Door locks don't always work 1999 V70 base

[Schiener]

1999 V70 base model, problem started about 3 years ago with 215,000 miles on the car. When really hot outside, the doors wouldn't unlock with either the fob or by double-rotating the key in the door. As soon as it cools, even with the AC on for awhile, they work again. This hot/cool problem repeated every time it was hot outside. No problems the next winter, until the first hot day. In the 3rd summer of this problem, some of the doors would unlock, some would not, depending on the temp - the hotter it was, the more doors wouldn't open, rather, some doors would become inoperable at a lower temp than before. Then the Left rear door got to the point that it will not work most times, no matter the temp. Now most doors will not regularly work, even when cold outside. While researching this site and others, it seems that other owners have had this same problem. When trying to find the relays, or to listen to the 'good' rear door for what a normal sound would be, after about 10 repeated 'lock' and 'unlock' presses of the door switch, all doors stopped working. After it sits for a bit, they work again, but start to fail after only a few lock/unlock cycles. It's as if something is heating up and becoming inoperable until it has a chance to cool. I figured a relay, but I don't see any mention of a relay in the system, and there is no relay or fuse in the fuse/relay boxes. I read about a Central Lock Controller, but also no mention of where that part is or how to test if. All of the locks work smoothly when operating by hand.

Thanks in advance...
Schiener

[abscate]

I’ve documented the circuit in a thread on my S70 that I’ll link to shortly. This is quite aRube GOLDBERG system so brace yourself before you look at the diagram

Both the interior lights and locks are controlled by 2/76 lock controller logic module

[850 LPT]

The problems you are encountering might point to failing door lock actuators. If they have never been replaced, they are due.

When they start failing, it's always only on hot days. There is information on this on this site, you will have to search for that.
But I would try and diagnose in this direction first if it was my car.

Dirk

[BEJinFbk]

Each door lock motor has a small protection device.
As designed, if the motor should draw too much current
and begin to overheat ( for instance, because of a stuck
switch ), the device opens the circuit to the motor to keep
it from melting down or damaging switch or relays contacts
in the system.

As these cars get older, and due to the way that these
protective devices are built in to the motors, they begin
to fail. And they fail when they get hot.

There are several ways to solve the problem and they’re
all here on MVS. New motor/latch assembly, tear the old
one apart and either clean the PTC connections, replace
the PTC, or put a hunk of copper where Volvo wants a PTC.

[Schiener]

Thank you! That’s very helpful & what I’m looking for: information on the circuit so I can look for the likely culprit. I actually spent a few hours searching this forum but didn’t find any solid fixes, either the wrong year or guesses as to what it could be. But I did see a few other owners who had the same problem.

[BEJinFbk]

Search "lock motor PTC".
That should get you started.