I have a three which I'm about to install, but I've been reading some posts that have been saying I won't see the full range of readings like I might on a five bar. Thoughts?

Whilst I agree with Don for the low end accuracy (a relative term for Volvo gauges) - I will add that higher pressure isn't always better - so in a car without an oil temp. gauge a 5 bar unit can be useful in talking to you when your oil is up to temp enough to "get into the throttle" (not advisable with cold viscous dino oil)....oil warming lags coolant warming...

An oil TEMP gauge is another interesting discussion - with some analogies to Don's viewpoint. An oil temp gauge can be used for ultimately how hot your oil gets (but often too late!) - it is best used to tell you when your oil is warmed enough for gunning the sucker.
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www.fidalgo.net/~brook4

I have a contrarian view.

While a good running engine will produce more oil pressure than the maximum reading on a 3-bar gauge, that only means that the 3-bar gauge will peg (at max) during normal driving. And do you really care if your engine produces 65 psi or 80 psi?

However, because of an expanded low scale, the 3-bar gauge does a better job displaying the low oil pressure that's typical of a hot engine at idle. I would prfer to carefully monitor my engine's oil pressure at the low end because this will alert me to any engine problems, such as worn bearings, worn oil pump, displaced delivery pipe or bad O-ring (not an unknown problem in the B21/23 engines), worn cam bearings, and so on.

I have several 5-bar gauges and senders to install in my various 240s but have grown accustomed to the 3-bar gauge in my '82 245T. I think it tells me more of what I want to know.

Yes, I *DO want to know if my low pressure has dropped from 25 to 20 to 15 psi.
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Don Foster (near Cape Cod, MA)

BTDT. Again.
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Don Foster (near Cape Cod, MA)

Senders are different. They may not be that accurate, as VDO specifies the 80 psi sender and the 5 bar sender interchangeably even though there is a 7-8 psi difference. I would go with 80 or 5 bar, because 70 to 80 is the most I have seen. No need to go to 100 psi, for example. 3 bar is only about 44 psi, and I see more than that on my freshly built B230FT when the rpm are up above about 2500 rpm. On the highway, I see about 50 psi. At hot idle, I see more like 20 psi. On a worn or older engine, you might subtract 20% from those readings.

If you want the gauge to show all operating conditions, get the 5 bar or 80 psi and matching sender. If you just want the gauge to show if pressure is high enough, the 3 bar gauge and sender will be fine, since all you really care about is that you have 1 bar at idle and a bit over 2 bars at 2000 rpm. You may not really care whether you have 35 psi or 45 psi or 55 psi at 3000 rpm.

Philip

One bar equals 14.5 psi
Three bar equals 43.5 psi
Five bar equals 72.5 psi
The Volvo Repair Manual Section 2 (21) Page 8 says: Oil pressure at 2000 r/min warm engine and new oil filter: 35.6 psi-85.3 psi.
You decide.

Can I just swap gauges from a 3 Bar to a 5 bar, or do I have to change senders as well? I just got the 240t running and the 3 Bar gauge is pegged at startup and only comes down a bit at idle after warmup.
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don hodgdon '89 744ti, '81 242t, '71 D-35

Gauge and sender go as a pair.
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Don Foster (near Cape Cod, MA)

You seem to be the one to ask about electonic stuff. My gauge (5bar) keeps showing low oil pressure ( 1 to 2 bar), but if I give the face of the gauge a whack it pops up to 3 to 5 bar so it is not the wiring or the sender. Any idea what gives? Personally I like the 5 bar, because even my beat up b21ft runs about 4 bar at 2500. The only time a 3 bar gauge would not be pegged on my cars is at idle. As Paul said, I know my oil is warm when the pressure drops from 5 bar down to 4, so I like the gauge for that.

If it's not a bad connection at the back of the gauge, then it's likely to be a bad gauge. I don't think VDO built those to NASA specs....

You could try to very carefully peel back the crimped-over bezel ring and slide the bezel and glass off the front, and then slide the guts out of the case. I've never disassembled an oil pressure gauge, so I don't knbow what you face or what you'll find or what you might (or might not) be able to repair.

When you reassemble, you can re-crimp the metal bezel, and the scratches and crimps will be hidden behind the plastic faceplate.

As I tried to explain..... in my opinion, precisely knowing your high oil pressure is less valuable than precisely knowing your (low) hot idle oil pressure and knowing when it begins to deviate from expected performance. You'll see this more easily with the 3-bar gauge than the 5-bar gauge.

But if it's high readings you want, then look around -- VDO sells a 10-bar gauge, with sender, that will drop right in.....
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Don Foster (near Cape Cod, MA)

Jesse,

I tried to pull one of my boost gauges apart and destroyed it :-( it didn't work anyway...

The gauges are so cheap at boneyards and on ebay that you might just want to pull one and replace yours.
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don hodgdon '89 744ti, '81 242t, '71 D-35

1 bar at hot idle may be OK. I had a 242T for 11 years, and with 10W30 normal oil at hot idle, the low oil pressure light would sometimes flicker. This is even when I first got the car and it had only 50k miles. The owner's manual said this flickering was OK. I think the low pressure light was triggered by a 12 psi sender, but I am not sure.

Anyway, the old 240Ts are notorious for poor harnesses, particularly where they come around the front of the block and go to the alternator and oil pressure sender. When I took mine apart, there were wires in there with insulation that just crumbled off. I patched it and it was OK for a few years and then I replaced the entire harness.

Philip Bradley