In the 1970s 107 items of SM30 class locomotives were modified to heat old passenger wagons, which were modernized with 500V electric heaters, and as a result their designation was changed to SP30 (meaning a diesel locomotive able to pull passenger trains). A prototype locomotive, SM30-232 was converted in 1970. Then, in 1974-1977, 106 locomotives were converted this way by the ZNTK Nowy Sącz (of those, 75 in 1974). They retained their original SM30 class numbers. Those machines remained in regular service until the late 1980s. Because of a withdrawal of old wagons, in 2000 all of them were again, after dismounting heating systems, returned to SM30 series. A similar modification was applied to SM42 class, resulting in the SU42 class.
Traction motors are series devices with 60 kW hour power. Main alternator (PABOM-186a type) is a DC machine with 600 V and 367 A. Its power rating whilst working with 1500 rpm. is 220 kW. During engine starting the main alternator serves as electric starter, powered from the battery. For the purposes of enlighting the locomotive and charging batteries an additional alternator (Pw-114a type) is used.
The only difference between SM30 and SP30 classes is a device to provide wagons with power from the main alternator. This device allowed the locomotive to heat up to three two-axled carriages.