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Wiring and speedometer issues, Update

1309 Views 72 Replies 7 Participants Last post by  ViolenceJack
Good morning gang. Since last time I was here my D250 is running good. I do however have some problems still and cant find a clear cut answer online.

So my fuel gauge works, but when I start to move the truck it bounces HARD left to right. I know the gauge technically works because I added about 5 gallons of fuel to the tank and it went up. The dummy light for "low fuel" is on though.

The oil pressure(?) gauge is at zero and doesn't budge. The truck has oil.

I'm not 100% sure if the temp gauge works either, I haven't really let it run super long to get that hot. I did however put a new thermostat in because the old one was rusty and stuck shut.

The speedometer and odometer do not work either.

The dummy light for the parking brake and ABS both stay lit. I have messed with the connector by the e-brake and it appears to be intact, but when I move it by hand the dummy light gets a little dimmer.

My pawpaw had most of the dash apart and I really have no clue as to what all problems he was facing. He did have the red plug for the dummy light panel plugged into the gauge cluster, and the black gauge cluster plug just dangling. I'm gonna attach a couple pics of some plugs that I'm not sure about that were left unplugged with (seemingly) nowhere to plug into along with some loose wires just laying in the floor. And I have a picture of a little silver fella that gets HOT even when the truck isn't running. I have it unplugged because I don't want to walk outside to a burnt up truck. Also I am for some reason terrible at tracing wires. Its the one thing that absolutely overwhelms me.

Any help is appreciated.



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First thing that comes to mind is check the dash ground, and make sure the cluster is plugged in (those round pins get lightly corroded from sitting). Everything you describe indicates a dash ground issue...
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From my understanding, by the time the Cummins was introduced, the cable-driven clusters were not used. I believe they started phasing them in around 88, by 89 they were all electric.
And last thing to add. I found this picture of apparently what it’s supposed to look like all connected. My VSS seems to be connected straight into the transmission without this ratio adapter part. Is the ratio adapter something that only goes on manual trans or something? Mine is the 3 speed auto. View attachment 975362
All of my 727 transmissions have a ratio adapter...
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Yes it has 5 volts like it is supposed to. Oh and another thing that doesnt seem to work is windshield wipers. Not sure if thats relevant
Add a stand alone ground wire to the windshield wiper motor...this is a pretty common fix if the motor isn't working. I usually attach the new ground to one of the screws at the end of the motor.
The newer Dodges use the rear wheel speed sensor for the Speedometer but not the 1st and 2nd Gen's.

Have you checked for volts at the VSS wires?

Just to clarify, the 2nd generation trucks use the ABS sensor for speed up through 30mph where the actual speedometer sensor in the trans starts reading.

That's how you can tell which sensor is bad: speedo up to 30 and dead, speedometer sensor; speedometer dead and comes alive at 30mph and up, ABS sensor.
So my last post was about how my speedometer and odometer doesn't work. My brake light and ABS light are both on.

I have since put in a new VSS on the trans. That did not solve my problem. The new VSS came with a new pigtail connector so I looked at the old wires and copper was showing through on multiple spots so i cut em and put the new connector on with wire nuts. (temporary as a test) Still nothing to the speedo or odometer.

I was reading through a older post about how someone had the same exact issues as me and their fix was the ABS speed sensor on the rear axle pumpkin. So I got a new ABS speed sensor and installed it today. Lights are still on and the same problems persist. I went ahead and unplugged the box behind the glove box so I am only dealing with the brake light being on. Who needs ABS anyways.

Next step will be pulling the gauge cluster and looking at the speedo/odo gears which I have been putting off in hopes that there is an easier fix.So are there any other potential avenues to go down with these problems I'm describing before I tear into the dash again?
Have you checked vacuum? The brake light will stay on if there isn't sufficient vacuum for the booster. It will also stay on if the emergency brake pedal is tweaked slightly, I had to bend mine.

I also unplugged my ABS box, the RWAL module on the frame is available from Rock Auto if you want to replace it.
the small white plastic gear set that is replaceable ( if the parts are still available ) are just for the odometer operation. the speedometer itself is run by a small electric motor that is controlled by the signal from the VSS. you do not have to pull the cluster from the dash to remove the speedometer. it pulls out towards the steering wheel when you get enough screws out (four if i remember right). it has to come straight towards you because of the electrical connections. the speedometer can be taken apart. parts of it are very delicate. when i took mine apart, the motor that controls the speedo needle fell apart in my hands. i found a used cluster and swapped parts until i had a working speedometer. you may want to go ahead and pull the cluster and closely inspect everything. the wiring, the connections, the circuit boards etc. if you haven't already done that. you can go ahead and upgrade your dash lights while you are there. i put in LED's. probably not worth taking apart just to do that - mine didn't improve all that much. the factory service manual has some good tips on trouble shooting the gauges and is good for helping to understand how it all works. if your odometer gears have never been replaced they are probably toast - the plastic on the original ones are not very good. make sure you shop around for those gears - some of the prices are outrageous!

There is also a company by the name Mr. Heaterbox that can replace any and all of the gauges in out clusters and replace the ODO gears. He uses a VDO speedo as a replacement. From what i have gathered, they are not cheap but what are your options besides used parts. I plan to send one to him as my speedo is very lazy but the ODO reads perfectly (eliminates the VSS as a problem)...
Wow thanks for all the info! So i’m sure there is a vacuum leak on the booster. I can hear it when i turn the truck off.

And for clarity sakes, when you say the 2nd gens use the ABS for up to 30 mph and then the VSS after, does that imply that the 1st gen doesn’t use the ABS speed sensor at all for the speedo? I am still a noob at all this diesel mechanic stuff
That is correct, the 1st gen soley uses the VSS for speed. The ABS sensor is only used for ABS.
actually disregard. i got a replacement speedo from a scrapyard today. the gears in it turned to powder when i took it apart. I am gonna go ahead and try to find some cheap ones
I found a set of ODO gears on Amazon for $15...
While I have the LMC relay harness, and it works well to date; I don't like the proprietary relays (they are not a typical icecube). The other thing is they expect you to run the passenger's side of the harness across the core support, I lengthened mine to fit the firewall.

It is a worthwhile mod...does make the lights brighter.

Those headlight pigtails are very common as they were used a almost every make and model up till the plastic headlamps. Dorman makes them as well as others. I have a few just in case one burns a terminal. I think I sourced them locally, but Amazon has them.
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La Speedometer Gear Odometer gear set

I bought these off Amazon...
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