Diesel Tach Not Working

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2zuzus
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Diesel Tach Not Working

Postby 2zuzus » Wed Jan 10, 2018 6:38 pm

Greetings Folks,
Well I am finally asking advice to a problem I am stumped with. And I know I am in a good place to do that.
My 1986 2.2 Turbo Diesel Trooper has a 3rd Gen. engine including add on oil tank. The upgrade was done 90k miles ago according to the previous owner who purchased from his uncle. When date-wise, where, and by whom the swap was done were not known. I bought this truck about a year ago now. The truck has the correct VIN code for a diesel engine.
The truck had the gasser tach in the dash. The fuel inj. pump had the screw in plug at the sensor location. (That pump does not have the fitting for the turbo sensing by the way, but does have the correct part #'s on the 2 attached tags.) The engine wiring was, and to a degree still is, in a bit of disarray. As far as I can tell the wiring from the sensor plug for signal and ground are still intact. It seems there was a power overload at some point, as the 3 fusible links to the battery are gone, plus several "fixes" were put in. The complete state of the wiring system absolutely, that's still an open question. However virtually all electrical systems in the truck are now working, and the truck runs great, as it did from the start.
I purchased a factory packaged sensor unit in "new" condition. The bag was opened, ostensibly for an eBay photo. That came from England. That was installed and "plugged in" months ago. It is the correct part as per package label.
I purchased a diesel tach recently as a takeout from an '86 Diesel Trooper. When I put everything together today, I didn't get so much as a bump from the tach needle.
I will do a continuity check to see if the sensor wire is good from the fuel pump plug to the plug at the dash, and from the other wire to ground, again.
My main questions are: How do I check the sensor to know it's condition? I tried a continuity check between the ground and sensing wires when I received it but got no reading. Given my limited electrical knowledge I don't know that there even is supposed to be continuity between those wires.
Checking the tach is the other main item. Where can that be checked to know it's condition? Can those items even be checked with a VOM? If I need an ocilloscope or such I'm out of luck.
I did spend over an hour searching for this problem in the archives but came up with nothing relevant.
Help with this problem will of course be most appreciated.
I would like to say that I had a source to offer for both the sensor and the tach, as I know those are items that folks are looking for from time to time. Unfortunately I cannot as there was only one of each in both cases.
Thank you for your time with this, Chris.

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Paul
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Re: Diesel Tach Not Working

Postby Paul » Thu Jan 11, 2018 12:16 am

Chris - -

The tachometer sender in the diesel pump is a variable reluctance gadget (meaning that it has a magnet and a coil) and the coil should have a resistance of about 1700 ohms. No reading (as you got) is bad news. Maybe you did not have your meter on the right range for this resistance reading. Better try again.

I have not worked on a diesel trooper tach but I think the circuit is the same as a P'up diesel. I have done some work on a trooper gas tach to convert it to diesel and I remember that it is in a round case and should have three connections on the back. One for the Bat, one for the Ground and one for the signal from the sensor.

So, check your sensor resistance and make sure that you have the three connections to the back of the tach. Also, one wire from the sensor must be connected to ground with the other one going to the signal input of the tach. Since you have two wires going from the sensor to the tach, I suggest connecting one of them to ground at the tach ground terminal.

The tach unit can be checked by powering it up from a 12V source and applying a signal to it from an oscillator (a variable frequency sine wave source). A frequency of 308.33 Hz should produce a reading of 1000RPM. The input voltage is not real critical but 1 or 2 volts should work well.

You might post some photos of your stuff to make sure we are talking the same parts.

Paul

PS - - Here is the circuit for the P'up diesel tach. Should be the same as yours. Click on image to get a better file.

P'up Diesel Tach Schematic email.jpg
'84 P'UP 2 wd diesel, 5 spd with 0.78 fifth gear and differential back to 3.73.

