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Intermittent No Start, Always Starts Using Fuel Pump Relay Jumper

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XJ40 ( XJ81 ) 1986 - 1994

Intermittent No Start, Always Starts Using Fuel Pump Relay Jumper

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Old 07-06-2018, 08:53 AM
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Default Intermittent No Start, Always Starts Using Fuel Pump Relay Jumper

On a 93 XJ6, there is an intermittent no start that happens when the fuel pump does not prime during the ON key position. However, I can always get it to start if I jumper 30 - 87 blades on the fuel pump relay which is located in the trunk near the antenna. It looks like an aftermarket relay with 4 prongs. I would like to replace it but need to know the proper part number to look up.

Can someone please tell me the part number for it or where to purchase one?
 
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Old 07-08-2018, 10:36 AM
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From the diagram at jaguarclassicparts.com, it looks like the OE fuel pump relay was one of the infamous "blue" Hella relays that were not sealed against moisture, oil and other contaminants, and failed over time. The original part number was DAC6786 [EDIT: DAC7686]:



In 1997 Jaguar issued a Technical Service Bulletin to address issues with the blue Hella relays and introduced a new sealed relay, part LHF6703AA:

Jaguar Blue Hella Relay TSB 86-178 8/97

Part number LHF 6703AA was subsequently superseded and the latest part number is JLM21842, which is still available from Jaguar as well as our sponsors like SNG Barratt, Coventry West and Jagbits.

Note that these relays have a resistor and flyback diode in parallel with the coil to protect against voltage spikes. Using a relay without this protection will result in shortened life of the relay coil. You can add a resistor and diode across the relay's coil terminals but will have to research appropriate values for these components (I think I recall measuring resistor values in the range of 600 ohms, and my assumption is that the diodes would have a Zener voltage of at least 24 volts).

Cheers,

Don
 

Last edited by Don B; 07-11-2018 at 12:08 AM.
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supra-raleigh (07-10-2018)
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Old 07-08-2018, 12:02 PM
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Jagbits: https://www.jagbits.com/product/LHF6703AA-OE.html
Jagbits 10% discount code: JAGFORUMS-10%
 
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Old 07-10-2018, 04:23 AM
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Thanks you both very much. That is why I love this forum. You have gone above and beyond what one might expect.

Don I think you may have meant DBC7686 instead of DBC6786, as you wrote. [EDIT: DAC7686] Thanks.
 

Last edited by Don B; 07-11-2018 at 12:08 AM.
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Old 07-11-2018, 12:11 AM
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Originally Posted by supra-raleigh View Post
Thanks you both very much. That is why I love this forum. You have gone above and beyond what one might expect.

Don I think you may have meant DBC7686 instead of DBC6786, as you wrote. Thanks.
Hah! You're right that I meant to type DAC7686 and failed, but check your own typing. We both must be tired!

I've edited both our posts so future readers will get the correct number, not that it matters much since it's been superseded at least twice.

Cheers,

Don
 
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