Jaguar Repair: My 1985 XJ6, voltage drop test, fuel pump relay

Question
QUESTION: Howard,
I asked you a couple of days ago about deep charge batteries and tonight I need to shift the questioning a bit after much study. First, it turns out my battery is the deep cycle spec battery "gold" level from a local parts chain, I was only able to tell be comparing the actual item number on the battery to the online listing. So, tonight I ran some tests and found that with the battery charger on, only because the battery was low from more start attempts, reaching in behind the air filter and opening the flap did not cause the fuel pump to turn on with the key "on". So next, I attached a voltmeter to the pump leads and observed that when the flap was opened that the voltage went straight to 14, then dropped to under 8 and held nearly steady. I repeated this many times and got consistent results. I can hear the relay on the firewall clicking sharply as I do this test as well. Last thing, just before writing, I connected the battery charger directly to the pump and it ran strong and smooth. Any known fault that you know of, or am I just left with finding the cause of this voltage drop?

Thanks!
Michael

ANSWER: Hi Michael,

When voltage drops when connected to a load (fuel pump in this case) it is an indication that either the load is excessive or there is a bad connection in the circuit between the pump (the load) and the battery. Since you you hot wired the pump with the charger and it ran good, that just about proves that there is a poor connection between the pump and the battery.

What you have done is a normal test called a "voltage drop test". Which is a test of a circuit.

Your next test should be to turn on the ignition and stick something in the front of the airflow meter to hold the flap open a little and then go to the fuel pump and check the voltage again like you did before and confirm that the pump is not running and that the voltage is under 8v and then go to the fuel pump relay and test the White wire while it is still connected and if the voltage is low go to the inertia switch on the passenger's door jam and test both sides for voltage (while still connected)

If the voltage is high at the white wire at the relay test the white w/green tracer wire while still connected, If the voltage is low, first disconnect the Aux Air Valve wire plug and test again and if still low, then you have a bad set of contacts in the fuel pump relay.

However the added load of the Aux Air Valve can apply an extra load on the point in the relay to make them fail so you would have to ohm test the Aux Air Valve to be sure it is not the normal load of the air valve that is causing the relay to fail.

Here is another quick test of the relay by just unplugging the double white wire from pin #30 on the relay and remove the double White w/green tracer wire from pin # 87 on the relay and make a jumper with a flat spade terminal on each end and plug the white wire into the White w/green tracer wire and see if the pump runs. If it does run you have a failed relay.

Let me know.

Howard

---------- FOLLOW-UP ----------

QUESTION: Howard,

Ok, inexplicably, the pump began running tonight with the key on/flap in mass air sensor housing opened. So, I skipped the electrical testing and went after it as a fuel delivery issue. The pump is bad. I set up my gauge just after the fuel rail and had no pressure, checked flow just before the regulator and there was none. Checked flow after the filter and there was none. Finally checked flow at the pump output, and there was none, but the input hose, when detached, quickly spilled gasoline in the trunk. That pump is just about a year old. Not top dollar, just an ebay deal to be honest, but new in a box. Are they just junk, would you consider this a fluke?

Thanks again,
Michael

ANSWER: Hi Michael,

There are some inexpensive pumps to replace the Bosch pump (which is expensive) and the method to spot them is the wire connectors. Also you can pull the pump and if it does not say "Bosch" it is an aftermarket pump. The Bosch pump have two flat spade terminals of two different sizes made into the pump where the aftermarket ones have nuts to bolt an eyelet type wire end or a bolt on adapter so you can plug the Jag female spade connector on to. In which case the rubber boot on the wire usually will not fit correctly.

Either one of the pumps are very susceptible to trash locking up the pump as the pick-up screens are not fine enough to stop all trash from the tanks and a roller cell pump can't handle even the smallest piece of trash.

If you find that the pump is locked up you can sometime get the trash out by removing the pump and connect the pump electric terminals backwards thus making the motor run backwards and on a few I was able to expel the trash out the inlet by running mineral spirits or such through it backwards.

Howard

---------- FOLLOW-UP ----------

QUESTION: Howard,

Thanks. Obviously then, the one I replaced a year ago, this one, and the one I just ordered again tonight are all aftermarket type, and not Bosch. Mine isn't locked up though, it is spinning and just not moving a drop. Same failure on the last one. Seems like you mentioned once that they get loud when they go bad. When I was applying voltage directly to the pump for testing, it did seem quite loud compared to when it was new. I guess I'll just swap in the new one when it arrives next week and carry on.

Until I break my Jag again...

Michael

Answer
The Bosch pumps usually last a long time but at the cost you can lease a new Jag for a month. I was offered aftermarket pumps by a Jag dealer for $25. as they were not going to handle them any more but they wouldn't tell me why so not knowing the history I bought two of them and put one on my car and put the other in the trunk as a back-up with the tools to replace it if it went out on the road. If it goes out in a short time I plan to get a Bosch pump and use the last cheep-o pump as a back up.

I love these old jags but I also know them and I stay within a radius of the reach of a local wrecker service in my town and I carry his number in my wallet. I also carry a trunk full of spare parts so even if it is a job that I can't do by myself I would have the part needed for any garage mechanic to install and I know the car well enough to guide him if he was afraid to work on it (common problem).

This should not reflect on the newer Jags (mid 90's up) as Ford did a good job correcting many of the electrical problems. I don't know much about how much influence the Indian company TaTa has had in the newest Jags.

I started working on Jaguars in 1960 in a Jag dealership and have worked for Jag dealers ever since on and off. As far as sedans go, I still drool when I see a MK II Jag sedan with chrome wire wheels and straight stick. The next best looking sedan is the series III sedan.

Howard