Hot Mess Jag Electrical Diagnostics

"Akio Ohtori - RIP Oppo" (akioohtori) 10/07/2020 at 21:53 • Filed to: The Hot Mess

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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.

!!! UNKNOWN CONTENT TYPE !!!

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).

Figure 1.2

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.

Figure 6.1

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.

Figure 6.1

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.

Figure 3.6

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!

Dr. Zoidberg - RIP Oppo > Akio Ohtori - RIP Oppo 10/07/2020 at 22:10

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Akio Ohtori - RIP Oppo > Dr. Zoidberg - RIP Oppo 10/07/2020 at 22:16

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BUT I GAVE YOU DOGGO!

Dr. Zoidberg - RIP Oppo > Akio Ohtori - RIP Oppo 10/07/2020 at 22:18

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What specifically did that mechanic list for that quote? I'm not sure it's an absurd price for the work involved... Although it is an absurd price in regards to spending the value of the car on repairs.

pip bip - choose Corrour > Akio Ohtori - RIP Oppo 10/07/2020 at 22:19

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Hot mess indeed

BaconSandwich is tasty. > Akio Ohtori - RIP Oppo 10/07/2020 at 22:20

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I'm enjoying the play by play. Please, be sure to post another update!

Akio Ohtori - RIP Oppo > Dr. Zoidberg - RIP Oppo 10/07/2020 at 22:23

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The price wasn’t that absurd, TBH. The bulk of the work is the 11 hours required to remove the dash to replace the expansion valve. Parts are about $1000, with $1250 in labor for the compressor + drier and the rest going to the expansion valve.

Realistically if I can isolate the problem to the compressor clutch, I’m out about $400 in parts for OE and a couple of weekends labor if I do it myself. My friend who owns a shop offered to do all the pumping down and flushing for beer.

However the dual control valve is known for failing in the Jaguar community, usually taking the climate module with it, so the possibility exists that is the problem.

It is also $400*

*I don’t know that for sure but I mean... it is a Jag.

Kiltedpadre > Akio Ohtori - RIP Oppo 10/07/2020 at 22:24

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I understood doggo;  after that you lost me.

Beefchips > Akio Ohtori - RIP Oppo 10/07/2020 at 22:25

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Hm, m aybe just roll down the windows?

Dr. Zoidberg - RIP Oppo > Akio Ohtori - RIP Oppo 10/07/2020 at 22:28

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How come you just don’t ignore problems like the rest of us?

[eye twitch]

Akio Ohtori - RIP Oppo > Dr. Zoidberg - RIP Oppo 10/07/2020 at 22:29

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:glances nervously at all my broken cars: Oh you know... I’m responsible.

... I need a drink.

ClassicDatsunDebate > Akio Ohtori - RIP Oppo 10/07/2020 at 22:35

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Highlander-Datsuns are Forever > Akio Ohtori - RIP Oppo 10/07/2020 at 22:37

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gmporschenut also a fan of hondas > Akio Ohtori - RIP Oppo 10/07/2020 at 22:38

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I ran into a similar issue years ago with turn signals that didn’t want to function . I went through a replaced every fuse.

MM54 > Akio Ohtori - RIP Oppo 10/07/2020 at 23:01

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I agree that it seems like a lot for a single 10A circuit; the A/C clutch isn’t too much power but the “aux coolant pump” could be something to look further in to. It’s strange to me that it runs the relay coil and the pump itself, I get that it saves the switching load but why not... you know, give something like a pump its own fuse?

I think you’re on the right track; with the removal of R8 saving the fuse I don’t think it’s hugely narrowed down, but at least we know it’s not a dead short in the still-hot parts of the circuits (horray?). It could still be an issue with the coolant pump drawing too much and the inrush for the a/c clutch just being the final straw (or it could be the a/c clutch shorting internally).

While you wait for the meter, unplugging the a/c clutch and putting R8 back in might be a worthwhile quick test (if you can get to the plug...) to see if the relay itself is somehow the problem, though I have doubts there.

In any case I really don’t think a failing compressor would have any impact whatsoever on the clutch; it’s an on/off solenoid to engage the pump to the pulley , not like it’s an electric compressor that is getting loaded down from internal friction. Usually if the pump seizes up, the clutch turns into smoke but the electrical load to engage it isn’t affected.

