Tach Erratic or Dead at Times

Travis Mobile Technician Wisconsin Posted   Latest   Edited  
Case Study
Network Communications
Tooling
2002 Chevrolet Trailblazer EXT LT 4.2L (S LL8) 4-spd (4L60-E)
Tach Erratic

Just a case study that shows the value of an OBD2 Splitter and testing a bit further before pulling the trigger. 

Called to diagnose this 02 Trailblazer for an intermittent no start and erratic cluster. Prescan showed many network codes in the PCM, ICM, BCM, and SIR modules. I could only duplicate the erratic tach so I went after that first. The RPM signal data PID in the PCM looked good but the RPM data PID in the ICM matched what the tach was doing. I hooked up to scan tools to monitor both modules at the same time just to confirm 100%. At this point it would have been easy to conclude I have a bad ICM causing the Tach issue and also at times pulling down the network and causing the no crank no start. 

I decided to scope the engine RPM signal into the ICM which ended up being erratic as well so my next step was to test the signal the the PCM. Just before the PCM I saw where an aftermarket remote start system tapped into the RPM signal with a quick connect. I decided to test my signal there and while handling the connector the wire broke. Problem solved. The vehicle had a weak battery so the shop replaced that and will touch base if the no crank condition resurfaces. 

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Helpful
Interesting
Glenn Owner/Technician
Texas
Glenn
 

Hi Travis, Just to clarify your comment, “Problem solved”. Did you repair the wire then start the vehicle and retest to verify? This one kind of sounds like a cliff hanger. As for aftermarket alarms, I typically advise my customers to have me remove them if the vehicle is a few years old or not a high theft model. Most are a money-making gimmick anyway Whoever invented crimp and tap splices…

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Dmitriy Analyst
Ontario
Dmitriy
 

Glenn, what do you think about this splice? diag​.​net/file/f20m1o1ql… ​

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Bill Owner/Technician
Michigan
Bill
 

Dmitriy that splice appears very dependable. Can't say I have ever seen this exact method. Might it be from the airline industry?

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Glenn Owner/Technician
Texas
Glenn
 

Hi Dmitriy, That is what I would call a textbook splice. In addition to wrapping the 3rd wire in, I also prefer to solder the splice, then double shrink tape. Unfortunately, people seldom use that method these days unless it involved detailed restoration work. Tap splice have been the normal method for many people. I don't use them! Anything that cuts into wires and breaks the strands to…

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Tony Owner/Technician
Missouri
Tony
 

I would tin the splice with a small amount of solder and wrap the companion wire next to the original wire for a couple inches. If the harness wire can be depinned from a nearby connector with reasonable effort I will do so to slide a shrink tube down both wires after splicing. I do not make that stress relief loop. There's usually precious little room in/around auto harnesses. For stress relief…

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Dmitriy Analyst
Ontario
Dmitriy
 

Glenn, Bill, and Tony — thank you for your comments and tips.

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Travis Mobile Technician
Wisconsin
Travis
   

Yes, I verify all of my work. Here is the scope capture after the repair and before I heat shrunk the connector. I was just having the shop get back to me on the intermittent no start that I was not able to replicate and which may have been due to the poor battery.

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Travis Mobile Technician
Wisconsin
Travis
 

I find this to be one of the best splices.

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Funny
Glenn Owner/Technician
Texas
Glenn
 

Hi Travis, That is a sure way to get direct power, but I would be concerned about moisture creating corrosion and dissimilar metals reacting. Do you coat or wrap a connection like that with anything?

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Travis Mobile Technician
Wisconsin
Travis
 

That was just a joke. I am not sure how well a screw would remain tight threaded into wire.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Travis, I've seen some wild DIY stereo installs around my area. The other ones I see are the Kawaski mules or John Deere gators loaded up with extra ‘patchworked’ electrical options and addons, then the owner wonders why the charging system burns up.

