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tech question mechanical oil pressure gauge |
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Speedway 650 
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Joined: 25-December-2005 Location: Clarksville, MI Status: Offline Points: 54 |
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 Topic: tech question mechanical oil pressure gaugePosted: 27-August-2014 at 5:12PM |
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As some on here may have noticed, I have been trying to sell my '73 Merc Montego, which has been listed with very low oil pressure. To try to eliminate the possibility of an artificially low reading on the dash mounted electric gauge, I bought a Suntune mechanical gauge on ebay. As my luck would have it, the kit came without the oil line and fittings. As I live in a little town with a poor excuse for a hardware and no place to get auto parts, I improvised a temporary install of the Sun gauge.
Now the question. The mechanical gauge reads pressure in whatever cavity it is attached to, correct? The line with the kit should have been 1/8" vinyl, and I installed a 5/16" vinyl hose over the 1/8" NPT threads with hose clamps. Does the diameter of the capillary tube affect the gauge reading? They aren't calibrated to a certain volume or size line, are they? This weekend, we'll get to town and I will buy a copper line install kit so I can mount the new gauge properly in the car, and not on a foot of hose draped over the cowl. The good news is readings are well over twice what the old electric gauge reads. But, while I don't believe the install affected the accuracy of the reading to any large degree, I am still nervous about that potential. I might check the accuracy of the tach with an induction type, as well. If it is off any degree, and it don't seem to be, I'll just reinstall the original base cluster. All the copper on it looked like a new penny. The upgrade cluster in the car now was the best parts from three of them that were really showing their age. The use of LED lights brightened the lighting, but I never got the ammeter to work right and, it would seem, the oil pressure gauge, either. I'll be looking for any advice. Somebody out there has to be smarter than I am. -Les in MI.
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californiajohnny
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Joined: 05-October-2013 Location: winlock, wa Status: Offline Points: 14609 |
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Posted: 27-August-2014 at 6:16PM |
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the oil gauge should have come with the line kit! no the larger size hose shouldn't make a difference. on the sport cluster- did the std cluster gauge read ok pressure? could be the sending unit? if you buy a new sender make sure you tell them it's for a gauge, not a light (there is a difference) does the fuel and water gauges read right? and as for the ammeter gauge look towards the top of the 72-76 forum about the sport cluster wiring it's got a sticky, follow that diagram (the part with the shunt 8ga. wire 30-36 inches long) seems weird but that's how it has to be to read correctly!
http://forum.grantorinosport.org/forum_posts.asp?TID=2933&title=info-72-79-gauge-clusters-and-wiring Edited by californiajohnny - 27-August-2014 at 6:18PM |
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JOHN
74 GRAN TORINO S&H CLONE 74 VETTE CUSTOM 90 S10 BLAZER 4X4 LIFTED 77 CELICA CUSTOM 75 V8 MONZA SUPERCHARGED 79 COURIER VERT. SLAMMED 75 VEGA V6 5 SPD 70 CHEV C10 P/U 68 MUSTANG FB CONVERSION |
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Montego01
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Joined: 16-December-2012 Location: Largo Status: Offline Points: 565 |
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Posted: 27-August-2014 at 11:19PM |
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I agree with John. Your oil pressure is probably all right if the new gauge seems to read a higher pressure.
You could replace the oil sending unit first and see if that helps the original gauge. If not, possibly replace the gauge to. I would think you would definitely want this reading accurately when someone comes and sees the car. |
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Speedway 650
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Joined: 25-December-2005 Location: Clarksville, MI Status: Offline Points: 54 |
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Posted: 28-August-2014 at 4:49AM |
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Thank you for the replies, gentlemen.
The mechanical gauge that I bought came from one of those bargain barn places that sells everything under the sun cheap that they can buy at liquidators and auctions. Their retail stores are actually in western Michigan, and I have bought tons of stuff from them over the years. They sell all kinds of automotive tools, chemicals, and accessories. Some of it junk, and lots of it good quality, but sometimes decades old. My "kit" was NOS, but came in a two piece plastic package that could be opened and then snapped back together. The wiring and mounting hardware was there, but not the plumbing. I used a printed wiring diagram for the specific car I have when I changed the clusters. It did not specify the ammeter shunt length, so the one I installed is short, if that matters. Otherwise, it should be good. I di install a new oil pressure sending unit to replace the switch that was there. The temp gauge is believable, but always reads a bit low, even on a hot day. The car has a huge radiator, though and is all stock. Again, I think I trust the fuel gauge. I did install a different voltage regulator on the back of the cluster, as the one on it when I first installed it was junk. So, if the shunt must be exactly like the link on the reply above states, that could be why it seems to read what the current is doing, but exaggerates what I think it should say. Other than that, only the oil pressure reading is suspect. I could try another sending unit, just in case the one I bought new was less than stellar. I have bought bad parts new before. If it is the gauge, I will have to just install this separate mechanical one somewhere else. The oil pressure now starts at about 75 pounds cold, as opposed to 35. It drops to 12-15 lbs at hot 700 rpm idle in gear, where it read maybe 5 before. Hot and 2500 rpms now reads 40-45, instead of 25-ish. These reading are obviously much more to my liking! I also replaced the vacuum modulator on the C-6, which was leaking. Now, hopefully somebody will come and buy the car who isn't looking to rebuild or replace the engine soon, but don't mind some body repair. Thanks again. -Les in MI.
