testing a pressure sensor????
 

guys,

is there a way to test a pressure sensor to make sure it's working. i'm still trying to figure out why my motor keeps hesitating when i hit the gas and it won't produce it's full power. it's a 72 1.7 with the original fuel injection. any ideas let me know,

thanks
scott

Worn out TPS?

To test the MPS you pull a 15hg vacuum on it for 5 minutes. It should hold that 15hg or maybe bleed down to 12hg, if it bleeds down to 0hg you are in trouble.
Do you smell gas? A bad MPS makes the car run rich...
My guess is a sticking dizzy or bad dizzy.
Timing is everything on these cars.

Geoff

Geoff,,,,a worn Throttle Positioning Sensor gives you a hesitation as well....

Blipping the throttle by hand will show you the symptoms....very common.

Of course, your mileage may vary....

MPS test.
1 Like geoff said. 15 lbs for 5 minutes.http://forums.pelicanparts.com/uploa...1112761724.jpg
2 between post numbers 15 and 7http://forums.pelicanparts.com/uploa...1112761881.jpg
and between 8 and 10 http://forums.pelicanparts.com/uploa...1112761934.jpg readings are in oms.
Joe

Geeeez..NO votes for a TPS????

Hope he posts and I'm right.....

Im with you Mike...A worn TPS could cause it...

Sticky dizzy weights would too. Oil the felt in the center of the Dizzy!

Ohhhh,,,scary...I owes ya a beerski....

Re: testing a pressure sensor????
 

TPS problems are usually around 2500-3000 rpms and don't keep the engine from reaching full power.
Timing problems often do....sticking advance plate or off timed cause hesitation and prevent the engine from producing power.

OR a really rich condition, which is easy to smell.

Geoff

thanks for all the advice guys, i'm going to try a few things tonight. i haven't set the timing since last year. would resetting the valves cause the timing to be off?? and a couple more questions, what is a TPS?? and where is the advance plate in the dizzy??

thanks
scott

well i figured out what the tps is but i still can't figure out where the advance plate is, where will i find it.

thanks

scott

The points ride on it...It covers the counterweights...

thanks bowlsby,

how do i find out if it's sticking?? and is there another name for the TPS. the name i found was throttle positioning sensor but i can't find it in pelican's catalog. i'm trying to figure out what it looks like and where it is on the motor.

scott

a worn pressure sensor will usually cause the car to run WAY rich. YOu can also do a quick check of it by sucking on the vacuum port to see if it's holding vacuum. What happens is that there is a sealed cell diaprhagm ion there that can seperate and trick the computer into thinking there is a false vacuum reading. I think there was a guy on the board that was repairing them, do a search.

as for the TPS, it's on the throttle buttefly. (the black box attached to the butterfly shaft) if you remove the cover, you'll see a arm wth electrical contacts that contacts the printd circuit track on the board. What typically happens is that the printed circuit track wears a flat spot. Sometimes, if you are careful you can clean it, or remove a bit of material to freshen up the track, then very slightly re-adjust the electrical arm to contact it.

also on 1.7's another common problem is that the throttle shaft becomes slightly bent and causes the butterfly to stick in it's bore. This was causing some hesitation and idling problems on my old car. I'm pretty sure this has to do with the way the throttle cable applies constant pressure to the top of the shaft, hence bowing it over time. You can check this by taking off the air boot, then opening the thorrtle by handf then letting it shut quick. if it sticks closed, then you know it's bowed.

I fixed this problem on mine by carefully disassembling the throttle, smoothing the edges of the butterfly and also lightly sanding the portion of the shaft where it sits in the throttle housing. this solved the problem for me.

THere's also the old problem with vacuum leaks. If there are any leaks, it will cause a bunch of problems. When i had my 1.7 i would replace the vacuum hoses every year.

well,

i tried to start it last night and the battery is drained. it seems like something is draining the power. i'll have to figure out what this problem is now before i start on the other. but thanks for all the suggestions, i'll try them as soon as i can figure out how to keep it running.

thanks
scott

Joe Sharp,

What values are being measured ? resistance (ohms?) or volts?

Resistance. You won't get voltage anywhere in the system unless you're hooked up to a power source, like a battery.