2zuzus
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Re: Diesel Tach Not Working

Postby 2zuzus » Thu Jan 11, 2018 2:43 pm

Hi Paul,
Thank you for your reply!
I have looked at your post on the tach conversion that you mention, as I was interested in the feasibility of going that route. It is very interesting information, and shows a daunting task for myself and my electrical circuitry experience, or lack thereof. I enjoyed it just the same, and was actually thinking about it and you as I wrote my HELP post. I was very grateful to see your response.
I spent some time checking what I could today. Your description of the tach does in fact match mine, the three posts, round unit etc. There are four soldered connections with three of them mating to the correspond posts on the circuit board. The fourth is for the tach lighting and that has no post for it. There are also four input wires in the plug on the tach: 12V power; ground, for lights and meter; light circuit; and tach signal.
I checked the sender on the fuel pump on the 1 K ohm setting and got 1.630 ohms resistance, which I am hoping is acceptable. I checked the harness plug sender end to the harness plug tach end on the signal wire and got continuity, no resistance. Same for the ground. A note here, there is not a continuous wire from send unit up to the tach. The sender grounds on the engine block, and is picked up elsewhere by the harness to the dash. I did not unfortunately have time to install a continuous ground wire from sender to tach ground post.
I plugged the sender back into the harness and checked the resistance at the harness plug tach end. The reading was a bit more than at the sender itself. interesting to me, I checked the sender with the engine running and got readings from 1400 to 1700 during a 30 second check.( Teeth of the gear turning past?).
The bad news is when I put it all back together, the tach still showed zero movement.
I am hoping the problem is a bad ground (although the readings don't bear that out).
Is there a way to check the tach with the only instrument I have, The VOM? At this stage I can't even get 1 rpm that I can tell, so checking it for accuracy isn't the issue right now. I would just like to know if this gauge can even display a reading at all. I wouldn't try to check the tach itself for continuity outside of the lighting circuit (that I checked and it is good, so ground is working there) for fear of smoking some component on the circuit board. If I know how to safely check the tach for a continuous circuit I will.
That's about it right now. I have to scoot out for a class on rigging we have to take every few years. Always something new to learn.
Thank you again most sincerely Paul.
Chris

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Paul
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Re: Diesel Tach Not Working

Postby Paul » Fri Jan 12, 2018 12:21 am

Chris - -

The 1.630 on the 1k scale would be 1630 ohms, I think and should be fine. Measuring the resistance at the tach is a good idea, but not with the engine running as the tach sender puts out a couple of volts and this will mess up the ohmmeter. Do the same with the engine off and you should get 1.630k ohms as before. However, with the engine running you should be able to measure the signal voltage at the tach. Put the VOM on AC volts and you should see about 0.5 volts or more and this will increase as the engine speeds up.

The sender ground through the engine block is probably OK, although not the best way as it is subject to more noise getting mixed into the sender signal.

At this point it does look like your tach unit is not working. I do not know of a way to check it with only a VOM. Checking it will require an oscillator (to produce the sender signal) and the 12V power. Also, if it is not working an oscilloscope along with the circuit diagram is needed.

Keep us posted - -

Paul
'84 P'UP 2 wd diesel, 5 spd with 0.78 fifth gear and differential back to 3.73.

2zuzus
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Re: Diesel Tach Not Working

Postby 2zuzus » Fri Jan 12, 2018 8:35 am

Greetings Paul,
Thank you once again for your reply and insights.
I will check the signal voltage at the tach today as per your instructions, and hopefully eliminate another possible problem.
At this stage I will hold off on the direct ground, but if I, scratch that, if someone get it working, I will definitely run that wire.
I have been scratching my head trying to think how on earth I can get this tachometer itself checked out, and the light bulb finally went on. The fellow I bought my USED VOM from was/is, (hold on to your hat because this is a bit archaic), a TV repairman. Yup, no kidding. Better yet, he's a straight shooter, because everything I have ever bought from him at his garage sales has been exactly as he said it would be.
I will contact him and see if he would be willing to check this out. I had already printed off your 1st reply and will do the same with this last one.
Is there any other info you have that I could pass on to this fellow that would help diagnostically, specs. or what-have-you? Seems like you have provided it already. This time, however, and hopefully, it will be seen by someone with a clear understanding of it, unlike myself.
I will get to that voltage reading today, and post the result. I have no idea how long this tach check might take as of yet.
Thank you again Paul. I would surely be dead in the tachometer water without your kind assistance.
Chris

