To all , OPS failure,what part fails.????

[simon]

I read some posts on the OPS and there is recommendations to let the OPS power a relay and let the relay power the lift pump.

The reasoning is that the OPS contacts are not beefy enough to carry the current,and burn out, some sooner, some later.

I replaced my OPS yesterday cause it to did not work no more.

I set out to find out WY.
I rigged up a 1/4" connector and put the old ops in one side and a quik coupler in the other and plugged it in the air compressor hose.

Then I tested the Ops for continuty accros the two out side terminals with the amp meter, tested OK, no restrictions. That was with 80 psi air pressure.

PUt it back on the truck, That is now a 5 min job. It did not work, no continuty.

Time to take it appart.
I cut a groove about a 3/32" deep Around the plastic part about 3/8 from the metal base and pulled the top off exposing the contacts.
The contacts looked brand new , no corrosion or burning what so ever.
They look to me heavy enough to carry that current easly.

It appears to me that the mechanism inside that is supposed to lift the contacts up against the terminals by the oil pressure is getting weak and does not move enough to bring the contacts together. At least that seems to be the case with this one.

The point is, if this condition causes the OPS to fail than installing a relay is not going to help dick all.

My question is, Did any one else go to the trouble to cut one open to find the cause? Than I sure like to know what YOUR findings where.???????

I can see the contacts frying if the liftpump shorts out, but the fuse should prevent that unless the fuse is to heavy.

By the way for any that don't know it , the center terminal is for the gauge only and has its own circuit that grounds trough the metal base.

My 2 cents on the subject.

[N9Phil]

When the contact closes it will draw an arc because of the current draw of the pump. This arc will cause the contacts to pit and after a while they will not make a good contact that will allow the needed current to flow through. When the contact closes it can some times make a good contact and not other times. When your engine starts you don't have 80 pounds of pressure to snap the contacts closed like you did with the air hose. I have taken a couple of these pressure switches apart and every time the contacts are the problem.

This happens on other equipment where there is a contact that transfers a large current when closing. Central Air conditioners have a contactor that switches the voltage for the compressor, Electric Ovens have contacts that switch the voltage for the elements, and Furnaces have fan relays that switch power for the motor. All of this type of equipment have similar problems with contactors relays and or switches and experience the same problems.

The snap action that you get from a relay allow for a higher current capacity than the oil pressure switch and therefore the best solution.

N9Phil

[Shikaroka]

That's interesting.
Thanks for posting your results.
Your's may be an exception, as I'd say the burnt contacts are more likely, but who knows.

[simon]

When the contact closes it will draw an arc because of the current draw of the pump. This arc will cause the contacts to pit and after a while they will not make a good contact that will allow the needed current to flow through. When the contact closes it can some times make a good contact and not other times. When your engine starts you don't have 80 pounds of pressure to snap the contacts closed like you did with the air hose. I have taken a couple of these pressure switches apart and every time the contacts are the problem.

This happens on other equipment where there is a contact that transfers a large current when closing. Central Air conditioners have a contactor that switches the voltage for the compressor, Electric Ovens have contacts that switch the voltage for the elements, and Furnaces have fan relays that switch power for the motor. All of this type of equipment have similar problems with contactors relays and or switches and experience the same problems.

The snap action that you get from a relay allow for a higher current capacity than the oil pressure switch and therefore the best solution.

N9Phil Phil9,

You are for the most part correct , with a few exeptions.

1, the liftpump takes only a little current versus 110 or more volt relays.
2, if you study the OPS you will see that it has a mechanism that snaps over center at a certain pressure setting, mimicking a relay snap action.

My OPS contacts looked like they had never seen current flow at all, thats how clean they are. I asked the former owner how long ago the ops was changed. He said never as long as He owned it . He put about 215.000 km on the truck

Unless some other dudes chime in with the same opinion I am not sold yet as to burned contacts as the cause, it could be the result of a fluttering contact due to failing of the hydraulic mechanism.