--DD

Generally, if the pressure sensor is the right one for your car, holds vacuum, has the correct values of resistance for the primary and secondary windings, and is unmolested (i.e. hasn't been "adjusted" by drilling or removing the epoxy seal on the end), then there is a very high degree of assurance that it is working properly. Check my web page for more details on the pressure sensor and testing of your D-Jetronic FI system.

All of my information comes from Mr. Paul Brad Anders.
Thank You Brad
Joe

This is the first post I've seen for testing the MPS electrically. Can't wait to test mine tomorrow. Might be the culprit for its crappy running!

Gonna get rid of them thar dam'd points too ;) got me a new 'lectric ignition thingy today :)

........b

well i got my car running again. it was just a bad battery. i drove it a little the other day before it started raining and it ran pretty good. it wasn't hesitating that much so maybe the battery was a problem. i'll know more when it dries up and i can test drive it again.

scott

Good luck Scott, as you can, see questions on the MPS go deep fast, You have had the best, in responce to your request.
Joe

You can borrow my Janbo sometime Bruce to trace down your problems.



Geoff

Thanks Geoff. You've offered it to me many times and I appreciate your generosity. I will take you up on it.

The weather should be changing for the good in a short time & I'll take a drive up your way to see my sister and stop by to have a beer wit'cha.

How does the Janbo work? and what is it connected to so you know what's wrong with the car. It's strickly FI trouble shooting, right?


.........b

You plug it in with the main FI plug to the machine. It has a pee brain so you can start and run the car while you troubleshoot. Pretty much a go-no go type of tester but it works by telling if everything is operating or not. Has some tests that displays readings too to evalue.
Its not a tuning device.

Geoff

Testing the resistance of the primary and secondary coils just verifies that the coils aren't open or shunted. It doesn't provide any information as to whether or not the MPS is actually responding to the vacuum signal. The Janbo tester mentioned has a rudumentary circuit that gives a range of response to a 15 in. Hg vacuum level, but not the detailed data needed to tell if the unit is calibrated properly.

Here's my line of testing/logic for an MPS if you don't have an analytical tester (see my testers page):

1. Test to see if it holds vacuum. A lot of units I see can be pumped down to 15 in. Hg, but quickly leak back up to atmospheric pressure in a minute or so. If the unit is an OEM MPS, and hasn't been rebuilt (OEM has rivets holding the case together, rebuilts have screws), then if you have a leak it's very likely due to a crack in the full-load diaphragm, and the unit will continue to degrade until it no longer works, and needs replacement. If it's a rebuilt, it could be leaking from a number of places. If you're brave, you can open the unit and reseal it, but it's likely this will affect the calibration of the unit. If you decide to do this, do NOT change the settings of the screws in the full-load diaphram, and to not tamper with the full-load limit plug in the end housing.

2. Test the coil resistances. If you find a shunt or open, the unit is toast and needs replacement. It may be that in the case of an open, a lead from the coil to the plug may have broken, and could be resoldered, but it doesn't happen often.

3. If you've passed tests 1 and 2, and the unit is either a) an unmolested OEM unit, or b) an unmolested rebuilt unit, then it's very likely the MPS is functioning properly. The only caveats to this are:

i) The MPS is the wrong part number for your application (check
my web page for part numbers)
ii) If it's a rebuilt, the unit may be miscalibrated for your
application. Apparently, the main rebuilder (Bret Instruments)
only uses one calibration setting for all of their rebuilts.
iii) The aneroid cells may have a leak in either one or both cells
iv) There may be a mechanical problem with the unit. This could
be the failure of an internal leaf spring or some other
component. It is also possible that debris/dirt may have
entered the end housing of the MPS through the vent slot,
and is jamming the movement of the full-load diaphragm.
Check by bright light inspection.

This is the most you can do without a tester. Good luck!

That's very good info. I doubt very much that I'll be taking my MPS apart. The closest thing I'll do is use Geoff's tester.

I installed the Pertronics last night. The car started and idled very smooth. I'm thinking the dizzy is worn so I'll settle for the electronics. Checked all my numbers of the components from your site Brad, and it seems they are correct.

thanks again all.... b