Hello,

My Dad is experiencing some problems on his truck. He has a Dodge Diesel 12 Valve year is 92. D250

Symptom
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When the cruise control is turned on the truck seems to surge a bit. Like it goes faster then slows down, goes faster slows down. The cruise does not seem to stay at a steady gentle cruise speed. I searched everything in this forum and cant seem to find any one else experiencing this problem.

We did find one problem with the electrical part of the system, where the recommended fuse is a gray 2 AMP fuse, but the previous owner had a 5 AMP in it. I don't know if this would create any problems though.

Thanks for any help we can get.

Chad

 

I think that the cruise is held steady with vacuum. At least on my 91, I believe it is. Could it be a vacuum leak?

 

Assuming Dad's truck runs an automatic transmission, check the throttle position sensor located on the front/top of the injection pump.

My mess presented with the very same symptoms and we found the throttle position sensor had a dead spot in it. Made things act stupid.

Hope this helps.

 

My guess is a vacuum leak too. However check the linkage real good. The throttle linkage has a couple of plastic bushings around the pivot shaft that go away over time and add a little extra slop.

 

Ok Update...

We found out the fuse is correct and doesn't cause any problems. Now about the cruise. We checked the line voltage for the sensor/pcm that controls the cruise speed. The voltage is reading 4.8Volts. According to records it should be in the 5volt range. So this look fine.

The plastic gear for the sensor looks good. No cracks or stripped teeth of any kind could be seen.

Hoses for the vacume seem fine, we looked at them and there is no rubber rot of any kind. The rubber on the hoses are in good shape, with no cracking. The connections and clamps are all tight with no twisting. No kinks either.

We are gong to check the linkages now as my dad said there is a little bit of play there. So we are going to investigate this. Will let you know how it goes.

Thanks a bunch!

Chad

 

Ok new update,

We purchased a new VSS (Vehical Speed Sensor) that goes into the Tcase. After replacing this we took the truck out, but it is still acting the same way.

So i think it's narrowed down to a vacuume leak or something faulty with the servo.

Need more input based off this new information.

Thanks.

Chad

 

Look REAL hard at the TPS.

Does it act weird at any speed running the cruise control or just at 55 or 60mph?

Try running it at about 45mph (thus lower throttle setting and resulting TPS position) and see how it acts.


If push comes to shove, measure the resistance from the TPS's (variable resistor) wiper to one side of the resistor and see if you have any dead spots.

 

Is your Dads truck an automatic or a 5 speed?

Bob

 

check power to the switches at the steering wheel, and power to the servo. If there is none and all else check out good, then it will be the clockspring. I am in the same dillema.

 

The answer will be different if it's an automatic vs. standard. The standard trucks can get very twitchy on the freeway if the truck is juiced. I am going to try a large fuel filter in the vacuum line to the servo to see if I can soften the signal.

 

Hey guys Sorry, The truck is a standard.

 

OK that brings up another possibility. The nut that holds the rear driveshaft yoke in place might need to be tighened. This nut holds the speedo drive gear in place with a spacer on either side, the shaft is not keyed so the nut needs to be VERY TIGHT. Just drop the drive shaft off this yoke the really crank the nut down tight. This cured my problem - same as your dads. Let us know if this works - Good Luck!!

Bob

 

But that trick would only work for a two wheel drive correct?

 

If the surging is really bad on flat or down hill sections of the freeway, then you are having the same problem as a bunch of us. So far I have not found the solution, and it really bugs me!

 

I am going to let him know about the yoke. I don't think the yoke is loose or anything like that because a while ago he replaced the output shaft bearing, and made sure everything was tight. The cruise problem was there before he did the output shaft bearing. When he took it off it was on tight.

Today we checked the vacuum and it is reading a very strong vacuum. The other test we did was disconnect the vacuum hose from the servo, and then connected a vacuum pump to the servo nipple. We then disconnected the servo connector from the servo. We took the Blue/Red wire and connected to power. We took the remaining terminals and connected them to ground. When the unit is connected in this way you can test the cruise servo to see if it is working correctly. We then disconnected the cruise servo linkage.

When the unit is connected, and then vacuum is applied, a click can be heard from the servo, no vacuum is created and the cable does not move, however when the electrical cable is disconnected and we apply vacuum the vacuum increases. As soon as we connect the power cables again a loss in vacuum is detected by the gauge.

Does this mean the servo is faulty? Or what do you think?

Look forward to hearing from you guys, and we really appreciate all the help.

Chad