Route of wires from VSS to speedo head?

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Route of wires from VSS to speedo head?

Postby Richie O » Tue Apr 05, 2011 7:16 pm

Mark, this may be up your alley. My 92 has the " some times " works speedo. When it has acted up I have wiggled, unplugged and frigged with the VSS connection. That does not correct the issue. I would say the speedo works about 3/4 of the time. Where does the wire harness route go that has that wire in it? I know it goes from VSS to PCM, but what group is it? I am thinking its a bad connection somewhere that will sometimes loose connection. I don't want to go fishing around and frig up other things while trying to fix this one. What say you?
1989 W250 727, 3.07 L/S, S300, P/S Intercooler, Stans exaust, Pump adjustments, 127k miles,297 hp
1993 W250 extended cab, rag, 4.10 l/s, 6x16's, HTT 62/71/14 piston l/p, Isspro EV series tach, fuel pressure, boost, oil pres, water temp, volt, pryo, 132k/ 301 hp
1992 W250 with NV4500, 3.54's, 16cm 60mm GDS H1C, ground stock cone, Isspro tach, pryo, boost, fuel pressure, slow, rusty, dented,180k
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Re: Route of wires from VSS to speedo head?

Postby Mark Nixon » Tue Apr 05, 2011 9:43 pm

From the VSS, the white and orange wire (VSS signal) runs to the GRAY plug connector near the wiper motor, which then plugs into the main engine harness.

IIRC, the speedo wiring has a 3 point splice in the main engine bay harness.
1 arm of the splice goes towards the ECM, another goes to the main bulkhead connector, while the 3rd goes back to the gray connector I mentioned earlier.
The wires to the ECM and the one up to the speedo head are both white with an orange tracer, as mentioned.

The ECM connector number for the speedo wire there is #47 and is the bottom row, 6th spot FROM the right, with the ECM in the fender.
This is looking at ECM from the passenger's side.

From the bulkhead connector on the ENGINE side of the bulkhead connector, going into the cab, the white with orange tracer wire is located on the 2nd ROW down, 2nd wire FROM the right.
For reference, there is a large PINK wire at the far right top row, the one you want is 1 row down, 2 to the left.

I would put my suspicions more towards the harness that runs from the VSS to the Gray connector, which incorporates the O/D wiring, 4wd and the NSS wiring, as well.
This harness is one that is subjected to the harshest conditions of weather, invasive repair work (often moved and abused during repairs) and just vibrations in general.

The other possibility is like what I am fighting right now in the Red/White flatbed.
I have intermittent ODOMETER operation, sometimes it counts miles right off in the mornings, other times it takes 10 or 15 miles to work, yet the SPEED part of it works just fine.
It's like it has to "warm up", then it's fine for the whole time I'm driving it.

I suspect it's an age-loosened connection in the "motherboard" of the speedo and this could easily be the case in yours, too.

Mark.
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Re: Route of wires from VSS to speedo head?

Postby Richie O » Wed Apr 06, 2011 6:39 am

Thanks Mark. A link to this is going in the sticky section. ;) I listed it for a 92 and 93 truck in the sticky area. Is that correct?? I thought in your thread with Andrew the early trucks were different. Thanks again.
1989 W250 727, 3.07 L/S, S300, P/S Intercooler, Stans exaust, Pump adjustments, 127k miles,297 hp
1993 W250 extended cab, rag, 4.10 l/s, 6x16's, HTT 62/71/14 piston l/p, Isspro EV series tach, fuel pressure, boost, oil pres, water temp, volt, pryo, 132k/ 301 hp
1992 W250 with NV4500, 3.54's, 16cm 60mm GDS H1C, ground stock cone, Isspro tach, pryo, boost, fuel pressure, slow, rusty, dented,180k
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Re: Route of wires from VSS to speedo head?

