This sticky thread will be a bit of a work in progress. Hoping others will contribute who may have a little more knowledge especially of the Europe market engines and the changes that occurred with them. But it seemed like it would be useful to have this as a reference and a place to consolidate info about how to identify what features a given engine may have.
So I'll try to start with what I believe is true. Please post if there are any mistakes in what is below so that we can correct them.
Note that, very generally speaking, the changes and evolutions that occurred with the D24 engine in Volvos is also reflected, for the same time periods, in the versions of the D24/D24T/D24TIC that were used in other vehicles such as the VW LT trucks/vans and the Pinzgauer all terrain vehicles. Of course any changes related to the rest of the car don't apply to those vehicles.
1980 (1979 in some European markets?): D24 engine is first introduced, available in the 200 series. Naturally aspirated only, with no emission controls, solid mechanical lifters. This early first-year engine was unique in that it used a different block, cylinder head, and cylinder head bolts than any of the later engines. The 1980 engines have 11mm head bolts that are narrower and shorter than the bolts used in later years and are standard type hardware, NOT TTY "stretch" bolts. They can be identified visually by a regular inhex head, not a 12-point head.
The 1980 engine is believed to also have used a slightly larger diameter prechamber than later engines.
In certain limited markets -- Finland and Italy mainly? -- there was also another diesel 200 series model available during these early years that used a 5-cylinder "D20" version of the diesel engine, to accommodate tax laws that penalized engines over 2.0L displacement. These were not popular and supposedly were used mainly for taxi service. I don't think we have ever seen a single real-life example of the 5-cylinder model show up in pictures or with anyone owning it in this web group, but the service literature does refer to them and they did exist. The D20 engine seems to have been only offered for one or two early years before it was discontinued due to lack of demand.
For the US market, 1980 model year diesels were sold in an unusual 264/265 DL trim. They featured the square headlights of the 260 GLE as opposed to the round headlights of the same year 240, but lacked the GLE trim features, having manual windows, etc, like a 240 DL. No equivalent gas engine Volvo was available with the diesel's trim configuration for this model year. 1980 was also the only model year where the D24 engine was available in the pre-update 200 series body with early, 140 series style taillights on the wagons and the early 140-style dashboard and instrument cluster.
1981-1982: The D24 engine continues production but is updated in 1981, along with the related 4 and 5 cylinder engines from the same general family, to now use larger and longer 12mm torque-to-yield head bolts that require a 12-point triple-square tool. Note that in the US there was no 1981 model diesel, only 1980 and 1982, due to emissions reasons IIRC. The rest of the world did get 1981 model year diesels as far as I know.
The diesel engine 200 series cars could have the manual M46 transmission or BW55 3-speed automatic. I think the automatic was not available in the first year 1980 cars nor on the 5-cylinder D20 version, but someone may be able to correct that.
In the US market, the 1981 model year was skipped for diesel availability, and the diesel was reintroduced for 1982 model year, now as a 244/245 GL trim and VIN, featuring full GL trim with alloy wheels, leather, sunroof on sedans, power windows, etc. It continued in this trim configuration through the first half of the 1984 model year.
1983 (1982 in Europe?): The Volvo 700 series is introduced and along with it, the turbocharged D24T engine becomes available in the new vehicle platform. This TD engine is not intercooled and has no emission control auxiliary devices, but it does feature a different block casting with higher nickel content, piston oil cooling jets, different pistons, an oil-to-water oil cooler, higher pressure (155 bar) injectors and different injection pump with boost enrichment functionality, and a different cylinder head casting using valves designed for higher temperature operation. Available only or at least dominantly with the M46 manual trans in this first year of the D24T, in terms of what we have actually seen out in the world, even though the service literature suggests the D24T 760 cars for 1983 could have also had the 3-speed BW55 trans, perhaps at least in Europe (although one guy on Turbobricks insists he once found a 1983 760 TD in an Oregon junkyard with the BW55 trans and was excited since he salvaged the rare 3.15:1 geared rear axle from it, so perhaps a few did get sold?).
