The car is a 1980 245DL B21F/M46. Originally would crank fine but wouldn't start. Jumped fuses 5 and 7 (in-tank and main fuel pumps, thought it was the fuel pump relay) momentarily, car started and ran. Then that trick didn't work the next time, eventually found the negative battery cable was disintegrated, replaced it, recharged battery, then would crank great but not start, jumped fuses 5 and 7, car started and ran great. Re-started a few times without a problem (didn't need to jump fuses), left it overnight, and now it won't crank, won't even try to crank, no starter click, there's no power at all - the clock is barely ticking and the accessories don't work regardless of ignition position. However, my meter reads 12.5 volts across the battery terminals and at the positive cables leading into the wire harness that runs into the vehicle. There appears to be no power at the fusebox in any ignition position, unlike before.

Do I check voltage at the ignition switch next and how would I do it ideally without removing the switch? If the switch is bad would it prevent power from getting to the fusebox? Anything else to check? Is there an easy continuity check for the harness into the fusebox? The ignition switch was replaced at ~190k miles, car now has 286k.

Thanks for any help, this car runs too well to be killed by an electrical gremlin.

Zack
1980 245DL M46 286k
1988 745T+ M46 223k

Ok so the battery is fully charged according to my charger, I get 12.5 volts between battery positive cable and ground and between all the spade terminals on the junction box and ground. There's no power at fuse #7 (main fuel pump, supposed to get power directly from the junction box). The clock is still ticking but erratically like it's getting less current than it should (it's on the same fuse as the central locks which stopped working due to low power). This leads me to believe there's a short somewhere in the harness between the junction box and the fuse box and also between the junction box and the ignition switch (the buzzer that sounds when the key is inserted and the door is open works once in a while, always worked before this issue surfaced).

The wiring harness runs a long distance from the junction box to the fuse box and is fully wrapped and inaccessible in spots - any advice on how to find the short (or other ideas on diagnosis) would be great - Thanks

Zack
1980 245DL M46 286k
1988 745T+ M46 224k

Take the negative cable off the battery (for safety) then remove the positive terminal and scrape the inside clean. Do the same to the outside of the + post.

Then check & clean the small red wire connection at the + terminal—and check that wire where it connects at the small, black junction block on the inner fender. That's where all the interior circuits are powered from.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Thanks Bruce - I checked the connections you mention, the meter read 12.5 volts between the small red wire where it connects at the junction box and ground, and also between each of the other terminals on that junction box and ground. If those connections were bad I would've expected to see a voltage drop, but given the condition of the negative battery cable before I replaced it (literally disintegrated in my hand) I'm going to give this another look.

Don't let a DMM reading lead you astray. It takes only a miniscule voltage potential to produce a meter reading—but corrosion etc. can choke off the needed current that really does the work.

That + terminal-to-post contact is essential for the battery current to reach the small red wire, and from there to ALL the car circuits. It is also every bit as critical in allowing the alternator current to charge the battery.

Putting jumper cables on the terminals completely bypasses that critical terminal-to-post junction, and may allow a start, but plays no part in battery charging. Even once the car gets running.
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Sounds like the battery is dead.
Did you leave something on overnight?
A dead battery will still show 12V across it's terminals unless it is completely drained of electricity.
Try charging the battery and replacing the fuel pump relay.

hope this helps
steve

Thanks Steve - I didn't realize a dead battery would still show 12 volts (still doesn't make sense to me) - Batteries last a long time in this car (10+ years in the past, this one is about 7 years old) and I know it used to start even when I read less than 10 volts across the terminals. Definitely didn't leave anything on overnight. Given the age of the battery and your suggestion I'm going to put it back on the charger one last time to see if that works, although last time with a full charge I still had to jump 5&7 for the first start (but not the subsequent few starts before it died again). I replaced the FPR with a used one before this (when I fully charged the battery last time) but I'm not 100% sure it's a good used one (I trusted Swedish Engineering to not sell me a non-tested used FPR but I should've asked).

What condition is the engine wiring harness in? IE. Do the wires in the gray plug on the firewall above the engine have crumbling insulation?
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1980 245 Canadian B21A with SU carb and M46 trans in Brampton, Ont.

Thanks for the reply - I replaced the main engine wiring harness (with the gray plug by the firewall) with the updated non-crumbling style around 3 years ago. The other wires look ok.

Don't know if you have done this but I would start by servicing the battery terminals and cables, checking ground connections, and re-seating all fuses.
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1986 Volvo 240DL wagon