Fixing the 308 Oil Pressure Gauge so that it Reads Accurately

This is on my list for this winter, I'll let you know what I learn.

 

I looked into replacing my electronic gauge with the guts of a mechanical. Two issues: no reverse reading gauges available except Smiths and none go 0-170 lbs. seems 100-110 is max. Ended that project. If anyone is aware of a possible please post it.

 

Frankly, the only thing I would do if I was concerned is attach a mech gauge and do some comparisons at different RPM and just take note of it. As long as the e-guage needle moves consistently, it doesn't matter where it points, you can simply be aware that, for example, when the mech gauge indicates 85 PSI, the e-gauge points to wherever it points. Same thing at, say 30 PSI.

As noted, the gauge in the car is clearly not much of an oil pressure gauge! But if it's calibrated against a mech gauge and you know where the needle points at the highest and lowest acceptable operational pressure readings, it's all you really need in order to know that the pressure is OK.

Obviously, if you need to know precise pressure for some reason, a different gauge would have to be fitted. The one in the car, even if it was absolutely accurate, doesn't have the necessary indices to do you much good in telling you that at X rpm there is 53PSI, for example.

FWIW, any oil pressure above 10psi/1000 RPM is a waste of engine power...

 

Does this help:

FCA Technical Forums - 308 Oil Pressure Adjuster Valve

Mike

 

All auto oil pumps have a relief valve so as to not blow the system (primarily the oil filter but also any related coolers/hoses, etc)) apart as the pressure rises with RPM. Some - like the 3x8's - have externally adjustable relief valves. All it consists of is a spring which is compressed more or less by some sort of screw fitting. As noted, all the relief valve does is set the maximum pressure that the pump can deliver. Changing its setting has no effect on the accuracy of the gauge.

 

I don't know why, but mine reads higher when the headlights are on.

 

There are several problems. One is being a electric gauge its reading is based on voltage. The difference between a predetermined "Correct" system voltage and the voltage to the instrument modulated by a variable resistor in the form of an oil pressure sender. Well the voltage source to the instrumments through the relay panel is anything but consistant. Next the senders themselves are terrible pieces of crap and no two seem to work the same. Finally out of frustration a few years ago we sent gauge and sender to a really good instrument shop (not the one in N California so many seem to love) and had the instrument calibrated to a different brand of sender and then rewred the power source to a large B+ buss that had more stable voltage. It worked pretty well. I still prefer a mechanical and expect to get the OE face installed on a quality mechanical on my own at some point.

 

The OEM instruments are for entertainment and amusement only.

 

Rather than a mechanical oil pressure gauge on the motor I plugged my sender into pressurized air with a good gauge and regulator. This let me sweep the pressure from 0 to 100 easily. Also I used a voltmeter on the circuit. Turns out the gauge was fairly consistent but the sender was very inconsistent. I opened it and found it full of crud, moisture and corrosion.

 

And that's why I swapped all of the Veglia ancillary gauges for VDO's many moons ago.

VERY accurate now - the oil pressure needle moves with rpm, like a tach.

Old thread: http://www.ferrarichat.com/forum/technical-q-sponsored-algar-ferrari/59100-308-gauges-out-veglia-vdo.html

Cheers - DM

 

Nice thread Dave, thanks for sharing.

 

How tough is it to put a a Veglia face on a VDO gauge?

 

i checked the voltage at the fuse box and at the top of the gauge - it's not the problem i have an 81 308 and i think the original sender was replaced with a newer one (? 308 qv). the resistance range is not the same. i measured my oil pressure with a mechanical gauge and it is twice what the veglia gauge reads. you can get the veglia gauge to read correctly if you put a resistance in parallel with the sender but you now only get half the lower scale. i.e. a true pressure of 80 psi will read 80 but zero psi reads 40. i like having all the instrument needles vertical at normal values so i live with the imperfect fix. i bought another sender from mccann but it doesn't have the right resistance range either. if anyone finds a source of senders compatible with the 70's and early 80's cars i'd buy one. jon s

 

Sp.uk sells both ranges early and late.

 

I don't think it will help us in Ferrariland, but an interesting thing I came across when fixing my old VW Scirocco guages is that they ran them all off of 6V (or some other <12V amount). There was a three terminal regulator to supply a steady DC supply so that even with a low battery or lights on or whatever the gauges were all still accurate.

OK, not really accurate, but didn't fluctuate with voltage anyway.

 

crowndog: thanks for that info. i'll check their site today. never hurts to have 3 senders. jon s

 

crowndog: the 81 308 parts catalog calls for part #114927 but the sp site says that number fits the 308 qv. any experience? jon s - thanks

 

Send an email to Ita at SP with exactly what you want and she will get back to you. At least she has always been very helpful to me. Sometimes the website isn't correct or confusing.

 

thanks, i will. jon s