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Re: Diesel Tach Not Working

Postby 2zuzus » Fri Jan 12, 2018 1:04 pm

I tested the signal voltage at the tach end of the harness. The reading is .47 volts AC at idle, with a steady increase as RPM rises, just as you said Paul. Looks like I better try to get in touch with the fellow with the equipment to test the tach. Rats! It is rarely a single trip for me, so this is nothing new.
The bad is now generally known. The Good? The sender seems to be working as it should, and the connection from it to the gauge plug is good, just could be made better at the ground. Best of all though, I went from scratching my head in the dark, to having a direction to move forward with in less than a days time of asking!
Take the proverbial bow Paul, you earned it!
I will post the outcome of the next step when it happens. Any further insight Paul will be welcome of course. By the way, I just wanted to mention that my computer skills are on par with my electrical skills. Basically I can get by. To that end, I don't currently know how to or even if I have the means to, post pictures. I would certainly like to be able to. They are worth a thousand words after all. When they are available, I will ask the younger crowd that passes through here occasionally if they can show me how. I wanted you to know Paul that I was not ignoring your request for pics.
For you folks that have looked in on this thread, I hope my dilemma has produced something useful to you, as other posted threads have done for me. Again the thanks goes to Paul.
Chris

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Paul
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Re: Diesel Tach Not Working

Postby Paul » Fri Jan 12, 2018 8:55 pm

Chris - -

Well, thanks for the many thanks :)

I hope your TV repairman can test this for you. Possibly, he can repair it also if needed. Make sure to give him the schematic diagram from above. Also, the gas unit you have may be helpful because it uses almost all the same parts. Hang on to that.

Paul
'84 P'UP 2 wd diesel, 5 spd with 0.78 fifth gear and differential back to 3.73.

2zuzus
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Re: Diesel Tach Not Working

Postby 2zuzus » Sat Jan 13, 2018 3:31 pm

Hey Paul,
I hope so too. I'll update as anything happens, good or bad. I will hold on to that gas tach as it will be a good spare in case of a problem with the 1988's unit.
And, you're welcome.
Chris

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Re: Diesel Tach Not Working

Postby 2zuzus » Sun Jan 28, 2018 7:20 pm

UPDATE
Greetings Folks,
Several people have been involved in trying to get this tach issue straightened out.
A second Tach was pulled and used as a tester from another diesel Trooper. This 2nd Tach was known to give readings intermittently. It was used to see if anything was coming through the wiring harness from the sender unit. Unfortunately in that regard, it didn't work any better than the first tach, but as testing would show, it proved a major fact.
The second unit was actually bench tested first with a signal generator at the prescribed voltage and frequency. This unit worked, intermittently, on the bench at first, but when the voltage was raised beyond the initial prescribed voltage range up to 7 volts, the unit performed great. The chip that processes the signal is the most likely culprit. As specs come in and time allows, it will be looked into further.
The original Tach was bench tested second, with the same equipment to the prescribed values. It yielded no readings at all. The bezel was removed from the face and after removing the hardware at the back, the works were pulled out of the case.
NOTE: The bezel is crimped on for most of the perimeter around a plastic lip. There are several ways to remove it. I use a bottlecap type opener,(also can use that style paint can opener), and I wrap the end with tape for a softener as it presses against the rim of the bezel when you pry. Put the tongue that would go under the bottlecap edge, under the bezel rim edge between it and the plastic side wall of the case, and pry up gently as you push in towards the rim to keep the tongue from rolling out from under it. Go slowly, move the opener in small increments around the perimeter, and lift in small amounts, going back over the perimeter several times until the edge has been straightened almost 90 degrees. If you raise the edge from the one mounting tab to the other, you will have more than half the rim straightened and you can carefully pry the bezel off,(and back on again),from there by sliding the unstraightened part out from the plastic rim, while lifting the straightened part for clearance.
Once the works were out it was found that the initial resistor was burned out. This acts as a fuse to protect the rest of the works, and it did. The resistor was replaced and that tach was retested. Everything worked as it was supposed to within all test parameters. Mike, the TV repairman, told me to install it. I asked him if I should test my circuits again in the Trooper and he said all is fine there. The other tach had been installed a few times by then and there were no blown parts in it, that resistor in particular. There was no problem in the sending circuit as the intact second tach showed.
I installed the tach this afternoon and it works as should, finally. AMEN!
A very big THANK YOU goes out to Paul for his knowledge, patience and commitment to a problem that was not his to begin with. He supplied all test values and schematics as well as techniques. Of course Mike deserves a big THANK YOU as well for taking the time to enthusiastically run the tests and be generally interested in this question.
As answers develop with the Second Tach, they will be posted.
I do hope this may be of some help to others.
Chris

