(Possibly) long post warning.
TLDR: Intermittent no crank investigated and still not sure why. Advice and opinions gratefully received.
Long version:
In May last year I had a late night FTP in the Tahoe. Had been parked at a train station whilst we were out for the day at the cricket and upon returning had a no crank scenario. Recovery was called and after some initial diagnosis (more electrons from jump pack, swapping starter and ac relays over) it was suggested that the starter motor had failed. The starter isn't accessible from above and the recovery guy didn't fancy rolling around on the floor so no further testing was done and, after having to push a 3 ton lump with two sleeping kids and a pissed off wife about 400m to the height barrier at the car park entrance which meant the recovery wagon couldn't get to us, we were recovered home.
A week or so later once I had time I went out to where the car had been left multimeter in hand to start some proper diagnostics, before connecting any leads however I gave the starter a perfunctory tap with a hammer and tried the key which saw it burst into life. Taking this as confirmation of a failing starter motor, though shut down and restarting did not require any additional percussive maintenance, I ordered a new starter and, upon arrival fitted it. Though being incapable of getting the parts I want from Rockauto I ordered a direct drive starter rather than the gear reduction starter. The direct drive being about twice the size and although still fits is somewhat closer to the exhaust manifold potentially reducing life span through heat.
All was well in the world after this with no further issues until the end of August when, on the last day of a weekend away camping, we jumped in the car to leave for home from a Sunday lunch pub visit and we had no crank no start. A quick tap of the (new) starter with a pair of pliers (in absence of anything more substantial being to hand, saw it again spring into life and we made it home without further incident.
A few local journeys didn't see any reoccurrence of the issue but, to be on the safe side, and assuming the new starter had in fact been cooked by its increased proximity to the exhaust manifold, I ordered yet another new starter; this time the 'correct' gear reduction version. This was duly fitted and on the first drive out upon trying to leave for home the now dreaded no crank condition reared its head again though as before some light percussive maintenance saw us on the move.
At this point my brain began to accept that the failure of 3 starters in a row seemed unlikely and something more was probably going on.
Well time passed and as I only really use this car in the summer and work. life and shitty weather got in the way of doing any further investigation until last weekend when I charged the battery, put the old 'new' starter back on and after a few twists of the key got it up and running and moved round to the garage so I could do some proper investigation.
I have been going out every day since last weekend to see if I can catch it in a no crank state, but until today it fired up right away every time!
Today however was the fateful day, just as I was checking the bus timetable and planning to head over to the NEC for the classic / restoration show I went out to do the daily check and it didn't start. A day of diagnostics, interspersed with some interior cleaning (having been sat all winter it was grim inside).
To the current (wish it had it) tests:
Test 1 - Is there power at the starter solenoid when cranking? After digging out the requisite 17 furlongs of multimeter leads so that I could connect to the starter (passenger side) and be in a position to observe the results from the drivers side I duly established that in fact NO there was NO signal to the solenoid.
Test 2 - Swapping the relay with the AC relay (the same) didn't result in any change, so unless both are dead it seems not to be a relay issue.
Test 3 - With the relay removed I confirmed the presence of a constant 12v and switched 12V from ignition when 'cranking'. It therefore seemed that the fault was either the relay (which I thought unlikely on the basis of the previous substitution) or in the continuity of the wiring to the starter solenoid.
Test 4 - With key on, applying a jump between the constant 12v feed and the feed to the starter solenoid in the fuse box = starting and running car. I was pretty much back at square one here mentally as surely that means the signal wire to the starter solenoid is OK and I had established the relay was getting the 12v command from ignition when trying to crank so does that mean it can only be the relay?
Feeling confused, but with a surefire way of 'hotwiring' the car to start I pulled it out of the garage and took a break from diagnostics to address the biohazard interior status.
With that job completed I 'hotwired' the car again and moved it back into the garage. It was at this point I noticed that the gear selection indicator on the dash was not indicating current gear. Hmm thought I the neutral safety switch probably plays a role in the starter circuit and a bit of google over a belated lunch suggested that a symptom of a failing neutral safety switch is a lack of gear indication on the dash.
Somewhat refreshed and reenergized I then returned for round two of diagnostics but this is where I have become more confused than before!
I again removed the relay and tested for 12v at key in cranking position, this again was present, which assuming the neutral safety is linked in the ignition circuit would seem to suggest that it isn't the issue, though I guess it could also be in the feed to the solenoid after the relay and affect things that way?
I then started thinking about the relay again and wondered if perhaps that wasn't the most likely culprit, as despite swapping with the identical AC relay, I still had no real idea if that one was good or not (as the AC has a massive leak and I have only used it for testing purposes (after I replaced the control panel which was also completely dead previously) getting it to turn on by jumping the low pressure switch.
At this point I very much desired some of those relay breakout boxes that I have seen used on youtube but as that is something I have yet to drunkenly purchase (or if I have, I have forgotten) I made my own attempt with a selection of needle probes:
With the command wire disconnected at the starter solenoid testing then commenced.
Again I, without fail, got a 12v feed to the relay from the ignition when 'cranking' and then, intermittently, managed to get 12v at the pin out to the starter solenoid. This however was improved upon by changing the earth location for the relay to the engine block and I did then manage to get 12v from ignition being transferred by relay magic to the 12v feed to the starter solenoid, this was confirmed by testing voltage at the solenoid end of the wire. In my mind this rules out the relay and the neutral safety switch being an issue as having 12v at the end of the wire where it connects to the solenoid would suggest that it receives power from the relay and then feeds the solenoid it is happy and allowing signal through.
Now thinking that it must just be a dodgy earth I had the not so bright idea to reconnect the wire at the starter solenoid, and with the relay connected by my 'breakout box' and a better earth being provided to the earth pin direct from the engine block attempted to start the car - this just resulted in melting plastic as, quite obviously in hindsight, the little needle probes couldn't really be expected to carry the current demanded by the starter solenoid.
After this failed test I have given up. I am not sure if it is a relay (as it seemed to work in testing) or if in fact it has been an earthing issue all along.
If anyone has any recommended avenues for further testing to figure this out one way or another, or to point out where I have made assumptions that could well be wrong in the above, I would be happy for any advice!