S3 XJ no start - wierd

Good morning all,

I stopped by my local independent Jaguar mechanic for a visit on Friday.

He was stumped with a S3 XJ6 no start. It was towed in after the owner said it died during a drive.

John started diag: good fuel pressure, injector pulse, grounds, spark, but no start. But, with the air flow meter disconnected, it starts (roughly)! While running, it dies immediately when the AFM is plugged in!

A used AFM and ECU were tried, but no change. Coolant temp sensor was okay.

He’s going to try swapping out the diode Plectron (spelling?) red relay-shaped thing near the firewall relays.

Any ideas?

That’s pretty weird.

The red Pecktron thingy is the “Diode Pack” for the fuel pump circuit. If the pump is operating properly in both ‘start’ and ‘run’ modes there is little reason to suspect the diode pack.

However, there is a tie-in from the diode pack to the AFM so maybe some odd feedback situation is occurring? I dunno. I’m just tossing that out. It’s a stretch.

I’m only on my first cuppa coffee :slight_smile:

More mulling needed


He has a point, Roger - but an uncertain one…

Basically, the diode pack is an interconnection to trigger the fuel relay while the engine is running. If one diode fails to block; power is applied to the starting circuit by the closed reed switch on the AFM. When the key is returned from ‘crank’ to ‘run’ power is still applied to the starting circuit - and the ECU will stay in ‘cranking’ fuelling mode…

However, this will not impede starting…

However, if the read switch fails to close with the flap open the fuel relay will disengage the key is released to ‘run’. Again, this will not interfere with starting - the fuel relay being engaged by the cranking circuit.

When the AFM is disconnected, the ECU only loses air flow data, and will drop to ‘minimum’ fuelling - either due to a built-in default, or an unintended fluke of its construction.

Disconnecting the AFM will not interfere with any other engine management functions, or the engine itself. But the mere fact that the engine runs with this disconnection also means that these functions should also work properly when AFM is connected - weird…:slight_smile:

One possibility is that the ECU persistently overfuel due to signals transferred through, but not ‘manipulated’ by, the AFM. Though the xk can stand a lot of fuel, there is a limit - a look at the plugs for sign of ‘wet’ overfuelling may be tried…

Or there may be more than one fault interacting - which certainly complicates matters…

I dearly like to know the solution to this - so keep us informed? When solved we will slap our collective brows and say, as usual; “Of course, why didn’t we think of that”…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

John reprts that swapping another Pektron unit fixed the issue.

Thanks Doug and Frank.

That’s one fault and cure to remember.