Lucas 5AM airflow meter

[Cider Andy]

As far as I can tell, this unit is supposed to be complementary to the Lucas 14CUX fuel injection ECU and is lifted straight out of the Range Rover 3.9. It was also fitted to a Saab (a 900 I think), along with the 14CUX. But I have in my possession a Lucas 5AM airflow meter from a Jaguar (3.2 & 4.0 litre AJ6 straight six engine), which is one of at least two different 5AM meters used by Jaguar (I'm told by a Jaguar independent).

So what's the story? Are they all the same? The Jaguar unit I have is physically identical to the Land Rover unit, but is it the same electrically? Certainly it appears to work properly, unless the original was faulty and the ECU is ignoring it, in which case I wouldn't notice a difference.

Does anyone know?

[GreenV8S]

I suspect Mark Adams would know if anyone does.

Are there any other distinguishing marks or part numbers on the one you have?

[Cider Andy]

Some further info, Peter:

Original unit:
Land Rover part number ERR 5198
Lucas part number 70410A
Date code 1699 (week 16, 1999, perhaps? Car is a 1999 450)

Jaguar unit:
Jaguar part number DBC10852
Lucas part number 70388B
Date code 1693 (cam from a 1993 car)

There's a couple of other things which made me think they were internally different: I've seen remanufactured units for sale at the same supplier at £150 for the Land Rover unit, but £275 for the Jaguar unit! Also, I've seen different prices for a replacement ERR 5198 versus an ERR 5198A, which would imply there's a difference.

[griffter]

GreenV8S said:

I suspect Mark Adams would know if anyone does. Are there any other distinguishing marks or part numbers on the one you have?

I've enquired long and hard and no-one seems to know! Among numerous conflicting reports, the emerging picture is that all 5AM meters are the same as each other, and are interchangeable. What's more, they're 'functionally identical' and thus interchangable with the 3AM 'Hitachi' (which precedes the 5AM but is still used in 14CUX applications). AFAIK the 3AM was used on RR and various late 80's Jaguar 6 cylinder engines. The 5AM was used on the later versions of the same models, and also the Saab 900 turbo. The aluminium collars which screw to the air flow meter itself, and to which the intake pipes are secured, may vary in diameter from application to application.

The only adjustment on the meters (and thus electrically variable factor) seems to be the CO idle trim value, adjusted by means of the screw on the afm 'box'. But - you guessed it, no-one can agree on what its value (measured in volts) should be, or what it does anyway.

All this is summarised hearsay - it might be totally wrong, but it's the best I've managed to glean. Be careful if you start swapping things on the basis of rumours and guesswork - you don't want to damage the ecu.

[Cider Andy]

griffter said:

...no-one can agree on what...it does anyway.

I can certainly agree on that! In the short time I've been investigating this, I've found to my horror that you can disconnect it AND the throttle pot and the engine still runs. This amazed me!

[GreenV8S]

I gather that with the bigger airflow meters there is a Jag unit which looks identical to the ones used on the RV8 but is calibrated differently to support a greater air flow on the supercharged Jags. I don't know if there is something similar going on with the 5AM but it seems possible.

If you have both units, it should be possible to compare the output under similar conditions to see if they are producing similar readings?

Alternatively you could just ask Mark, I'm sure that he knows if anyone does.

[Cider Andy]

Yeah, the Jag Man showed me a 5AM off a 4-litre XJ6 circa 1990, but with a different Jaguar part number to the one I have. He reckons that's off a 6-cylinder XJS. Your calibration, or characteristics theory makes sense. Maybe I should make some voltage comparisons.

In fact, the reason for all the substitutions is to diagnose a rough-running problem. So far, I've built up quite a range of original ignition components as spares, all of which work, if not quite as well as the new parts. Having messed around with the throttle pot. this weekend I'm pretty sure I've found the offending item. Once I've found a replacement that's not at an extortionate price I'll be able to eliminate it. Then all I have to do to get it as sweet as a nut is the manifold gaskets...

[griffter]

I spoke to a guy who reconditions afms, ecus and ignition amplifiers for all makes and models. In respect of ignition modules (I didn't ask directly about afms) he confirmed that he thought that like parts are functionally identical, and that varying p/ns simply denote different vehicle applications, different batch nos, and probably parts prices (ie Metro - £, TVR = £££). I suspect the same is true for afms. After all, isn't the whole point of the fuel map in the ecu to tailor the fuelling to the particular vehicle? - there should be no need to manufacture differently calibrated sensors for a given engine management system (eg 14CUX) as the specific application is tailored by the fuel map, from generic sensor readings.

In summary I think 14CUX uses bespoke fuel maps and generic sensors, not the other way round. But I'm prepared to be proved wrong!

[ntel]

Some good reading here www.rpiv8.com/carbs-2.htm#setting-up

[griffter]

ntel said:

Some good reading here www.rpiv8.com/carbs-2.htm#setting-up

RPi are about the only people to offer this sort of detailed information, for which respect is due. Problem is my CO trim is 0.003v with no ill effects, and when a mate of mine increased his trim to 1.0v (from an initial 0.05v IIRC) the car ran terribly it was running so rich. So is the info correct? I can't find it elsewhere, and I can't say it's wrong - but IME it doesn't seem right in practice.