Hot Mess Jag Electrical Diagnostics - Part 2

"Akio Ohtori - RIP Oppo" (akioohtori)
10/10/2020 at 17:08 • Filed to: The Hot Mess

When we last left my Hot Mess, my cheap Jaguar S-Type R that I love and hate in unequal measure, it was staring down a $3,600 repair bill for a suspected bum compressor. Finding this suspicious, I did some shade tree diagnosis, but was let down when the current fuse in my multimeter was found to be blown.

!!! UNKNOWN CONTENT TYPE !!!

As is our pact, I’m going to ease you in with a doggo picture.

We pick up that action several days later. The parts fairy has bestowed upon me a cheap AC/DC clamp meter which will allow me to passively read up to 100ADC, rather than the 10A my Fluke can read in-line.

Since my electrical diagnostics setup was geared for in-line reading, through the meter, and the new clamp doesn’t do that, the first step was to make a current loop for the meter to be able to read. Since we’re dealing with a faulty circuit, I figured throwing in a fuse wouldn’t hurt either, so I killed two birds with one... wire and used a in-line fuse holder as my current loop.

Janky, but it’ll work.

Knowing the current setup was blowing 10A fuses, I threw a 15A fuse in there, just in case we were looking at a catastrophic short situation.

I plugged my new rig into the socket for F32, fired up the Jag, and with the climate control off, took a reading.

0 Amps

Ok. Make sense that everything on this circuit deals with HVAC. With the climate control off you’d expect nothing to be causing a current draw.

Next I turned on the climate control and set it to 70F. With R8 out, this would attempt to turn on the AC but wouldn’t be able to because the relay is out.

Ok well technically -1.78ADC because I had the meter reversed, but still...

1.78ADC

Acceptable.

Not sure what is drawing that, but I’m not intimately familiar about when the auxiliary coolant pump runs so... could be that?

Alright. Time to put R8 back in and take a reading. R8 is the relay that controls the AC compressor clutch, so with it in the HVAC system will be fully operations.

5.80ADC

Also acceptable? I actually have no idea how much current a compressor clutch should draw. Google says between 2.0 and 4.0ADC, so we’re in that range... if only just. The current did drift down as the AC remained on, eventually settling at around 5.2ADC.

Ok. So... that isn’t enough to blow a 10A fuse for sure.

I guess it is time to turn up the heat

Literally.

With the driver’s side climate control still set to 70F, I set the passenger side to max heat, 89F.

13.86A

Bingo.

But... what did we learn?

Well... with the heat on one of two systems is now in play: the dual coolant control valve and the auxiliary water pump. The former is known to fail and take the climate control unit with it. The latter just got replaced when I was doing my big maintenance refresh.

I know which one I’d put my money on!

!!! UNKNOWN HEADER TYPE (MULTI-LINE BREAK?) !!!

The next step would, ordinarily, either be to buy a replacement dual coolant valve, or, more logically, unplug it to confirm it is the problem.

Unfortunately I have people sized hands and more or less the entire cooling system is in the way of me and that valve. Ditto the auxiliary coolant pump.

Equally unfortunately, as we found out in Part 1, the dual coolant valve is directly run off the fuse and the climate ECU. Nothing I can do to disconnect it with direct physical access.

Time to pack it in I guess?

...

But wait! In the process of writing this article (I knew I did these for a reason...) I remember that while yes, the dual coolant valve is directly driven and cannot be easily disconnected, the other two loads on that circuit are relay driven. I can pull R7 and R8 and then the dual coolant valve will be the only load on the circuit!

So I did that. Leaving the climate control in dual mode, I pulled R7 and R8 and measured the current.

And you know what? The current draw was fine.

Wait what?

Yeah totally. Like... 1.6 amps.

I threw R8, the AC compressor clutch relay, back in and got an equally acceptable 6.21 amps.

Ok... well what happens if I put R7, the auxiliary water pump relay, back in?

CURRENT! BADNESS! PROBLEM!

Huh. Ok. That is... good?

So... sure is looking like the replacement auxiliary water pump or associated wiring is to blame.

However R7 itself is also driven off F32. It is a long shot, but maybe the relay coil has failed in such a way that is causing it to draw a lot of amps. Unlikely and I assume if the relay itself was drawing 6 amps, or whatever, it would have melted.

Still, as a man of science, I figured it was worth a shot. I grabbed one of my replacement relays and plugged it in to the socket for R7.

0.00A?

Wait what?

Oh. The 15A fuse in my current loop blew.

Conclusions

Well I feel like my diagnosis is pretty sound. After blaming everything from the compressor to the cooling fan, the issue appears to be the auxiliary coolant pump.

Good news? The pump is closer to the surface than the dual coolant valve and I have the old one, which there was nothing wrong with, still sitting in my parts bin.

Better news? I bought the pump through my mechanic friend, so the possibility exists they’ll be able to warranty it for me and I can get a brand new one for free. I’ll probably still stick the old one back in over a apparently shoddy replacement, but still nice to have the option.

Best news? With R7 out I can still drive the Jag. With AC ! My understanding is without the auxiliary coolant pump the heat won’t work worth a damn at idle, but I can live with that for the time being. Also it is 2020 and I live in Oklahoma. We’re like... two months from needing heat.

This... is a relief.

The possibility still exists the pump is fine and it is a problem with the connector or wiring, but point being the AC can continue to work, I don’t need to replace the climate module or compressor, and better yet I don’t have to do to anything immediately.

I can work on my other cars!

So I think I’ll leave you and do just that!

DISCUSSION (9)

EngineerWithTools > Akio Ohtori - RIP Oppo
10/10/2020 at 17:21

Well that’s a great development! Ok, maybe 80% great because something that was just replaced seems to be messed up, but still.

Congrats on the AC! And thanks for discouraging me from looking at Jaaaaaags.

CompactLuxuryFan > Akio Ohtori - RIP Oppo
10/10/2020 at 17:23

Lesson: preventative maintenance is futile (harmful?) if part quality control is bad. That or British cars are actively spiteful

dieseldub > Akio Ohtori - RIP Oppo
10/10/2020 at 17:36

Excellent bit of sleuthing!

SilentButNotReallyDeadly...killed by G/O Media > CompactLuxuryFan
10/10/2020 at 18:04

Lesson: With respect to British cars, DO NOT fire the parts cannon until the target is already demonstrably dead. This lesson may also apply to some vehicles of French and German origins...

BaconSandwich is tasty. > Akio Ohtori - RIP Oppo
10/10/2020 at 18:22

That's some good detective work. Vehicle electric systems are still a dark art to me. Good stuff!

Beefchips > Akio Ohtori - RIP Oppo
10/10/2020 at 18:37

As a perhaps soon-to-be owner of an old used luxury car I’m reading these posts with renewed interest. Trepidation even. But you are encouraging me, I can do it!!

MM54 > Akio Ohtori - RIP Oppo
10/10/2020 at 19:48

Nice work! Is the connector to the aux pump itself accessible that you can unplug it to rule out the wiring somehow being only-a-litt le- shorted? Otherwise, sounds like a sound diagnosis.

pip bip - choose Corrour > Akio Ohtori - RIP Oppo
10/10/2020 at 20:08

well done

Urambo Tauro > Akio Ohtori - RIP Oppo
10/10/2020 at 21:18

Good find! Electrical stuff can be daunting, but with t he right meter and a good diagram, the worst part IMO is really just gaining access to the component and /or connector.