HMMWV A2 - Transmission/Tachometer Issues

[MaverickH1]

I'm hoping these issues link to the same root cause, so I'm grouping them together. 6.5L GEP engine, 4L80E transmission.

I have a tachometer that SOMETIMES acts like this as well as sometimes just pegging all the way at full RPM even though the engine is clearly not at that speed:

https://youtu.be/Ujm57lZa9TI

Also, the transmission randomly downshifts for a split second. This started probably 2 months ago. The most consistent time it would happen was around cruising throttle at 45 mph. It seemed like a computer couldn't decide which gear to be in. RPMs would jump slightly for a split second like it shifted down and then back up immediately or something. It almost sounded like a slip. But slips should happen under load. It never slips under load. Transmission fluid was inspected, it smelled a little burned and was a little high, so I drained it and changed it and things seemed to be better for about a week. But it was a problem that would only happen sometimes anyway.

It came back (or perhaps never went away), and I decided to just live with it because it didn't seem damaging. I started buying the supplies to completely change the fluid and the filter.

Then yesterday it felt like the truck was stumbling a little bit under load. The engine sounds healthy, but you could feel it not having a consistent acceleration. Every time the tranmission has a problem, the tachometer is acting up like the above video.

Also in the last few days, it developed a new problem where it would act like it was in 2nd gear while you were at low RPM in a parking lot or something low speed, and it would all of a sudden take off as if the throttle position sensor gave a high reading, at the same "cruising" throttle position it would drop into a lower gear and the engine would throttle up. It would jump forward a little tiny bit, not enough to be a safety concern but enough to scare you.

In any case... I started trying to find out what would be going on this morning, as the truck is parked until this is more understood. It appears I wired my tachometer to a Mechanical Tachometer on the back side of the engine as the directions indicated. The assumption is that this mechanical tachometer gets its engine speed signal from a mechanical attachment to the engine itself, and then creates an electrical signal that I'm tapping into with the in-cab Tachometer. But I'm not seeing where that electrical signal goes or how it is used.

I *THINK* the Transmission Control Module utilizes a throttle position sensor as well as an engine speed sensor to perform its calculations and choose the gear accordingly, as well as the A2 brake position sensor factoring in somehow. Is the mechanical tachometer not used at all for this? The 6.5L engine parts list shows a dedicated Engine Speed Sensor near the front/left part of the engine. I'm assuming that's the actual signal to the TCM. I also assume that the mechanical tachometer is purely used to go to the diagnostic connector next to the transfer case selector, and is actually not used at all during normal operation.

Suggestions/expertise welcome. My gut feeling is it's something like these possibilities:

1) Throttle position sensor or engine speed sensor failing. Mechanical Tachometer signal does not go to the TCM, and the In-Cab Tachometer I have is just a bad unit, or the wiring isn't as solid as I'd like.
2) Mechanical Tach IS used by the TCM, and it's failing. Causing both the In-Cab Tach issue and the transmission issue.
3) TCM failing.
4) Transmission already on its way out. Perhaps a bad rebuilt unit.

The truck is at my shop where I'm trying to start up a WaterJet cutting business, so I don't have it here with me to look at right now. Just me and the TMs.

 

[MaverickH1]

I'm back at the shop today, so I'll be looking into this issue a bit and sharing findings.

 

[MaverickH1]

Tachometer issue - I didn't remove the radio shelf and the dog house to look into the tachometer connections. Wiring diagrams confirm that the Mechanical Tach wires only feed to the STE/ICE port by the transfer case selector. I might hook up the RPM function of my voltmeter to that and leave it on in the truck while driving. If the Mechanical Tach sender unit gives steady RPMs but my In-Cab Tachometer gives crazy ones, I'll just check connections. If the connections are good then I'll replace the In-Cab Tach. Which I hate to do because I wanted to keep a track of the hours on the engine without having to do silly math. Maybe I can trick the tach into thinking the engine is running and reinstate the run hours...

Transmission shifting - The Engine Speed Sensor located at the front/bottom/left of the engine tested fine for continuity and the wires to the TCM are all in good shape. Nothing appears to be wrong with the speed sensor. The Throttle Position Sensor located on the fuel injection pump area tests fine for continuity on all wires to the TCM, but I don't have the specialty jumper cable or specific gage block to check its adjustment. At the moment, I suspect it needs to be adjusted. Parts are ordered to do this at a later date.

I will change the trans fluid and try to do a complete flush of the system and add a new filter. I believe I'm running Dexron VI in the two fluid changes I've already done. Research leads me to believe Dex VI was developed to be lighter and give better fuel economy numbers in newer GM vehicles. It is not suited well for HMMWV applications due to increasing wear. I will be replacing it with (I THINK) full synthetic Dex III which is what the HMMWV was designed for. I've been averaging somewhere between 12-13 mpg consistently up to this point. We'll see if running Dex III changes that. This truck only has 6,000 miles on it at this point (supposedly).

 

[papakb]

I'm still old school with a 6.2 and a TH400(3L80) but think your problem might be due to one of the sensors for the 4L80E. That trans has a TIS (trans input sensor) and a TOS (trans output sensor) that feed the TCM. If any one of those 3 components (or the associated wiring) was failing I think you'd end up with the symptoms your listing. I'm not sure which one feeds the tach but the sensor on the back of the engine that's driven by the oil pump drive is normally only used to provide a diagnostic signal to the STE/ICE.

 

[MaverickH1]

I'm still old school with a 6.2 and a TH400(3L80) but think your problem might be due to one of the sensors for the 4L80E. That trans has a TIS (trans input sensor) and a TOS (trans output sensor) that feed the TCM. If any one of those 3 components (or the associated wiring) was failing I think you'd end up with the symptoms your listing. I'm not sure which one feeds the tach but the sensor on the back of the engine that's driven by the oil pump drive is normally only used to provide a diagnostic signal to the STE/ICE.

We're on the same thought track. Thanks for being assuring!

Confirmed on the STE/ICE. I thought I was going to have to take it for a drive, but it turned out that it gave me the issue right in Park. Using the RPM setting on my voltmeter, it gave consistent numbers at the STE/ICE while the In-Cab Tachometer jumped around like crazy. So I've confirmed either a bad In-Cab Tach after only 200 hours of operation or a bad connection.

A note about using a voltmeter for measuring the RPM... mine has an RPM position for 8 cylinder, 6 cylinder, or 4 cylinder. The idle speed of the Humvee is ~700 rpm, and the tach reads nice and smooth at that speed. On the 8 cylinder setting, the voltmeter was reading about 120 RPMs or something at idle. Switching to 4 cylinder gave me right at 350 RPMs. When I was reading it at the 4 cylinder selection, it seemed like I just have to multiply by two. The key is that when I throttled up to ~2,000 RPM on the In-Cab Tach, the needle jumped around like crazy in the above video, but it read a solid number on the voltmeter varying by only +/- 10 RPM.

I've also ordered the parts to properly adjust/check the Throttle Position Sensor.

I will be changing fluids and the filter tomorrow also.