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1990 K5 Speedo malfunction

I have always heard that you should maintain twisted pairs for the speed sensor. A little section at the splice is no problem, from what I understand.

I have used a drill to twist wires before, I just use a longer length since I end up mashing the insulation on each end.
 
A section as big as that I’d twist. I’d also make sure your splices are good and water tight to avoid corrosion and thereby high resistance that will change your value again.

I usually go with solderless heat shrink style connectors with an added layer of heat shrink tube at least 3/8 to 1/2 “ past the end of the connector on each side.
 
If there’s a chance that it’ll come in contact with or in the vicinity of the exhaust, then use an automotive rated wire since the insulation will have a hire heat (and chemical) rating.
 
rampage said:
If there’s a chance that it’ll come in contact with or in the vicinity of the exhaust, then use an automotive rated wire since the insulation will have a hire heat (and chemical) rating.
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*higher* :doah::haha:

Sorry, guess my edit time expired.
 
I know since I had to add my wiring in, it's not twisted as nice as the factory stuff is, and I've not noticed any problems, but just do what you can.

I'll second using GOOD (not Chinese garbage, they like to copper plate aluminum wire to trick you) automotive grade wire of the proper diameter. Application like this personally I'd probably solder and heat shrink, just due to the size of the wire, but crimps are possible too. When I've got to do multiple wires normally I'll try to offset the splices (cut the wires at different lengths), splice and heat shrink each wire separately, then use a large length of heat shrink tubing to cover all the splices. Extra layers of abrasion resistance are good.

Not sure how well it really works, but some of the heat shrink tubing with heat activated adhessive/sealant hopefully will do a decent job of sealing the wire from the elements.

Glad you found the hacked wiring. That harness running up the frame rail is normally hard to inspect.
 
So I preferer to solder the connections and do the heat shrink approach. I am interested to see if anyone has and different approach on the soldering. Always on these old trucks when I have repaired any wiring, the wire was always oxidized and corroded which makes tough to solder. What I have found best that works for me is I strip the wire back, fan out the conductors and scrap them with sand paper and or razor to get corrosion off.

Anyone have a different approach or technique
 
Justin Fleming said:
So I preferer to solder the connections and do the heat shrink approach. I am interested to see if anyone has and different approach on the soldering. Always on these old trucks when I have repaired any wiring, the wire was always oxidized and corroded which makes tough to solder. What I have found best that works for me is I strip the wire back, fan out the conductors and scrap them with sand paper and or razor to get corrosion off.

Anyone have a different approach or technique
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If your wires are oxidized or corroded internally then you should replace the entire wire.
 
I just cut it back as far as necessary until I get to good wire. In actuality, crimping is better than soldering apparently, But with thin/fine wire it feels to me like it's easier to cut the wire when crimping.

If the wire is bad, you are probably better off replacing as much as necessary. In the past there were a couple places on eBay selling GM color coded wire for not too much.
 
I think the VSS is just a variable reluctance sensor, so you can check it with an ohmmeter (probably a couple hundred ohms?). That also means you can unplug the connector at the DRAC and measure the VSS resistance from there and verify all the wiring is good. If you want to get fancy testing the sensor, set the DMM to AC volts and flip some steel past the tip. I do this on ABS sensors by spinning the wheel.

For wiring, if you don't have enough good wire left to splice, that connector pigtail is probably still available. You can probably also get new terminals and do the whole thing without splices. It's generally quite easy to back the terminals out of the connector with a pick. Speaker wire won't perform a lot worse than twisted pair, but the temp rating is terrible. The function of twisting is to keep the 2 conductors as close to each other as possible and minimize the area of the loop between them. Zip cord wire is somewhat similar.
 
Actually, the purpose of twisting pairs of wires is to eliminate picking up noise. When you have straight sections of wires, they will act as an antenna to receive all kinds of electrical RF signals. When you twist the pairs around each other, the noise gets cancelled out because both sides of the circuit are receiving the same signals and there is no voltage difference between the two legs.
This is why you can run unshielded CAT5 wire through all kinds of electrically noisy environments without getting noise on the lines.
Each pair is twisted many times per inch and that causes it to reject noise. Also why when making up a CAT5 cable a split pair is such a problem.
That occurs when you accidentally use the wires from two different pairs to send a signal down. Since they are not twisted together, there is no shielding effect.

Way back in the old pre-cable days, a common mistake was to run 300 ohm flat lead TV antenna wire down the pole from the antenna straight and flat.
The correct way was to twist it between the insulated stand-offs. If you didn't, and many folks did not because it looked like a neater job, it would act as an antenna which not only picked up noise from motors and what have you, but it would also tend to pick up TV signals that had bounced off objects and were a few microseconds behind what the antenna picked up. So you got all kinds of ghosts and blurred images.
 
Fordum said:
Actually, the purpose of twisting pairs of wires is to eliminate picking up noise. When you have straight sections of wires, they will act as an antenna to receive all kinds of electrical RF signals. When you twist the pairs around each other, the noise gets cancelled out because both sides of the circuit are receiving the same signals and there is no voltage difference between the two legs.
This is why you can run unshielded CAT5 wire through all kinds of electrically noisy environments without getting noise on the lines.
Each pair is twisted many times per inch and that causes it to reject noise. Also why when making up a CAT5 cable a split pair is such a problem.
That occurs when you accidentally use the wires from two different pairs to send a signal down. Since they are not twisted together, there is no shielding effect.

