Speedo failure

[Pat]

So, I am cruisin' along in the rain with a couple thousand pounds of tile in the bed of my P/U when the engine shudders and recovers but the speedo slowly dies over a period of several seconds. I don't know why a diesel would shudder because of the speedo but there are speed signals generated and wired around to various places. Way more complicated than I understand.

This is a '97 Dodge Ram with original 12 valve Cummins turbo diesel. Ideas?

[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]

[egon]

Just guessin: would an electricl failure in the speedometer circuit give a temporary jolt to the cruise making it want to slow down the truck. You couldn't be so lucky as to find a faulty relay, fuze or connection.

The ac clutch just went out on my truck. About $300 to replace it.

Egon

[fivestring]

Pat, when I saw the title, I wasn't thinking speedometer failure! [img]/forums/images/icons/grin.gif[/img] [img]/forums/images/icons/blush.gif[/img]
LOL!!! [img]/forums/images/icons/grin.gif[/img] [img]/forums/images/icons/grin.gif[/img]

[Pat]

Egon, Who knows? Not me. I wasn't using Cruise Conrol, I was just crusin' under manual (fhould that be footal) control. The cruise control is not working, A G A I N , for the jilionth time. I have 100,000 mile warranty but 50 deductable for EACH problem. If you take it in to get three minor nits fixed it is $150. I have had the cruise fixed 5-6 times. I don't think it is fair that I should have to pay for the same thing over and over but...

So, your A/C went out... What difference does that make up there in the frozen north? Oh yeah, it reduces the efficiency of your defrosters. Sorry, I almost forgot. Bummer. I can recall what it was like to drive an air cooled VW in North Dakota. Manual defrosters were common in the form of a passenger with an ice scraper set on continuous operation.

Pat [img]/forums/images/icons/smile.gif[/img]

[Pat]

FiveString, Not a good thought! Keep your mind out of the gutter and make room for mine.

Pat [img]/forums/images/icons/smile.gif[/img]

[egon]

Frozen North ehhh. I'll have you know it hit +32 Celsius yesterday.

And we planted our meagre garden the day before.

Driving those air cooled/heated VW's scrapin was the only thing kept you warm.

From the many years spent working as a plant rat we learned to tap, jiggle and swear at all electrical and even some mechanical devices. Sometimes it worked. Cruise has to be connected to the speedo somewhere just to get a signal.

Egon

[Autotech]

I'm not familier with the system used on Dodges of your vintage, but your hesitation, speedo failure, and lack of cruise are probably all related to the failure of the speed sensor. By 97 most vehicles were using an electronic sensor mounted in the trans tailshaft where the speedo cable would normally be. Without this speed signal your speedo has no data to read, the cruise has no information, and I suspect when the sensor failed it created a momentary signal glitch which caused the engine management system to give you that little hiccup. Diagnosis and repair should be pretty straightforward for any reasonably competent shop.

Gregg

[egon]

Just finished crawling under my 96 dodge. There is an electrical connection on the tailshaft.

Egon

[Pat]

Egon, In Minot North Dakota during the worst of winter cold a standard air cooled VW would have its dipstick reading raised by a half a quart just driving 15-20 miles (gasoline infiltraton past pistons due to choke staying full on and the pistons and cylinders not fitting very well at -40 (F or C take your pick it doesn't matter) Even at significantly warmer temps, the defroster on full with all flow diverted to windshield and sometimesthe best it would do is sort of make the ice and frost on the inside of the windshield sort of clear and transparent right in the corners of the windshield where the "warm?" air came out.

The cruise conltrol has been out for a couple months but I haven't gone to the bother to have it fussed with. I didn't say fixed because I don't think it has actualy been fixed in years, just fussed with till it sort of temporarily works. It is possible that the speedo failure is related but not neccessarily.

The speedo is mechanical as is the odometer and trip meter. Well the speedo is actually mechanical-magnetic/electric kinda sorta. The mechanical speedometer cable rotates at a rate proportional to speed as it is driven by a gear off the tranny output shaft. There is a spring that pulls the speedo needle to zero. A force proportional to speed is generated by a piece of ferromagnetic material that is in close proximity to a copper ring in which eddy currents flow inducing a counter EMF and opposing magnetic field which are proprotional to the rotational speed of the speedo cable. The result is a torque on the part of the mechanism attached to the speedo needle which counters the spring and rotates the speedo needle toward bigger numbers the faster you drive. All of this is done with no curent used from the automobile electrical system (except to light the instrument at night.)

A seperate speed transducer (from the Latin to "lead across" as in lead the energy across from one form to another. In this case mechanical rotation is converted to an analog electrical signal proportional to speed) is driven by the same rotating speedo shaft. This speed signal is used to do all sort of things. Among these are such things as knowing when to unlock the torque converter so you can stop without killing the engine and stuff like that. This speed signall is labelled by Dodge, Vss.

"+ 32, planted a garden..." We have eaten our apricot crop and have been getting strawberries for months. A few blackberries were ready weeks ago but the main ripening is weeks away. Mulberries are ripe. Pears are getting to be the size of 50cent pieces to silver dollars.

Pat [img]/forums/images/icons/smile.gif[/img]

[Pat]

Gregg, Your understanding is real close. I have both a mechanical speedo cable and a speed signal generator (transducer) The speed signal generating transducer is located by the tail shaft of the tranny and the speedo drive cable runs through it.

The speedo instrument is the traditional hysterisis driven unit and the speed signal Vss is used for various things like unlocking the torque converter at low speeds. You are most likely correct about the source of the brief engine hesitation. I got side tracked by a brief bit of NON-RAIN and tried to tractor some but will get on my creeper and have a look at the wires. It could be something as simple as driving through rain water at high speed on the interstate or ...

Pat [img]/forums/images/icons/smile.gif[/img]