and valve gaps are all spot on. Metering unit is timed correctly, all injectors are spraying a good cone pattern with no dribbles or bleed back.

I have replace or rebuilt all of the ignition system at least twice. New coil, leads, cap, rotor arm, plugs, points, (tried electronic ignition too).

The fuel system is OK, pressure seems pretty constant over 100 psi on the electronic gauge. The gauge seems to only want to read up to a max of 102 psi despite it being a 160psi one. The injection seems to work OK as she will run right down to 80psi or less before she stops or chugs.

So, back to the alternator - why do I suspect it? Well it's a recent recon ACR type of unknown origin or output (although I think it's an 18ACR). It replaced the one that came with the car (also and ACR) that I suspected was faulty, it wasn't charging properly - but now I'm not so sure!

Anyway, the misfire is still there but if the car is running and I switch the lights on, I get a pronounced misfire. I get the same type of misfire if I wound the horn (air horns) for a 2 or 3 second burst.

So, I test the alternator to see if it's failed or not. Now it gets interesting - vehicle electrics are not one of my best areas of skill - I admit to being a bit of a thicko here. Electrickery is one of the great unknowns for me and operating a multimeter is a challenge. Getting the test results is OK but what do they mean and are they normal?

Well here's what I got.

  • Ignition off - voltage at the battery 13.06v - seems healthy
  • @ idle - voltage at the battery is only 12.9 volts but....
  • @ fast idle - voltage at the battery is 14.17 and it doesn't get any more at higher engine speeds

So that would seem to indicate that the alternator is charging OK.

So then my mind turned to what was the voltage at the coil whilst all this was going on - now I'm not too sure what these readings mean but I took them so I'll thrown them out to the Triumph intelligenci and see what comes back.

  • Coil resistance is 3.7 Ohms, correct for a non-ballast resistor set-up
  • Voltage at coil with engine at idle is 5.7 volts and it drops to 3.2 volts at fast idle
  • Voltage between battery negative and coil positive at idle is 6.29 volts and it drops to 4 volts at fast idle
  • Voltage between coil positive and battery negative is 12.55 volts at idle and rises to 13.6 volts at fast idle

It's fair to say I haven't a clue as to what all that means but it doesn't look right to me.

The next fun is the wiring - here's where a factory diagram isn't much help. As a Mk1 PI it should have a Lucas alternator with a separate control box. It doesn't have anything of the sort, it's got wires bundled together and a few brown ones not plugged into anything at all, just taped up out of the way.

There are three wires connected to the alternator, the chunkiest brown one goes straight to the battery positive, a thinner yellow and brown wire goes from the centre terminal on the alternator into the loom. There's a thinner still wire which is brown and green and also goes into the loom.

So a few questions for anyone interested in helping me make sense of this.
  • As I see it, the Brown/Green wire is the warning light wire and should not have any effect on the efficiency of the alternator. If it's only used for energising the alternator then when defective the alternator will not charge. As mine does charge then my logic says leave it alone! Am I right - the wiring diagram shows this should be a Brown/Yellow wire!
  • The Brown/Yellow wire, being chunky, must carry some of the charging current (rather than being the energising wire) but where does it go? I can trace it into a cheap and nasty after market crimped joiner near the fuse box on the other side of the engine bay but where it goes after that I'm not sure. Does it supply fuse number 2 - headlamp flasher relay, horn and interior light/Cigarette lighter?? Am I right - the wiring diagram shows this should be a Brown/Green wire!
  • The heavy duty Brown wire goes straight to the battery, that's easy.
I can test some of these theories by disconnecting things but I don't want to do that with the engine running or I'll screw up the alternator properly. Can I do it with the ignition on but the car not running?

Anyway, enough of such things, I'm off to fiddle again but this is baffling me! All suggestions gratefully received!


Paul Bodiam said...

Hi Jason

Your alternator is fine. The readings taken at the battery +ve terminal (and also at the coil +ve terminal) confirm this.

Don't worry about the readings taken at the coil -ve terminal: these are pretty much meaningless. I am guessing that you used a digital multimeter to take these readings. DMMs don't cope well with rapidly fluctuating voltages when trying to measure DC voltages. The -ve terminal of the coil is connected to 0V when the points are closed and then rises towards battery voltage when the points open as the condenser charges. This is happening at twice crank speed, so your poor little multimeter is being confused by the rapidly fluctuating voltage.

I think you will have to look elsewhere for your misfire.

You observation about the misfire being worse when you operate the horns or turn on the lights does point to voltage issues. The alternator is fine so I would be looking at losses in the wiring loom. Have you measured the voltage at the pump on idle and when revving. I would not be at all surprised to see some significant voltage drop.

I remember reading somewhere of a PI fix to overcome voltage drop problems involving running a new +12V supply directly from the battery to the pump using heavy gauge wire, controlled via a relay in the boot, using the existing pump feed to fire the relay.

hope this helps

Jason said...

Thanks Paul, I'll have a go with my multimeter next time I have a fiddle - there is a fix with a relayed supply to the pump direct from the battery and I think I might give it a go. At the moment I don't seem to get a corresponding drop in fuel pressure when I put the lights on - well at least not a serious drop. I have seen 1 or 2 PSI drop only and that's not sufficient to cause the kind of reaction I'm getting.