Way back in the old pre-cable days, a common mistake was to run 300 ohm flat lead TV antenna wire down the pole from the antenna straight and flat.
The correct way was to twist it between the insulated stand-offs. If you didn't, and many folks did not because it looked like a neater job, it would act as an antenna which not only picked up noise from motors and what have you, but it would also tend to pick up TV signals that had bounced off objects and were a few microseconds behind what the antenna picked up. So you got all kinds of ghosts and blurred images.
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Actually... if you want to get technical, you should keep it in context. You just compared 100Mbps and VHF to a ~70Hz VSS signal. It's possible the DRAC module has some susceptibility to RF, but it's unlikely to be an issue. In fact, it's already been posted in this thread that the system was working fine with various wiring configurations. At low frequency with the low impedance circuit the larger risk is from magnetic field and this is rejected by minimizing the loop area, as I stated above. For the record, I do recommend twisting these wires, I was just explaining why zip cord was working in the past, not making a generalization.
 
Vehicle speed sensor (VSS) circuit consists of a magnetic induction type sensor, VSS buffer (DRAC) module and wiring. Gear teeth pressed on the outside diameter of the output carrier assembly induce an alternating current in the sensor. On 2WD vehicles, this current is transmitted to a DRAC module where it is passed on to ECM. DRAC compensates for various axle ratios and converts signal to a square wave for use by the speedometer, cruise control and anti-lock brake system.


That's the definition from the service manual.
 
Seems you guys brought this thread back up it reminded me I needed to get back at replacing the damaged wire. So I replaced that section, soldiered in new wire and provided the twist to match existing, however this did not solve the problem. I really want to ring the wires out from sensor connector to drac module to make sure there is not other issues with in the wiring. To be continued......
 
Justin Fleming said:
Seems you guys brought this thread back up it reminded me I needed to get back at replacing the damaged wire. So I replaced that section, soldiered in new wire and provided the twist to match existing, however this did not solve the problem. I really want to ring the wires out from sensor connector to drac module to make sure there is not other issues with in the wiring. To be continued......
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Not sure what you mean by "ring", but you should run a continuity test, and also check resistance end to end. If the question is the wiring, those are the two tests required if the problem is consistent.

On long runs I'll just run a jumper wire to ground on one end of the wire so I don't have to deal with extending the meter leads.
 
dyeager535 said:
Not sure what you mean by "ring", but you should run a continuity test, and also check resistance end to end. If the question is the wiring, those are the two tests required if the problem is consistent.

On long runs I'll just run a jumper wire to ground on one end of the wire so I don't have to deal with extending the meter leads.
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That's an electrician term. Lots of DMMs have an alarm that sounds when the resistance measured is close to 0 ohms.
Guys will "ring a circuit " to check the ohms and to verify no shorts to other wires or the conduit, or ground.
 
Blue85 said:
Actually... if you want to get technical, you should keep it in context. You just compared 100Mbps and VHF to a ~70Hz VSS signal. It's possible the DRAC module has some susceptibility to RF, but it's unlikely to be an issue. In fact, it's already been posted in this thread that the system was working fine with various wiring configurations. At low frequency with the low impedance circuit the larger risk is from magnetic field and this is rejected by minimizing the loop area, as I stated above. For the record, I do recommend twisting these wires, I was just explaining why zip cord was working in the past, not making a generalization.
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I'm just too darn busy doing nothing these days. Its strange, I don't really have any pressing things I need to do, but somehow I'm so busy I barely have time to get things done.........
Need to work on that........

I agree, my examples were not exactly on topic, but I was rushing and those were the first definitive examples I could come up with off the top of my head to illustrate noise and twisting of conductors.
Even so, don't let the low frequency and low impedance lull you into thinking the signal is fairly immune to noise. I have seen noise problems on DC power lines.
Its just that I have spent so much time chasing problems over the years only to discover it was a bad shield or twisted pair that I tend to err on the side of caution.
My rule of thumb, is that if you have to replace any kind of cable or wire, make sure the replacement has the same level of noise rejection as the original.
If its shielded, replace with shielded. If its twisted, make sure the replacement is too.

As for it working the way it is, you just never know. I have seen what seemed to be a perfect cable repair that just did not work and the whole cable had to be replaced, to rigged up systems that could not possibly work that did just fine.
The most extreme example of that I can remember was a friend many years ago that asked if I could "splice in" a short piece of RG8 to his antenna feed. He needed about 2 more feet. I told him I could do it with a couple of PL259s and and a SO259 barrel connector. All new cable would be better, but that would work.
When I walked up, I was a little stunned to see a piece of heavy zip type cord spliced between two sections of RG8.
But, after I thought about it, I figured it would pass enough signal for him to hear calls.
I made the comment it was a lucky thing he did not try to transmit that way, and he said he had been talking on it for over a month, but he just did not seem to have the range he used to have..........
Before I left, I took a rag and polished up his transceiver really well. He asked why, I told him that it deserved some TLC after surviving that kind of abuse. Still don't know how the final survived.

Darn phone is ringing "off the hook", wonder how long that saying will survive, so I gotta go, but I just had a thought. Alternator whine on the power cable of a car radio. Possible example of noise on a low frequency (DC) cable. Of course, doesn't have much to do with twisted pairs though....
 
Ever end up fixing this? My Speedo just went out on my ‘90 k5 and I’m just gonna order the vss to replace before trying the speedo cable.
 
So following back up here. when I had taken out the bad wiring and I had drove the truck the speedo was no longer bouncing around on the dash at all, it was just simply not working. Then once I repaired the bad wiring the speedo was bouncing around again randomly as it was from my original issue that I was trouble shooting. As of today my new sensor arrived and I installed the unit in the bitter cold/snow because I really wanted to close this issue out. That being said the new sensor has solved the problem and the speedo is working normal once again.

my scenario appeared to be bad wiring and bad sensor. One wonders if the bad wiring caused the sensor to fail or I just happen to have two issues rearing there head at the same time.
 
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