How much vacuum should the vacuum pump suck.

Discuss (and cuss) the Nissan LD-series OHC Six diesel engine, popularly available in the US in 1981-83 Datsun/Nissan Maxima Sedans & Wagons.

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asdex
Posts: 33
Joined: 10 years ago
Location: Dunedin, New Zealand

How much vacuum should the vacuum pump suck.

#1

Post by asdex »

Hi at idle my pump sucks at about -3.13psi and when I rev the motor it goes to -3.84psi. This is with a digital gauge and measured by taking off the small hose to the auto transmission and measuring at the back of the pump.
Does this sound correct?
Thanks
rlaggren
Posts: 541
Joined: 14 years ago
Location: San Francisco

#2

Post by rlaggren »

I think vacuum (in the US anyway) is measured in inches of water. As I recall 16-20 inches is normal for a gasser (as measured with a common gas pump/vacuum tester gauge available over here) and I suspect that Nissan did not modify it's vacuum controls just for the diesel so the vac pump output s/b similar. If you go to a online conversion site you can probably get the value in various other units.

Rufus
82 Maxima wagon
windsock
Posts: 144
Joined: 12 years ago

#3

Post by windsock »

G'day asdex,

I assume we are discussing the vacuum pump at the rear of the alternator?

If so, converting your psi scoreto inHg (inches of mecury) or inH2O (inches of water) we get 6.37-7.82 inHg or 86.6-106.3 inH2O.

I have just connected my vacuum gauge to the vacuum pump and it was off the scale so my one pulls more than 25 inHg or 700 mmHg. Inches of mercury or water is the most common notation of vacuum I know of.

Your one seems a tad low by comparison.

Hope this helps.

Cheers,

Phil
Good roads lead to bad fishing.
rlaggren
Posts: 541
Joined: 14 years ago
Location: San Francisco

#4

Post by rlaggren »

Thanks for correcting my guess, , Phil. Maybe those old gauges I remember read in in-Hg... IAC sounds like you have healthy vac pump.

Rufus
82 Maxima wagon
asdex
Posts: 33
Joined: 10 years ago
Location: Dunedin, New Zealand

Vaccum

#5

Post by asdex »

Thanks very much for those answers.
I think I will look into it a bit further because as the automatic works off vacuum a low reading may make it change gear later which seems to be the case. I feel it should be changing sooner after I start off.
I have just had the brake booster reconditioned so it should have a good seal.
Cheers,
asdex
windsock
Posts: 144
Joined: 12 years ago

#6

Post by windsock »

Hi asdex,

When you say you feel the gearbox should be changing earlier, what sort of rpm is it changing you think? These things redline and are limited at 5400rpm. In my 2000kg land rover 5-speed manual my changes go something like (rpm range is mood driven) - 1st-2nd=1250-2500rpm, 2nd-3rd=1250-3000rpm, 3rd-4th=2500-3800rpm, 4th-5th=3000rpm. Max torque is about 2400rpm. Our vehicles most likely have the same aerodynamic profiles... :lol:

If it is still needing to be checked and stating the obvious, I'd check each tube coming off it for seal. If that doesn't fix things, I'd check the oil supply to make sure it is delivering sufficient oil to keep the cogs in the pump lubed and sealed for vacuum. There is also a large o-ring between the body of the pump and the body of the alternator, check this but I am thinking if this is leaking you'd see the oil leak. I have a pump that has several elbows off it from a car and I replaced all the copper washers on mine a while ago to make sure of a good seal.

Hope you can fix it easy. It is a messy business mucking about with the pump down there with all the oil and angles. If you remove it in-stiu watch out for cog movement within the pump, it can be a real bother to get the splines all lined up again to reinstall.

I noticed a couple of alternators for sale down in Invercargill on Trademe - ~$125 I think so quite pricey.