Meanwhile production of the 200 series with D24 continues without changes, with either automatic or manual trans.
European market 700 series cars supposedly also had the non-turbocharged D24 engine available.
1984: The ZF 4HP22 automatic transmission with lockup torque converter becomes newly available in the 700 series only, with the D24T engine (and maybe with the regular D24 also in the rest of the world). Most (maybe all?) D24 and D24T engines gain an auxiliary system for the first time in this model year, which we refer to as "altitude compensation" that uses a barometric pressure switch and a solenoid on the side of the injection pump to provide an automatic advance in injection timing when the car is operated at elevations over 3000ft/~915m. Prior to the introduction of this system, the static injection timing was supposed to be mechanically adjusted with a service procedure on cars that were going to be operated at high elevations.
"1984.5": Several changes that were destined for 1985 model cars (for the US at least) began to show up halfway through the 1984 model year. Thus, both for 200 series and 700 series vehicles, there are two distinct versions of the 1984 model year that can be found -- the early type and late type.
For the 200 series, 1984.5 changes were mostly seen in the rest of the car and the D24 diesel engine remained the same, EXCEPT that the early 1984 240 diesels used the same early style glow control system (two relays -- a main power relay under the hood, and then a separate timer/controller in the dash that operated it), whereas the 1984.5 240 diesels changed over to an updated glow control system using the same single integrated controller/relay unit mounted under the hood that had already been in use on the 700 series from the beginning. This is important to identify if servicing the glow system on a 1984 titled 240 diesel. Other changes for US market 1984.5 240 diesels were a switch from 240 GL to 240 DL trim (manual windows, no sunroof, no leather, no alloy wheels) and a revised instrument cluster. 1984 was the final model year for the 240 diesel on the US market, but the 1984.5 cars were really 1985 models with 1984 VINs and they were supposedly sold at dealers well into the 1985 calendar year due to weakening demand for diesel cars by that point in time.
For the 700 series, US market 760 GLE "1984.5" model year changes also were mainly in the rest of the vehicle (interior changes, significant redesign of front suspension parts, significant changes to electrical system, new instrument cluster design, change of standard alloy wheel style from 10-spoke "Nova" to dinner-plate "Rigel" design). Like the 200 series, these were really 1985 models sold with 1984 VINs and titles. Under the hood of 700 series diesels, the only change for 1984.5 was the introduction of EGR systems for emission control, at least on US models. These engines have altered exhaust and intake manifolds, an EGR valve in between them, and a primitive control system based around switches on the throttle lever of the injection pump, a vacuum temperature switch in the tee returning from the cold start device, and some switching solenoids and relays to manage EGR operation. The EGR engines these 35 years later rarely have correctly functioning EGR systems and the combination of EGR gases and soot mixed with oil vapors in the intake manifold can create carbon buildup in the EGR equipped D24T engines. Note that EGR was never used on the naturally aspirated D24, and also note that due to different emissions requirements, the D24T engine also continued to be sold without EGR systems in many other markets in the world.
1985: The lower trim 740 series is introduced in the US. Overseas I believe the 740 may have started a year earlier in 1984. For the US market, 1985 was the widest year of availability for the D24T engine. It could be had in 760 GLE sedan or wagon with ZF automatic trans, or 740 GLE sedan or wagon with automatic or manual trans. This was the only year in the US that the turbodiesel engine could be combined with a manual transmission and a wagon body style from the factory.
The 240 diesel was no longer available in the US beginning 1985, although as noted above, many 1984.5 models were supposedly sold in 1985 and were in effect 1985 spec models. 240 diesel sales continued in the rest of the world though.
No changes occurred to the D24T engine from 1984.5 to 1985 EXCEPT that beginning in 1985, the engines were no longer stamped with the "D24T" code on the upper front of the cylinder head at the mounting boss for the power steering pump bracket, as the earlier D24T and D24 engines had been up until this time. A very minor change.
- To be continued in a second post below due to length limitations. -