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Old 04-18-2014, 04:06 PM
SMN3D SMN3D is offline
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Join Date: Aug 2013
Location: Portland, OR
Vehicle: 1984 245 D24 M46 (converting to D24T auto), 1982 244 (installing missing D24 and M46 from wagon)
Posts: 13
Default Converting Volvo 240 VDO Gas tach to Diesel (Distributor to Alternator)

I recently acquired a 6Zyl Volvo 240 tach (redline at 6000, not 6500 like the 4 cyl or 5500 like the proper diesel), and am in the process of making it work with the D24 and the W output of the alternator.

I've traced out the components based on the wonderful work of regcheeseman at http://www.vwdiesel.net/forum/index.php?topic=3741.0 but being a 6zyl and not a 4zyl (and also being a Volvo and not a VW tach) there are a few differences:

. There is no cap C2 as there is on the regcheeseman schematic

. The 22nF cap (labeled C?1 on my image) between IC pin 1 and the R5/R7 junction IS cap C4 on the regcheeseman schematic

. There IS a 10nF cap (labeled C?2 on my image) between IC pins 1 and 3 (no equiv on either regcheeseman schematic)

. I can't ascertain from the markings what the value is for cap C5 (the lavender one)--nothing resembling 220nF or 47nF out of 0,15K100-/ (my analog circuit skills are quite rusty...)

. My R5 value is 33kΩ (unknown in the regcheeseman schematic, but removed anyway)

. My R4 value appears to be 160Ω (162Ω in regcheeseman schematic, close enough)

. I have a 47K resistor labeled R?1 that is not in either regcheeseman schematic between the R6/VR1 junction and the +

The posting over at that VW site started with http://www.vwdiesel.net/forum/index.php?topic=3096.0 which started with a Volvo (not VW) tach, and then got VW'ed when regcheeseman ascertained that the original mods by kuggkrans for the Volvo tach could never work (and he analyzed the circuit himself).

I'm looking for Volvo-specific expertise before I blindly follow the regcheeseman fixes, because I'm not sure of the following:

-- Is the alternator ratio on the D24 pulley the same as it is for the 4-cylinder VW engines, or will I have to adjust the values of any of the resistors/capacitors to compensate? My understanding is that the D24 engine itself is mostly the same as the 4-cyl VW version (with 2 extra cylinders), but I don't know if that applies to the pulley/belt ratios.

-- There is a W marked very clearly on the PCB of the Volvo tach (see the illustration), near where the current signal wire is connected, but not currently connected to anything else on the PCB. In a true Volvo 240 diesel tach, is that where the signal wire attaches to the PCB, and the components are re-arranged? I delude myself that all I would have to do with THIS tach is unsolder the signal wire from where it is now and attach it to the W terminal and all is good (delusions are FUN).

For those that have made it this far through this, thank you, and if you have any advice for me before I start unsoldering and replacing parts, I would appreciate it!

Scott
Attached Images
File Type: jpg Volvo240_Tach_6zyl_01_labeled.jpg (14.0 KB, 28 views)
File Type: jpg Volvo240_Tach_6zyl_01_unlabeled.jpg (12.5 KB, 17 views)
File Type: jpg Volvo240_Tach_6zyl_02_pcb.jpg (14.3 KB, 15 views)

Last edited by SMN3D; 04-23-2014 at 11:38 PM. Reason: reassesed pcb
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240, tach, volkswagen, volvo

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