The car (1995 XJ6 US) has been sitting for over a year and I'm wanting to get it back on the road..
So I just plugged my ECU back in after replacing the black 36 pin connector (keep in mind I did the soldering myself, so that's a possible area of failure).

Battery is new (mfr date 4/23) and fully charged. Shifter "P" glowing red.

Turn the key and I get a heavy "Clunk" sound, as if the solenoid engaged, but the starter will not turn.

I noticed the voltmeter bottoms out when the key is turned to start, so I'm wondering if it's a connection problem causing a voltage drop.

I see in the archives talk of a brass bolt on the firewall between the battery and starter to check, etc.

My questions:
Could this be a failed starter ( I haven't seen symptoms of diagnosing a bad starter in the archives, yet).
What other connections to check for?

More background:
The 36 pin connector was replaced because the fuel pump pin was corroded and vanished.
In my effort to resurrect the car, I was cranking a lot, so the starter was working a couple of weeks ago.
I went through the "Gentlemen, No Start Your Engines" threads pretty thoroughly.
I did not hear the fuel pump activate when turning the key on, so checked and jumpered the fuel pump and found it works.
Then went and checked the ECU -- I just knew *MY ECU* could not be corroded, but it was.-- when went to blow on it, my 36 pins became 35 pins.

I replaced the connector on the circuit board and if I messed it up, I can look for a replacement and transfer the proms, etc. but I feel pretty confident it went well.

Thank you

 

If the starter clunks, then it's not a fault with the ECU, BCM or Security Module.

It is
a. A seized engine (unlikely)
b. A dead starter motor/solenoid (possible)
c. Poor electrical connection/battery (likely)

Firstly, put cables directly from battery to starter motor and test. If it spins the engine then it's good.

Do some voltage drop tests from the battery along the cable path to the starter to check any loose/corroded connections. (The cable goes from battery to "Mega Fuse" under the rear seat, to bulk head connector (most likely culprit), to starter)

Also, check earth strap from engine to body. (On LH side from starter to lower inner wing/chassis rail)

 

This. I probably unnecessarily removed my V12's starter and got it rebuilt recently. When I went to put it back in, I spent several minutes removing corrosion from the big positive ring terminal using sandpaper. I had previously tried to clean it with a handlheld wire brush, but obviously that was not enough.

The stud on the firewall is (probably, I don't have an AJ16 to look at) easier to get to, so start (see what I did there?) with that.

 

G'day Brendan,

With the battery in the boot/trunk the battery lead to the starter motor is very long and made up of several parts with bolted joints. All these joints can loosen over time and cause a voltage drop between the battery and the starter. So you need check and clean and tighten all the connections to avoid the voltage drop.

Starting at the battery, all the connections here, both positive and ground need to be checked and the fusebox attached to the positive terminal needs to be opened up and the fuses checked.

The next connection is under the RHS rear passenger's seat and again there is a fusebox here which needs to be checked.

The next connection is in the RHS front footwell on the firewall. Here there is a stud which passes through the firewall and connects to the actual lead which goes to the starter motor and then the alternator.

All these connections need to be clean and tight so you don't lose voltage for both starting the car and for charging the battery.

Cheers,
Jeff.

 

Yes the distinctive clunk sound is the starter solenoid moving the starter into the ring gear

The voltage drop is the starter solenoid making contact with the heavy battery power contacts inside the starter solenoid and it has a path through to ground

Something a starter shop brought up was the power contacts can burn and be compromised but is a task to replace the solenoid so would look at easier things first

The fuel pump on for 3 seconds on key rotation is from pin X of the ECU as it is between the ignition switch and fuel pump relay

This same pin X on the ECU is the same used as the engine is rotating that turns the fuel pump back on for the duration of your drive

If the starter sag is too much the ECU will not be powered properly so maybe not giving you the fuel pump relay enable in the form of a ground by the ECU

Pin X is............

Black 19 as the Pink / Brown wire

 

Either starter jammed or internal short in the starter motor.
12 volts will typically drop to 8 volts under cranking load.
Any more than that is a major problem in the starter system.