You ready to look at some schematics?
I know, I know...
Here is a picture of a dog, Dog A, to start.
Ok. Ready now?
Good.
In the ongoing saga that is my personal mistake in buying a high performance Jaguar and thinking “how hard could it be,” the Jag’s AC went out for unknown reasons.
I took it to the AC shop and for the low, low price of $115 they offered to let me pay an additional $3,600 to replace :checks notes: everything.
hardno.gif
What they actually found was the AC compressor clutch fuse had blown and in their professional opinion that meant the compressor was going and everything needed to be replaced. That... didn’t sound quite right to me (and many oppos) so... time to dive into what is actually going on.
First off, there are two fuses associated with the AC compressor clutch:
At first I thought F32 was blowing... then the shop said it was F38... and upon further inspection it was actually F32.
What a fun journey we’ve been on!
For better or worse the Jaguar official workshop manual doesn’t really cover electrical stuff, so I poked around the internet until I found an electrical manual.
Quickly, using my good pal ctrl-F, I found F32 in the wiring diagrams. Somewhat unsurprisingly, the only reference to it on the schematics doesn’t actually show what it powers. However, helpfully, it does give a power reference number (boxed numbers) and say which diagrams it appears in (right arrow numbers).
Neat!
So from this I knew I needed to look for 70, 71, and 72 in diagrams 6.1 and 3.2,4, and 6. By chance, I found 6.1 before any of the 3's, so I started there.
Oh hey look at that. It is a 72 feeding the common and coil of a Auxiliary Coolant pump relay, and going on to feed the coolant pump itself!
Interesting. What else though? Oh hey, also on 6.1 (remember there were two references to 6.1 on figure 1.2) we’ve found source 70 and it is going to the dual coolant control valve, oddly with no interposing relay.
Curious.
Well that is 70 and 72 down, lets hunt down 71, which we assume is on figure 3. I’ll spare you the details, but figure 3.2, 3.4, and 3.6 are the same diagram for different engines. The one I am after is 3.6 as it is for the supercharged V8.
Bingo.
So 71 feeds the relay common on R8 and goes on to power the AC clutch.
Huh.
So we’ve got one 10A fuse powering:
- The AC compressor clutch
- The Dual Coolant Control Valve
- The auxiliary coolant pump
- The auxiliary coolant pump relay coil
That seems like... a lot for a 10A fuse to do.
Interestingly the feed for the R8's coil comes from F38. The rest of F38 seems to go on to power stuff in the ECU, so I guess I’m glad that isn’t the one blowing!
Anyway... so now what?
Time to take some measurements?
First off, I went ahead pulled R8 after the 2nd time F32 blew, in case the AC guys weren’t wrong and the compressor was about to puke glitter into my everything. As a precaution (and because they’re returnable) I ordered two new relays. I figure it is a long shot that R8 or R7 are bad, but stranger things have happened.
With R8 pulled, F32 didn’t blow a second time. I even swapped it out for a 7.5A fuse and that didn’t blow either. That said, with the AC dead the blowers blow hotter than ambient air, which means I had the whole climate control system powered down and all the stuff associated with this fuse is unlikely to be operating with the HVAC off.
Probably.
So I guess what I’m saying is I learned nothing from that exercise.
Ok... measurements...
Knowing this was coming I’d purchased some fuse adapters that allow you to connect multimeter probes.
Smart.
I connect everything up and checked the voltage with the car off.
15.25V
I’m sorry the fuck what?
15.25V
Ok so something strange is going on there... maybe this is the problem? I’ll check the battery....
15.25V
Ok... no. No.
I go and check a spare battery that is sitting in the garage.
16.0V
Ok well apparently my trusty Fluke has finally shit the bed. Oh look a low battery symbol on the meter?
After replacing the battery in the meter I read a much more reasonable 12.5V with the car off.
We’re good to go! I swap the leads around into amp mode, connect the meter in series, fire up the car and....
Ok... not unexpected as the AC was off.
How about with it on?
0.00A
Ok so long story short my current measuring fuse was blown too. Furthermore the meter is only good to 10A anyway, so I’d be looking at blowing more fuses during this endeavor.
A quick tour on Amazon tells me I can get a DC clamp meter for $50. I’ve been wanting one of those for years anyway so...
I guess we’ll have to return to this after the parts fairy drops that off.
Until next week!