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puttputtinpup
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Re: Diesel Tach Not Working

Postby puttputtinpup » Sun Jan 28, 2018 7:58 pm

this would have been great youtube material! Maybe you could add the value of the resistor that was blown and replaced. I hope you got some pictures to post.
Currently P'up-less, but keeping my eyes open for a good deal
Click this link for misc. diesel technical bulletins & other info

2zuzus
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Re: Diesel Tach Not Working

Postby 2zuzus » Thu Feb 01, 2018 6:51 pm

Greetings puttputtinpup,
Thank you for your post. I am not yet that computer capable to upload those items, though I have been digesting your tutorial on how to post pictures, and I think it was a great idea to enlighten folks with that. I didn't take pictures as I went, as I didn't do the tach repair work. However once I get the picture posting worked out, I can identify what had been the offending part inside the tach and other items of possible interest. I will find out what the resistor value is and post that with the pictures.
Take care, and Thank You for posting your tutorial.
Chris

2zuzus
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Re: Diesel Tach Not Working

Postby 2zuzus » Fri Feb 23, 2018 7:46 pm

UPDATE #2
This is an update as regards the second Tach.
Mike, who did the electrical test work on the initial Tach, became involved with other situations, and his time wasn't available any longer. Paul, who has been graciously providing so much insight and information, took on the mystery.
The Tach was tested, and it's readings fell within the parameters for a properly working unit. A mechanical inspection showed an interference between the needle, which had bent upward on it's end over time, and the inside face of the bezel. The needle was heated and removed, heated to be straightened, and then had it's bore for the shaft opened slightly so it would slip onto that shaft. Shoo goo was used to ensure a good fit.
Resistance was also noticed in the shaft rotation itself, and a small amount of lubricant was applied to correct this.
The needle had become very brittle with time, so a thinned Shoo goo "paint" was applied to reinforce it. Time had also dulled the needle's color, and nail polish was used to brighten it up.
At this point the needle was moving very freely, the electrical values were good, and the tune-up was determined to be complete.
I received the Tach back and noticed how nicely visible the needle now was, and completely free the needle now moved.
The unit was installed in the same Trooper, and worked great from the moment the engine was started. It is still in there and has been working fine for a week now.
The conclusion to this tach working well initially at 7 volts, is very possibly because of the extra drag that was present before the needle was straightened and lubricated. After the needle was freed up and reinstalled in the case it has been working very well and to spec.
In reference to the first Tach's resistor that blew as a fuse would, it's values are: 47 ohms 1/2 watt That is in answer to Andy's well reasoned
question of that part ID.
The mechanical problems with this tach are applicable to most if not all of the gauges in the Trooper's and the P'ups. These are other possible answers as to why a gauge is working poorly.
The resistor "fuse" value for the Diesel P'up Tach is 47 ohms 1 watt, as per the schematics. It is slightly different from the Troopers.
Lastly, and most importantly, a HUGE Thank You to Paul for his knowledge and expertise with these things.
Chris


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