ATD > Akio Ohtori - RIP Oppo 10/07/2020 at 23:01

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Have you replaced the dual climate control valve? They are notorious for leaking internally and the leak shorts them out which gives you full heat. Also no relay because the power to the dccv is always hot, the climate control computer   creates the ground to close the Solenoid so if the fuse blows you get full heat through the dccv. Check out www. jaguar climate control.com.  I had an STR for 7 years, loved it but always something broken. 

EngineerWithTools > Akio Ohtori - RIP Oppo 10/07/2020 at 23:42

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A brief story... some years ago, when leading the design of a piece of mobile equipment, we put forth a proclamation: Ye controls engineers shall not use low side drivers (essentially switched grounds) evermore lest shorts and other ground problems verily lead to unanticipated function activation.

Now, I don’t expect auto manufactures to avoid low side drivers because they can save some dollars in controller outputs and wiring by using them, but darn if they don’t make troubleshooting a pain.

Anyway, that’s why your coolant valve doesn’t have a relay and why your aux water pump are wired as they are: they’re controlled by the ground.

But, on that dual coolant valve: Is it proportional? (Another reason it wouldn’t have a relay.) In other words, can the controller run the two coils at some variable setting, less than 100%, probably via PWM (essentially rapidly switching the ground on and off) ? If so, a short to ground on the...ahem...ground side of that valve might cause it to A) open both valves fully, B) pull more current than intended and C) if B is true, burn out the valve coils.

Or...your aux pump is pulling more than it should, and you’re right than there probably isn’t much margin on the 10A fuse.

ITA97, now with more Jag @ opposite-lock.com > Akio Ohtori - RIP Oppo 10/07/2020 at 23:51

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Normally the “ subscribe to our newsletter!” ads detract from an Oppo reading experience, but I (sadly, for your free time and wallet ) feel like they actually work for your Jag posts if such a newsletter existed.

RacinBob > Akio Ohtori - RIP Oppo 10/08/2020 at 07:54

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Again i doubt it is the clutch as it’s current draw is not affected by load. The clutch current would be the same whether it was disconnected on your work bench or actually installed on a compressor that was locked up . I doubt personally that a clutch winding has ever caused an over current event unless there is some mode of mechanical failure that actually causes the insulation on the winding to fail and burn out. Even then, it would be a clutch failure and not a compressor failure. To rule the clutch out, drive it with the heater pump disconnected and see if it blows.

My bet is it is the heater pump, Pump bearings get tight, and that extra load will cause the pump to pull more current. That failure would be something I would expect could happen.

TheBloody, Oppositelock lives on in our shitposts. > Akio Ohtori - RIP Oppo 10/08/2020 at 08:37

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Exchanges like this is why I keep coming back to Oppo after all these years. Also.

DO. IT. 

Akio Ohtori - RIP Oppo > TheBloody, Oppositelock lives on in our shitposts. 10/08/2020 at 10:24

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While I don’t regret buying the S-Type R over the XJR , I’ve certainly warmed up to the XJR over time.  I like the the XKR is , of the three, the most proper Jag of that era, but also like the S-Type having a little funkier (if Ford-ish) styling.

Akio Ohtori - RIP Oppo > EngineerWithTools 10/08/2020 at 10:28

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I agree with all of this but haven’t had enough coffee to articulate additional commentary.

Oh ok one thing: According to the Jag people the dual coolant valve tends to fail and take other stuff with it on a semi-regular basis, so I’m cautiously optimistic that is the issue.  Once the new meter comes in I’ll know for sure though.

Akio Ohtori - RIP Oppo > gmporschenut also a fan of hondas 10/08/2020 at 10:29

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Sometimes that is a good answer to electrical gremlins.  When my friend got his Cabriolet he went ahead and replaced all the fuses.  Didn’t help but I’ve never seen grosser fuses in my life haha.

NojustNo > Akio Ohtori - RIP Oppo 10/08/2020 at 14:05

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oh yeah the golf / rabbit platform was Notorious for water leaking on to the fuse box. there was a drain under the hood that would get clogged and then it would back up into the cabin.