Converting Volvo 240 VDO Gas tach to Diesel (Distributor to Alternator)

[SMN3D]

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

[v8volvo]

Unfortunately 200 series 80mm diesel tachs are pretty rare, so may be hard to find one to refer to. You might have to do some experimentation.

I don't know whether the pulley ratio is the same on the D24 vs the VW motor, but I suspect it is not. From recollection, the crankshaft drive pulley on a 4cyl VW is significantly smaller in radius than the dampener on a D24 is, and the Volvo does not have an especially large alternator pulley -- I don't recall the VW alternator pulley being any smaller. Don't know what you would have to do to the tacho circuit in order to get the calibration right if there turns out to be a difference.

I have a spare 1.5L VW and probably have an alternator for a VW somewhere too so if you're in a bind I could make some measurements next weekend...

I know I did one time plug a 760TD tach into a running VW alternator (in order to determine which of two identical and unlabeled terminals was W and which was D+) and remember that the tach showed a generally plausible value for engine speed with the VW motor idling, so if the ratio is off it is at least not very far off.

You probably are already aware of this other alternative, which costs some money but works well:

http://www.dakotadigital.com/index.c...rod/prd128.htm

I have used these on a few different diesel conversions to get gasser tachs to work and have been generally pleased with their ease of use and performance.

Keep us posted on what you find out.

[SMN3D]

I measured the diameter of the pulleys on the D24, and this is what I got:
Crankshaft: 15cm
Alternator: 40mm

From a few minutes of research online, I found the diameters on the Mk1 VW diesel to be:
Crankshaft: 14.9cm
Alternator: (varies, either 30mm, 40mm, or possibly 70mm)

Since I wasn't using a micrometer (and didn't remove either the lower shroud or the radiator), I'm willing to guess my measurement of 15cm is pretty close to the specified 14.9cm of the VW Mk1 Diesel. :-)

According to the incomparable regcheeseman at a posting here:

http://www.vwcaddy.com/archive/index.php/t-4749.html

"The original spec of the W terminal I recieved was incorrect, instead of 25 pulses per rev, I've seen quotes of 14.5 pulses per rev quoted elsewhere.

There are at least two pulley combinations to drive the alternator so this can compound things further.

However the degree of calibration available on the standard tach is considerable and should be able to cope with adjustment btween theese variables."

So when I get some time in the next few days, I'm going to wire up the tach exactly as specified in regcheeseman's schematic, and then test it out on the bench with a drill of known RPM, use the 14.5 multiplier, and calibrate the tach on the bench using the pot already on the PCB. I'm not looking for super accuracy here, and I can always pull the cluster and recalibrate later (hopefully not snapping the speedo cable nut in the meantime!)

I will keep you up-to-date, and if all goes well, there will be an easy way (if you can solder and have the correct W-terminal alternator) for anyone to get a functional tach into our beloved volvos with very little cost (and how it came from the factory)...

Scott

[v8volvo]

Well, those pulley sizes sound pretty close, so maybe it will be right-on.

If you're successful with this, you could probably get a very small cottage industry going making replicas -- I bet there are at least a half-dozen 240 owners I could think of who would be excited to buy a plug-and-play diesel tach!

I have a couple of spare 700 series diesel tachometers; although their accuracy is nothing great, if you would like to get a calibration reference specific to the Volvo application at some point after you get this working in order to dial it in, I'd be happy to loan you one by mail -- contribution to R&D.

[Mick13]

hello
Nobody had try to convert gas to diesel with the DLS1 Dakota module ?