I do respect your opinion though.

And than again mayby my OPS was a fluke?


Simon

[N9Phil]

Simon, I would believe that yours might be a fluke. The oil pressure switch does have a snap action where a relay has a wiping action that helps to keep the contacts cleaner. The relays that are being used by most have a higher current handling capacity than the oil pressure switch.

Here is a picture of the failed OPS that I just changed the other day.

http://www.thedieselpageforums.com/photopost/data/517/medium/ops_1032.JPG

N9Phil

[simon]

Simon, I would believe that yours might be a fluke. The oil pressure switch does have a snap action where a relay has a wiping action that helps to keep the contacts cleaner. The relays that are being used by most have a higher current handling capacity than the oil pressure switch.

Here is a picture of the failed OPS that I just changed the other day.

http://www.thedieselpageforums.com/photopost/data/517/medium/ops_1032.JPG

N9Phil Hi Phil,

Yea that does not look to well.

What you say about the relay has merrit.

It would be interesting to know how many are flukes, versus contact failures.
We may never know, at least it made for conversation eh.


Simon

[tommac95]

Simon-
People on this forum dismantled these back in 1990's and discovered problem, which has i think been found worse on older hot engines with low oil press.

Beginning 1996 the OPS signals a relay {or the CPU/PCU energizes directly??} and the OPS is never/less-frequently problemmatic.

Both the PMD/FSD and LP are driven by transistors which tend to fail by shorting out in constant ON state. In these events the amount of current flow in the circuit increases , and probably draws down the voltage in the circuit , which further tends to strain electronic components included. At any rate there have been many FSD/PMD and LP replacements which failed again in short order.

Inductive loads (as solenoids and ignition coils, motors {circuits with coils}) are well known to cause difficulties when switching off , due to the tendency to arc at separation points.
*
Though straining by memory (credibility) , i vague recall a post contending what you discovered (sluggish/inadequate point closure).

For practical purposes it's been suggested to replace OPS if voltage to LP low.
>>The point is, if this condition causes the OPS to fail than installing a relay is not going to help
Agreed.
And restoring reliable operation on a well-used (pre-'96) vehicle suggests installing a relay BEFORE replacing the OPS [which i did as a preventive measure].

[Shikaroka]

Am I reading that right?
The 96+ trucks have a relay between the OPS and LP? So, I don't need to add a relay to mine?
I knew they had a relay to energize the pump at start-up, but I thought the OPS took over and fed the LP directly after that. I had planned on building a relay harness, just hadn't had the time. So, I don't need to worry about it?

Sorry to highjack! ;)

[JohnC]

Back around 1996 I cut open 2 of those switches and found both to have burnt contacts. Both were replaced and coupled with relays. No more failures in about 200K miles.

Further testing indicated the lift pump is pretty inductive, so I put a supression diode on each relay. My guess is a diode on the OPS would likely eliminate the need for an additional relay.

My bet is that if your contacts looked like they never carried current, that's probably the case; it was dead from day one.

As far as I know, the later (ODB II) trucks use the relay only during initial power up and the oil pressure switch does the dirty work while the engine is running. Same as the earlier trucks except the PCM controls the relay instead of the starter solenoid circuit.

[simon]

Back around 1996 I cut open 2 of those switches and found both to have burnt contacts. Both were replaced and coupled with relays. No more failures in about 200K miles.

Further testing indicated the lift pump is pretty inductive, so I put a supression diode on each relay. My guess is a diode on the OPS would likely eliminate the need for an additional relay.

My bet is that if your contacts looked like they never carried current, that's probably the case; it was dead from day one.

As far as I know, the later (ODB II) trucks use the relay only during initial power up and the oil pressure switch does the dirty work while the engine is running. Same as the earlier trucks except the PCM controls the relay instead of the starter solenoid circuit.
JohnC

I am a sucker when it comes to the nitty gritty of transistors, diodes and such , i know they are there, I don't understand what makes them thick.
What the heck does induction mean?