Phil
Good roads lead to bad fishing.
rlaggren
Posts: 541
Joined: 14 years ago
Location: San Francisco

#7

Post by rlaggren »

Don't recall if you have an FSM. There _may_ be a vacuum regulator in the circuit. IAC, it may be worth the $50 odd USD to get a hand vacuum pump w/gauge so you can apply vacuum to your system at various points and see whether you have a seal; might find it cheaper online. Applying it to the hose (disconnected) from the pump would test the whole system. The hand pump is useful for various other stuff also - see many of Al's posts about the Mighty-Vac.

Rufus
82 Maxima wagon
Carimbo
Posts: 463
Joined: 15 years ago

#8

Post by Carimbo »

Here you go:

Automatic Transmission problem 82 wagon
http://nissandiesel.dyndns.org/viewtopic.php?p=780#780

You're welcome.
asdex
Posts: 33
Joined: 10 years ago
Location: Dunedin, New Zealand

Sucking

#9

Post by asdex »

Hi, thanks for the info. I have a manual so will check that.
The engine has a rev sensor so I'll look out for a rev counter too.
I saw those alternators and that could be an option later once I've done the tests and checks you suggest.
$125 isn't too bad I just had to buy a rebuild one for my Toyota Corona 2c diesel which cost $580.
Cheers,
asdex
windsock
Posts: 144
Joined: 12 years ago

#10

Post by windsock »

Hi asdex,

Maybe drop by a couple of nissan wreckers and ask if they have a C32 Laurel car with intact dash. You can get the tacho out of that and wire it in to the rev sensor behind the IP front mount housing. Tacho to go...

Just a correction from an earlier post I made. These redline at 5000rpm not the 5400 I errantly stated. I looked at mine on the way to work today and realised the error of my ways... still nice to hear the roar well up to 4000 from a diesel.

cheers,

Phil
Good roads lead to bad fishing.
asdex
Posts: 33
Joined: 10 years ago
Location: Dunedin, New Zealand

#11

Post by asdex »

I have my vacuum gauge on and going now. It reads 6"Hg. I'm not sure how the chart for this works though. I see at 700 rpm it chart says 23.62"HG and at 5000rpm it shows 7.87"Hg.
I though the faster the vacuum pump would spin the higher the vacuum that would be created. I will test now by closing off thew various hoses to check for leaks before looking at another alternator.
Thanks

Link to chart.

http://nissandiesel.dyndns.org/viewtopic.php?p=780#780
nogrip
Posts: 46
Joined: 10 years ago
Location: hokitika, new zealand

#12

Post by nogrip »

my 4 speed auto seems to change up quite late also..
i dont think theres an issue with it,
kick down seems to be quite quick when required..

maybe its just changing late due to more load being needed to haul the transit...?? thats the conclusion ive come to on my transit anyway...
ld28t in a 81 transit van
asdex
Posts: 33
Joined: 10 years ago
Location: Dunedin, New Zealand

#13

Post by asdex »

Yes that might be so. If I find the vacuum is ok at that amount, I'll leave it at that. I guess there should be some sort of regulator to keep the vacuum constant but I don't see one. My kickdown and overdrive are switches on the dash. I was told the van has a Zephyr diff to which helps with gearing.
I will also keep an eye out for a rev counter but don't want to spend to much untill I take the van for some long trips over Christmas. Engine and auto are in an unknown state although the oil pressure runs about 60psi which sounds good. Cold it was 80psi.
Thanks
nogrip
Posts: 46
Joined: 10 years ago
Location: hokitika, new zealand

#14

Post by nogrip »

i have no idea what the diff is in mine, i believe its not standard,its got a square axle tube..its got discs on the rear too

ive got an aftermarket rev counter that works well..
its a model often found in buses..
ld28t in a 81 transit van
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asavage
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#15

Post by asavage »

asdex wrote:Finally figured out vacuum. My gauge will be 29 inches of mercury at zero with the motor off. Thats normal air pressure. When I start the motor it goes to 6 inches of mercury. So it's increased the vacuum from 29 to 6 which is good for the vacuum pump.
Cheers
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.
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