Need help on 1993 w250- CPS or PCM???

[captjackm]

1993 W250 Diesel.

Most of the time my truck is not charging. The A/C doesn't work. The overdrive and the cruise control do work. And if the CPS is bad then this isn't supposed to be how these things work. I say most of the time because they intermittently works as they should.

I do not have a scope to view the CPS sensor return, but it does get 8 volts, it does have a good ground, wire resistance is within specs (I believe it was 5 ohms) between the PCM terminals and the sensor connector. I do not have a DRBII to read the computer either.

According to the factory manual the CPS provides the signal for the PCM to turn on or off the Charging circuit, A/C, Cruise Control, and Overdrive. I am not 100% that this is accurate, however a couple of mechanics I discussed this with stated that they did also believe that this was the case. If the CPS is bad,

So, how do you test a CPS if you don't have a scope?
Why would only the two circuits and not all four circuits be compromised?

I suppose if the CPS is good, then logic would state that the PCM is bad.

Any help? Please?

Thanks in advance.

[Philip]

Here is how to test a CPS.

http://www.cumminsforum.com/forum/89-93 ... ensor.html

[captjackm]

Thanks Phillip, I did see that post. I don't have an "O" scope or a signal generator. I had seen some youtube videos about testing other CPS', which had to do with resistance measurements too. But I have no specs for the resistance on these Dodges. The OEM manual states to use the DRBII which I do not have access to. I do have a good ground and a 8.6 volts at the supply side. I don't know what to make of the readings I get on the reference signal on my VOM, they are all over the place, which would make sense I suppose but I don't think that's a trustworthy test.

I swapped the computer out of my other truck and still no charging or A/C. (the only reason I didn't do that sooner was that it was 270 miles away-but I have to put new rear leafs on it anyway, so I brought it home yesterday). I've done a pretty darn good job of checking the wiring, since I've owned the truck since brand new I've not done any wiring hacks, so it's all factory. Good grounds where there's supposed to be, no grounded / shorted wires that I've detected. The only other thing I did was to replace the clock spring (see my post on that). At this point I've ordered a new CPS from Napa A/P. It's supposed to be here tomorrow.

As I read the OEM manual, I am getting the sense that the only controls that the computer has over the overdrive and the cruise control, is recognizing correct transmission temperature to allow shifting into overdrive, and the computer recognizes brake pedal depression to turn off the cruise control. It doesn't seem like the computer cares if either of these do their own thing. It seems to me that the computer only "controls" the field circuit in the alternator, and it does control whether or not the A/C compressor is on or not.

Does anyone know if the internal wiring of the computer would allow overdrive and cruise control to work while not allowing the alternator field current and A/C compressor due to a bad CPS? Up to now I've led myself to believe that the computer does control all four, but after reading those manuals for three days, I am not so sure.

[captjackm]

I bit the bullet and paid $140 for the CPS. That was the problem. I'm going to search Craigslist ads for a small used "O" scope. Had I had one it would have been a matter of minutes to come to the conclusion of a bad sensor.

I was just very confused as to why the speed control and overdrive would continue to function. I've come to the conclusion that the manual is a bit vague on those two points. As I first read the manual I concluded that if the CPS was bad then all four circuits would not work. To have half of them work and half not was confusing.

So I conclude that if the charging system and A/C quit, and yet OD and Cruise Control continue to work, it is possible that the CPS is bad. I mention this because of the two very experienced mechanics I discussed this issue with were under the same impression, that all four systems would not operate.

[Philip]

The PCM doesn't provide the speedo signal. It only reads the signal going to the speedo from the VSS. OD is controlled by the PCM. When I lost the CPS on my truck years ago. I didn't loose OD.

You could have tested the CPS with a volt meter. That was the way I made the tests.

[captjackm]

RE: Speedo signal - I was referring to the speed control, or cruise control, not the speedo. In reading the factory manual I understood that the computer controlled the OD and the cruise control. It actually does neither of those. The computer on this truck seems to only control alternator field ground and A/C clutch engagement. Both of which seems to be a stupid reason for a computer. Perhaps it makes other decisions on temperature and air heaters, but it's not a very smart computer.

[captjackm]

RE: CPS testing with voltmeter: The CPS sends out a digital signal that truly can't be measured with a volt meter. A DVM may show voltages jumping around a bit perhaps, but I wouldn't trust it. I've seen some folks say that it's done, but I'd much rather invest the time on an oscilloscope before I'd condemn a digital electronics part with a volt meter. I have a few friends that teach computer diagnostics so I'm going by their instructions. I did buy a Tektronix 2335 "O" scope for $100 on Craigslist. Now I don't have to guess any more.

[Philip]

The CPS sends out a square wave on/off signal. I have had it on a O scope. That type of signal can be checked with a volt meter with the engine off. Just turn the engine by hand till the open spot on the balencer is under the CPS. Then the CPS closes sending a voltage signal to the PCM. Also the PCM supplys an 8 volt feed to the CPS. The CPS returns a 5 volt signal. When the CPS fails it will not send a return voltage to the PCM. I have checked a few known bad CPS with a volt meter. I have also checked known good ones also.

The CPS is nothing but a electric on/off switch. Just like the old rocker arm mechanical switch still used in the industry today. It sends a signal for each engine revolution. With the notch in the balencer you could use a rocker arm type switch. The PCM doesn't care all it wants is one 5 volt momentary signal for each engine revolution.

Teaching electronics isn't the same as spending decades in the maint fields working on many different types of rolling stock and factory machinery. BTDT