XJ40 ( XJ81 ) 1986 - 1994

No Start? No Fuel Pump Prime? Try This

Old 05-08-2019, 07:06 AM
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Join Date: Dec 2012
Location: San Antonio Tx USA
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Default No Start? No Fuel Pump Prime? Try This

If your not getting any fuel pump prime but the fuel pump still runs when you jumper the relay base on the one in the trunk by the antenna area, try swapping the relay with the green base (a/c clutch) with the one with the yellow base (oxygen sensor heater). These relays are located under the hood on the passenger side alongside the firewall.

I had been troubleshooting an intermittent no start for YEARS until my dad read somewhere that the fuel pump prime is controlled by the same relay as the oxygen sensor heater relay. I swapped the relays and the car now fires right up every time! This is just another one of those things that makes one wonder with frustration why Jaguar named this relay as the Oxygen sensor heater relay while leaving out a reference to the fuel pump prime function. I think that knowing that this relay controls fuel pump prime is more important to know than knowing that the relay controls the oxygen sensor heater. It is stuff like this that causes these cars so much down time.

I certainly hope this information can help someone and save alot of time and money.
The following 2 users liked this post by supra-raleigh:
Don B (05-08-2019), Lawrence (05-08-2019)
Old 05-08-2019, 09:19 AM
Join Date: Feb 2014
Location: The Beautiful Mid-South, U.S.A.
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Hi supra-raleigh,

For readers who want to see how the O2S Relay is involved in the fuel pump circuit, they can download the Electrical Guide here:

Jaguar XJ40 Electrical Guide 1993


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