Postby Mark Nixon » Wed Apr 06, 2011 7:24 am

Richie,
Does your truck have cruise and does it work even when the speedo acts up?
If it works, then it's likely not a pre-bulkhead speedo wiring issue.

Mark.
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Re: Route of wires from VSS to speedo head?

Postby Mark Nixon » Wed Apr 06, 2011 7:28 am

As far as differences...

In actuality, the layout of the SPEEDO CIRCUIT is similar for '91.5 and the Non-Intercooled '90 and '91 trucks.
There's still 2 pathways from the VSS, 1 to the grid timer for shutting down the pre-heaters after a pre-set speed, the other to the speedo.
I have not investigated the 1989 wiring as much as the later years, but it is largely on it's own due to the speedometer being cable-driven and I know the transmission harness plug near the heater motor is TOTALLY different in appearance than the '90-early '91 version.

The wiring and it's layout is identical between 1992 and 1993.
I've been over the wiring for both years and I have not found any difference in the engine, transmission (auto or manual, 4wd or 2wd, T/T sensor in auto), or taillight harness.
The only thing that is different is the part numbers on the ECMs and they will still interchange between the 2 years with no ill results.

1991.5 definitely has a unique ECM, engine and transmission harnesses.
There are similarities in the engine harness in the ICed '91, with the '92-'93 versions, but some items won't work properly with 1991 harnesses connected to '92- '93 harnesses.
BURNOUT is not a possibilty if the 2 somehow get interconnected, it's just that some items won't work.

The 1990 and 1991 Non-ICed engine and transmission harnesses both are identical and will play well with each other.
The dash harness is different between the 2, due to column design.
Personally, if I had a '90 truck and the column shot craps, I'd swap in a '91 column and dash harness.

The dash harnesses between '91 Non-ICed and Intercooled are the same general layout between each other.

Mark.
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Re: Route of wires from VSS to speedo head?

Postby Richie O » Wed Apr 06, 2011 8:03 am

Mark Nixon wrote:Richie,
Does your truck have cruise and does it work even when the speedo acts up?
If it works, then it's likely not a pre-bulkhead speedo wiring issue.

Mark.


No cruise on that truck. She's a stripper.
1989 W250 727, 3.07 L/S, S300, P/S Intercooler, Stans exaust, Pump adjustments, 127k miles,297 hp
1993 W250 extended cab, rag, 4.10 l/s, 6x16's, HTT 62/71/14 piston l/p, Isspro EV series tach, fuel pressure, boost, oil pres, water temp, volt, pryo, 132k/ 301 hp
1992 W250 with NV4500, 3.54's, 16cm 60mm GDS H1C, ground stock cone, Isspro tach, pryo, boost, fuel pressure, slow, rusty, dented,180k
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Re: Route of wires from VSS to speedo head?

Postby PToombs » Wed Apr 06, 2011 5:29 pm

Oh boy. What is it with you and those strippers? :roll: Spend some real money and get a nice piece! :mrgreen:
pete

Just enough power to break everything behind the crankshaft.
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Re: Route of wires from VSS to speedo head?

Postby Mark Nixon » Wed Apr 06, 2011 6:22 pm

PToombs wrote:Oh boy. What is it with you and those strippers? :roll: Spend some real money and get a nice piece! :mrgreen:


Huh.:roll:
Used to be you could get a nice "piece" for $100, now, it's $200 for a low quality one and you're likely to get some undesired "extra" things in on the "deal". :?

OH, WAIT! :shock:
You're talking about a TRUCK, aren't you Pete? :lol:

I shoulda' stayed outside and put more scrap Dodge junk in the truck. :twisted:

Mark.
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Re: Route of wires from VSS to speedo head?

Postby PToombs » Thu Apr 07, 2011 6:28 pm

I was talking about the truck! Why do you guys always think I'm talking about smut? :roll:


Oh yeah, I usually am. :mrgreen:
pete

Just enough power to break everything behind the crankshaft.
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