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Glenn Owner/Technician
Texas
Glenn
 

What brand OBD splitter did you use? Do you have a purchasing web link for it? I have had a couple of times where this adapter would have been handy. Have you ever used it with a DLC BOB as well?

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Travis Mobile Technician
Wisconsin
Travis
 

Here is the OBD Splitter link for you Glen. - amzn​.​to/3oNTEXb . $11 on Amazon, hard to beat that. Feels pretty well built especially for the price.

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Thanks
Glenn Owner/Technician
Texas
Glenn
 

Thanks Travis, I am curious to see how my Snap On Verus and Autel compare, this can also work as a quick breakout pin check device.

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Raymond Mobile Technician
Michigan
Raymond
 

Hi Travis, I would like to know part number, name, and where to buy splitter please. Question, does this splitter and running two scan tools at the same time interfere with data and operation of both scan tools or vehicle network? Thanx for sharing diagnostic info

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Travis Mobile Technician
Wisconsin
Travis
 

Raymond, I posted the link for the splitter to Glen's post but will post for you as well. In my experience, yes they can effect each other especially if you are looking at data in the same module. Data starts to lag or freeze. amzn​.​to/3oNTEXb

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Raymond Mobile Technician
Michigan
Raymond
 

Thanx, just ordered it, yes I will be mindful of data speed and modules thanx for heads up

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Doug Owner/Technician
California
Doug
 

But the excellent Travis I was wondering if that was even possible. Just purchased one myself! Wanted to compare scan tool side-by-side for quite a while.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Doug, I had to do a little more reading on this subject and as it turns out the typical vehicle computer will only allow one scanner to prioritize functions at a time. A 16 pin DLC is not like a USB for a typical nonautomotive computer. Some of these splitters are simple plug-in power taps utilizing only the power and ground pins to power user devices. (They are handy for vehicles without a…

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Doug Owner/Technician
California
Doug
 

Hey Glenn, I guess we'll find out huh.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Doug, I am going to experiment on a couple of old running Junker's before I try that on a regular customers driver vehicle, or one of my own. I had a bad experience a couple of years ago on my 95' Ford F350 7.3 when I connected my Autel MS906BT to it. The information was garbled and just would not work with that vehicle. Has I learned later, the early 95' diesel models for Ford were not…

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Interesting
Travis Mobile Technician
Wisconsin
Travis
 

Those Powerstrokes I believe you always had to be KOEO when starting communication. I had one lock up once and became a no start but a battery disconnect “reset” it.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Travis, The early ones could be very tricky. I have seen many people jump the gun throw IDM's at the problems and not resolve anything. I actually went to the trouble of connecting my Ford Rotunda breakout box and pin tested all the inputs and outputs to prove out the PCM was indeed at fault.

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Tony Owner/Technician
Missouri
Tony
 

Many years ago Matt Ragsdale mentioned hooking up two scanners with a splitter on iatn. I've never tried it but believed it works because he said so :-). The value of this is questionable but it might be possible to activate two controls at once, say two evap solenoids, on a platform that doesn't allow for this normally. Techstream has had multi module data display capability for years, I assume…

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Travis Mobile Technician
Wisconsin
Travis
 

In my video attached below, you can see both scanners reading data :)

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Travis Mobile Technician
Wisconsin
Travis
 

With the one I linked, yes. Both of my scanners will work at the same time. I did notice delays in data especially when both were connected to the same module.

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Lee Owner/Technician
Missouri
Lee
 

Just curious Travis what tablet are you using to run the EScope? I have the ATS EScope also but bought the complete setup from them. Yours looks more streamlined.

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Travis Mobile Technician
Wisconsin
Travis
 

In this case study I was using my Dell Latitude Tablet. I use a surface pro as well.

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Thanks
Geoff Diagnostician
Hawaii
Geoff
 

I'm shocked to see that a 2002 GM has dedicated RPM, and VSS, signal lines to the IPC, instead of bussing that info on Class 2.

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Travis Mobile Technician
Wisconsin
Travis
 

You and me both. Another reason I suspected the IPC before I looked at the wire diagram.

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