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**GordMay** · 08-11-2003, 12:31

A couple of questions I've been asked about the MD3B. Anyone have any further information on the Volvo's?

1. When you install a Gauge, i.e., fuel level, water temp., oil pressure, tachometer, etc. etc. do you ALWAYS have to have that manufacturer's sending unit or are sending units universal innature?

Senders are matched to the Instrument Gauges by Resistance (American &
European), and by the number of Stations (Single Gauge, or Dual
Gauges) they will operate - hence any manufacturer’s appropriate
sender will operate any other manufacturer’s resistance matched gauge.
See the Charts I sent.

2. How do tachometer's sense the engine revolutions? I
have a Volvo-Penta MD3B in our boat. The Tach does not work. I found four (4) sending units on the engine. I can identify the water temp, and oil pressure but damned
if I can figure out what in the world the other two are for. I wonder if
one of them is for the Tach? Would a tach ever operate off of oil
pressure, similar to the way an oil pressure gauge works (sensing the pressure mechanically and then converting that to electrical going to the gauge)?

Diesel Tachometers work by counting the number of pulses generated by
a sensor (Alternator, Magnetic (Hall Effect) Pick-up, Mechanical
Pick-up), and converting this to Revolutions Per Minut

delmarrey · 08-11-2003, 21:27

I'll bet the other two are the water and oil alarms. Electricaly, they are NORMALLY CLOSED at normal operating temp. and with no oil pressure which sounds the engine alarms before starting the engine.

Most tach sencors are mounted over the flywheel for pulse sencing.
....................................._/)

pjfsail · 10-11-2003, 08:06

Gord, thanks for you thoughtfull reply. I'll bet you are exactly right. It sure had me stumped. I even went to the local Volvo-Penta dealer and the parts manager was most helpfull but did not know the answer. She called Volvo-Penta and they couldn't tell her what the answer was! At this point I put the question on the back burner until you came along. I will continue to investigate this possibility as time permits. Thanks again for your help. Peter

**Steve Rust** · 30-12-2003, 20:30

I have a Volvo MD7B that has the fitting for OEM tach sender. It is located on the rear of the engine on the upper gear housing that runs the water pump and fuel pump I think. I am guessing it reads the gear revs either by mechanical or magnetic means. Due to the high cost of the Volvo sender ($175) and tach ($375) I ordered a Morse Teleflex tach from West Marine ($80) and hooked it up to the alternator. All you need is to have the W terminal on your alternator. You have several settings to pick from on the tach depending on your alternator but once I picked the right one I found it to be very accurate.

**GordMay** · 31-12-2003, 04:25

Thanks to Del & Steve for your valuable inputs.

Further to Steve’s post: The Alternator’s “W” (Stator AC) Terminal might also be labeled “R”, “AC”, “N”, “STA”or “Tach”.
If your Alternator has any of these terminals, you can utilize a code “T” (Alternator Type) Tachometer. If not, your alternator shop can perform a simple modification to bring out an AC tap.
In order to select & set up the Alternator Type Tach’, you will have to know (& tell your supplier) the number of poles your alternator has, and the Alternator to Crankshaft Pulley Ratio. The Teleflex (or others like VDO, Stewart-Warner, Faria, etc) instructions will explain the details.

If you're confused; take the Alternator and the Crankshaft Pulley measurement to a good Alternator Shop. They can sell you the appropriate Tach, and perform any required mod's to the Alt', and provide additional specific advice. These guys can become a valuable resource.

Regards,
Gord