But slooooowly I am learning.
I am a practical mechanic, and always want to improve things instead of just replacing. Always looking for the cause of failure.
It seems like the world want you to replace and replace on and on.

you suggest my OPS was dead from day one. Then I wonder what gremlin was making the liftpump run for 5 sec or more, after killing the engine, till the day the OPS failed .

Read my post, 95 starts ok, dies, then starts and runs fine.
The liftpump worked after shutdown too, but intermittent I recall

Time to call fox mulder [x files]

Simon

[JohnC]

Induction is an electrical property usually associated with coils and components that contain coils. The lift pump has a coil of wire that pulls the pump solenoid.

Induction is sort of like electricity in motion wants to stay in motion. When current is flowing through the lift pump and the oil pressure switch opens the electricity wants to keep flowing so it arcs at the switch contacts. A supression diode gives the current an alternate path backwards through the coil so it can disipate quietly.

I agree, your switch is X-files material...

[simon]

Induction is an electrical property usually associated with coils and components that contain coils. The lift pump has a coil of wire that pulls the pump solenoid.

Induction is sort of like electricity in motion wants to stay in motion. When current is flowing through the lift pump and the oil pressure switch opens the electricity wants to keep flowing so it arcs at the switch contacts. A supression diode gives the current an alternate path backwards through the coil so it can disipate quietly.

I agree, your switch is X-files material...
thats interesting, shows again one is never done learning.
Now where do I put that supression diode in the wiring, and where can i get one, and what amps is needed.

I know the workings of most components, it is the stuff you find in computers and such that have me stumped to no end.

Now what is mulders ph #

Simon

[tommac95]

John C.-
Thanks for the clarification , i thought past '95 OPSs were safe.

Simon-
Some versions of the Bosch automotive (12V) relays include a (prewired) diode, worth it in time if you can find them. Waytek used to stock Bosch, you can check the pdf catalog ... they have/had a penalty fee of ($5??) if the order is less than $25 i think. http://www.waytekwire.com/index.htm
Relay(??) : http://order.waytekwire.com/CGI-BIN/LANSAWEB?WEBEVENT+L0D442A2E652404006F55042+M37+ENG
*
Two sites explain/illustrate the diode installation/wiring for relay point protection:

http://www.bcae1.com/relays.htm

http://www.kpsec.freeuk.com/components/relay.htm
* *
The most basic passive elements of electric circuits are :
conductors , resistors , inductors , capacitors...
Conductors allow electric currents to flow. Resistors brake/restrict it.
Inductors exhibit inertia , like water-'hammer' , or torque required to start a shaft spinning , and braking effort to slow it.
Capacitors store electric charge ... they resemble a hydraulic accumulator , or a spring.

[simon]

John C.-
Thanks for the clarification , i thought past '95 OPSs were safe.

Simon-
Some versions of the Bosch automotive (12V) relays include a (prewired) diode, worth it in time if you can find them. Waytek used to stock Bosch, you can check the pdf catalog ... they have/had a penalty fee of ($5??) if the order is less than $25 i think. http://www.waytekwire.com/index.htm
Relay(??) : http://order.waytekwire.com/CGI-BIN/LANSAWEB?WEBEVENT+L0D442A2E652404006F55042+M37+ENG
*
Two sites explain/illustrate the diode installation/wiring for relay point protection:

http://www.bcae1.com/relays.htm

http://www.kpsec.freeuk.com/components/relay.htm
* *
The most basic passive elements of electric circuits are :
conductors , resistors , inductors , capacitors...
Conductors allow electric currents to flow. Resistors brake/restrict it.
Inductors exhibit inertia , like water-'hammer' , or torque required to start a shaft spinning , and braking effort to slow it.
Capacitors store electric charge ... they resemble a hydraulic accumulator , or a spring.

Thanks, very informative, great job