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californiajohnny
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Joined: 05-October-2013 Location: winlock, wa Status: Offline Points: 14609 |
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Posted: 28-August-2014 at 2:59PM |
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the cluster regulator--that's why i was asking about the other gauges, but if your engine has miles and never rebuilt ,stock pump, etc. that's probably about right for pressure
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JOHN
74 GRAN TORINO S&H CLONE 74 VETTE CUSTOM 90 S10 BLAZER 4X4 LIFTED 77 CELICA CUSTOM 75 V8 MONZA SUPERCHARGED 79 COURIER VERT. SLAMMED 75 VEGA V6 5 SPD 70 CHEV C10 P/U 68 MUSTANG FB CONVERSION |
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Rockatansky
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Joined: 30-July-2010 Location: On The Road Status: Offline Points: 6072 |
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Posted: 29-August-2014 at 10:33AM |
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replacement senders aren't calibrated the same as the original OEM stock ones, the oil pressure will read low with a newer sending unit
jillions of threads all over the netgoing back probably 10 years or more seems like there's a wide open market for stuff that works right
Edited by Rockatansky - 29-August-2014 at 10:34AM |
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72 GT Ute
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Billy C
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Joined: 10-February-2010 Location: Pittsburgh, PA Status: Offline Points: 948 |
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Posted: 30-August-2014 at 4:08AM |
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I don't know if it's any different for the sport cluster because I don't have that. On the base gauge cluster there is a dummy light on the dash that will light at 5-6psi. For example it will be on when cranking the motor until the oil pressure builds in the galleries. If the light isn't tripping on while running it's probably good. On my stock 460 I see oil pressure as low as 7-8psi at idle hot. Not low enough to trip the light. When you spin it up as long as the oil pressure goes up you are good. I was worried about it at first because all the non-big block people are always telling me it's way too low but if you think about it the engine doesn't need much oil pressure at idle, just enough to keep things lubed up. A 700rpm idle isn't doing much.
Shame you gotta sell your Montego, but good luck anyhow and I hope it finds a good home.
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-Billy Conturo
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Speedway 650
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Joined: 25-December-2005 Location: Clarksville, MI Status: Offline Points: 54 |
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Posted: 31-August-2014 at 7:56PM |
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Thanks for more help. I didn't know that the new senders made the OEM gauges read low. I spent $18 on a new sending unit anyway, just to eliminate the possibility of that being the problem. I installed it in the top of a "T", with the 1/8 inch copper line for the mechanical gauge plumbed in the side. As I didn't really have a place to mount another gauge in the dash and wasn't looking to cobble a mount for the mechanical in the passenger compartment, I mounted it to the inner fender so it can be observed with the hood open from the throttle plate side. It was a lot of trouble for demonstration purposes, as the gauge you can see in the dash is still reading way low. If somebody buys the car, they can mount it anywhere the want. I may just unhook the sender for the electric gauge and install a sender and light, which would, if accurate, flash a warning of major loss of oil pressure that you could see going down the road. I could mount a light anywhere without cutting up vintage plastic. For the record, regarding the tech question: the new mechanical gauge reads exactly the same with 5' of 1/8" copper line as it did with a foot of 3/8 vinyl.
I do have another issue which I think I'll put in another tech question. Again, thanks to several of you for your advice. -Les
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Montego01
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Posted: 01-September-2014 at 12:27AM |
That's a possibility. When I broke in the motor this weekend , I had a mechanical gauge sitting on the firewall. It was reading 75-90 lbs. Once broken in I put a new sending unit in for the stock gauge. It ready about 55-60 lbs' I think I still have the original unit. I may stick that in and check it out. |
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Rockatansky wrote: