M1152 Low Top Speed

[01GRANDER]

Did any of the m1152’s come with really low gearing? I’m having trouble with my new 52, I’m at 3300rpms at 63-65 when multiple other people say theirs is 2700-2900 at 70-75.

I believe I feel lockup, either way, it wouldn’t drop that much. I have shifted from 3rd to 4th to make sure it actually was going into 4th.

My tps plug was corroded but I cleaned it off, no difference. I also cleaned the fuel filter, it was HORRIBLE. I opened up the tank and it actually wasn’t bad, I think the fuel filter was so bad because there was some water and algae formed.

Any ideas? I’m not sure if they had ultra low gearing for the comms trucks, mine was one, tons of wiring originally.

 

[Coug]

All the ECV/REV trucks should be geared 3.08. I haven't heard of lower gearing than that in any military HMMWV, but I suppose it's possible somehow.

Always the chance the wrong gear for the speedometer is in the transmission, or maybe you're getting stray signals to the tachometer causing it to read higher.

I'm sure you've already done it, but verify the your speedometer with a GPS unit or some other method to make sure it's not just a bad reading there.


Doing the math, 1.9 hub gears, 3.08 differential gears, 37" tires, .75 overdrive gear, puts it at 2600 rpms at 65mph
This sounds about right for what everyone else is claiming.

with the same hub gears, rear diff, and tires, in 3rd gear (1.00) it puts you at about 3500 rpms at 65, or 3300 at 62mph.
That sounds a lot closer to what you are saying.

To have 3300 rpms at 65 mph with 37 in tires and 1.9 hub gears would put the differentials at about 3.92:1 gears. I don't see that ratio for sale, so it would either have to be 3.73 or 4.10 if it really does have different gearing. I find this highly unlikely though.


You do mention that you shifted from 3rd to 4th to verify so unless something is really off, I'm back to suspecting either the tach or the speedometer is off. Speedometer is easy to verify, so that makes the tachometer the likely suspect.

 

[01GRANDER]

All the ECV/REV trucks should be geared 3.08. I haven't heard of lower gearing than that in any military HMMWV, but I suppose it's possible somehow.

Always the chance the wrong gear for the speedometer is in the transmission, or maybe you're getting stray signals to the tachometer causing it to read higher.

I'm sure you've already done it, but verify the your speedometer with a GPS unit or some other method to make sure it's not just a bad reading there.


Doing the math, 1.9 hub gears, 3.08 differential gears, 37" tires, .75 overdrive gear, puts it at 2600 rpms at 65mph
This sounds about right for what everyone else is claiming.

with the same hub gears, rear diff, and tires, in 3rd gear (1.00) it puts you at about 3500 rpms at 65, or 3300 at 62mph.
That sounds a lot closer to what you are saying.

To have 3300 rpms at 65 mph with 37 in tires and 1.9 hub gears would put the differentials at about 3.92:1 gears. I don't see that ratio for sale, so it would either have to be 3.73 or 4.10 if it really does have different gearing. I find this highly unlikely though.


You do mention that you shifted from 3rd to 4th to verify so unless something is really off, I'm back to suspecting either the tach or the speedometer is off. Speedometer is easy to verify, so that makes the tachometer the likely suspect.

I should have mentioned, I did follow with gps and it surprisingly the speedometer is correct or almost. I have 40’s and I probably got closer to 65 at 3300rpms.

I’m getting very frustrated, I just got this because I was tired of the limited range of the m998 and I’m not much better off until I figure this out.

I don’t think the tach is off, I’m not an expert but it’s definitely not 2700.

 

[Coug]

okay, so with 40 inch tires if you are in 3rd gear it would put the differential ratios about 3.18 at 3300 rpms. If the tires are actually 39" diameter instead of 40", then it puts it right back at 3.08 where it's supposed to be. 67mph at 3300 rpms would be 39.9" diameter tires back at 3.08.

If you're really in 4th gear you'd be about 4.24 gears, so most likely it would be a 4.10 ringset. Extremely unlikely unless you managed to get a super secret test bed HMMWV somehow, but that's a really long stretch. Anything they'd need that low of gearing for would be better served mounting to a larger truck type.


I'm going to go back to either you being in 3rd gear (because that matches the speed/tires/rpms) or that the tachometer is reading incorrectly.

 

[TOBASH]

You may already know this…

Fast and easy differential check. Chain or chock vehicle securely. Jack up front end. Put transfer case and tranny into neutral and use secure jackstands to maintain safety. Mark a wheel and mark the drive shaft in crayon or chalk with a single line. Spin a wheel and see how many times the drive shaft rotates. look for 3 :1 ratio or higher. If your differential unit spins just three times, you have a weird tranny or transfer case or both.

 

[Action]

What does your truck do with stock tires size? I dont recall you mentioning 40” tires the first time you brought this issue up.

 

[01GRANDER]

What does your truck do with stock tires size? I dont recall you mentioning 40” tires the first time you brought this issue up.

Yes, it does it on the 37’s and 40’s. I gained 2-3mph with the 40’s.

 

[01GRANDER]

What does your truck do with stock tires size? I dont recall you mentioning 40” tires the first time you brought this issue up.

And there are no codes on the transmission.

 

[Guyfang]

All the ECV/REV trucks should be geared 3.08. I haven't heard of lower gearing than that in any military HMMWV, but I suppose it's possible somehow.


look in the parts TM. If the military had the same trucks with different transmissions, it HAS to be in the TM. Having said that, you can not rule out someone at some point, putting something in different. But not the military.

 

[Coug]

All the ECV/REV trucks should be geared 3.08. I haven't heard of lower gearing than that in any military HMMWV, but I suppose it's possible somehow.


look in the parts TM. If the military had the same trucks with different transmissions, it HAS to be in the TM. Having said that, you can not rule out someone at some point, putting something in different. But not the military.

the ECV/REV manual only has one part number each for ring and pinion.

 

[01GRANDER]

the ECV/REV manual only has one part number each for ring and pinion.

Any idea if the tps would cause something like this? Last night I accidentally left it unplugged and I could only go 55mph, once I connected it, I was able to go higher 63-65. Wasn’t sure if adjusting the position would affect rpm’s in a significant way.

 

[Mogman]

Any idea if the tps would cause something like this? Last night I accidentally left it unplugged and I could only go 55mph, once I connected it, I was able to go higher 63-65. Wasn’t sure if adjusting the position would affect rpm’s in a significant way.

If you unplug the TPS the transmission should go into limp mode, I think on the 4L it is second gear, maybe third.
I doubt VERY seriously anyone changed the diff gears, should be 3.08 already the highest ratio installed in the HMMWV (the "street" would call this the lowest gears)

 

[01GRANDER]

If you unplug the TPS the transmission should go into limp mode, I think on the 4L it is second gear, maybe third.
I doubt VERY seriously anyone changed the diff gears, should be 3.08 already the highest ratio installed in the HMMWV (the "street" would call this the lowest gears)

It was still shifting. It would only go up to 3rd gear, no Od.

 

[Coug]

2nd gear with 40 inch tires would max out at 4000 rpms at 55mph. This sounds like where you were at.

What did the RPMs read on your tach?
doh, would be over max.

How about this to check it out.

Put the truck in first gear.
Drive at 25mph.
Your tach should read right about 3k rpms.


Or do second gear, as that's what it defaults to if something is wrong
40 mph in second gear should read 2900 rpms.

If either of the above speeds match the RPMs listed (or relatively close) then you're not shifting into 4th gear when driving.
If the RPMs don't match but read much higher, then it's the tach that's off.


EDIT: and I'll say this again, the odds that you have different gear ratios in the differentials than every other M1152 ever produced are so high as to not even worth discussing anymore. While it is a non-zero probability, you'll have to use a microscope to find the odds.

 

[01GRANDER]

2nd gear with 40 inch tires would max out at 4000 rpms at 55mph. This sounds like where you were at.

What did the RPMs read on your tach?
doh, would be over max.

How about this to check it out.

Put the truck in first gear.
Drive at 25mph.
Your tach should read right about 3k rpms.


Or do second gear, as that's what it defaults to if something is wrong
40 mph in second gear should read 2900 rpms.

If either of the above speeds match the RPMs listed (or relatively close) then you're not shifting into 4th gear when driving.
If the RPMs don't match but read much higher, then it's the tach that's off.


EDIT: and I'll say this again, the odds that you have different gear ratios in the differentials than every other M1152 ever produced are so high as to not even worth discussing anymore. While it is a non-zero probability, you'll have to use a microscope to find the odds.

It’s definitely not in 2nd, I can go 55 and it’s 2700 or 2800 rpm’s. I also can run up to like 45 in 3rd and then shift it to 4th and it definitely changes.

how would I confirm? Because if I just assume it’s wrong, I’ll blow the engine. I don’t think it’s off, it’s screaming at me and I only drive manuals before this, I have a reasonably good grasp of rpms, I use to not shift by looking, just sound.

 

[01GRANDER]

2nd gear with 40 inch tires would max out at 4000 rpms at 55mph. This sounds like where you were at.

What did the RPMs read on your tach?
doh, would be over max.

How about this to check it out.

Put the truck in first gear.
Drive at 25mph.
Your tach should read right about 3k rpms.


Or do second gear, as that's what it defaults to if something is wrong
40 mph in second gear should read 2900 rpms.

If either of the above speeds match the RPMs listed (or relatively close) then you're not shifting into 4th gear when driving.
If the RPMs don't match but read much higher, then it's the tach that's off.


EDIT: and I'll say this again, the odds that you have different gear ratios in the differentials than every other M1152 ever produced are so high as to not even worth discussing anymore. While it is a non-zero probability, you'll have to use a microscope to find the odds.

In 1st gear, I’m hitting 3k at 15mph. I don’t think it’s wrong. I could be mistaken but I’ve had to drive some diesel work trucks and like I said, I’m pretty accurate. How would I verify?

 

[Mogman]

What is the highest RPM that you have seen?

 

[Coug]

In 1st gear, I’m hitting 3k at 15mph. I don’t think it’s wrong. I could be mistaken but I’ve had to drive some diesel work trucks and like I said, I’m pretty accurate. How would I verify?

Now you're talking about a differential gear ratio of about 5.05:1

A work truck will "feel" a different speed than a hmmwv. You're at different heights from the road, different viewing angles, different lots of things.
A work truck built for the civilian market will have a lot different "feel" than a government contract vehicle with no identical parts. Without knowing what work trucks you are driving that are diesel, we don't know if it's a really old pickup with a 6.2, or something with a straight 6.

We can keep going round and round on this all day.

At this point you say you have verified the speedometer reading compared to GPS and are confident that it is accurate enough.
You have 40" tires
You, as far as you know, have a stock M1152 which would have 3.08 rear end.
You have a 4L80e transmission which will have
2.48:1 first
1.48:1 second
1:1 third
0.75:1 fourth/overdrive


Doing the math, you appear to have different gear ratios depending on what speed you are going.

1st gear appears to have 5.05:1

in limp mode you max out at 55mph, which would put the 2nd gear 4k rpms with 3.08:1 (4k is the max no-load RPMs for this motor)
If you WERE in third gear with the TPS disconnected for no lockup, you're talking a 4.56:1 gearing

3rd gear we have no idea because we haven't managed to tell the difference between 3rd and 4th

If you were in 3rd it would be 3.08:1
If in 4th gear appears to be about 4.10:1




The only thing that keeps changing in all of this, is the RPMs aren't matching up with what you expect them to be, and for some reason you aren't shifting into overdrive/4th. If the RPMs are off far enough, you may very well be shifting into 4th and having lockup of the torque converter like you are supposed to, which just leaves the tach reading being off. That is the only thing I can see from the data that all the data would agree with. Anything else and you're talking black magic.

 

[Coug]

So how to verify any of this?

Block the front tires, both in front of and behind, to keep it from rolling anywhere.

Put the transmission into neutral. Can instead put the transfer case in neutral instead, but transmission is easier usually.

Jack up one rear tire of your truck.

Put a mark on the driveshaft as it enters the differential.

Spin the tire until the mark goes around in a full rotation.

Count the number of times you rotated the tire.

Come back on here and post the number of rotations.

My guess is it will be just slightly under 3 rotations. If so, then your differential is definitely 3.08:1, and your tachometer is the issue.
If not, then you might check your local area for exorcists to come fix your truck.

 

[Coug]

This will sound a little rude, and I apologize if it does.

Everything points towards the tachometer giving you a bad reading.


This is the second thread you have started for your issue.

The information we have to go off of is:

You bought a truck with a hacked up wiring harness.

the reading you are seeing on the tachometer is higher than you expect to see for the speed you are going.

You seem to think that because it "feels" like work trucks you sometimes drive, that the tach must be correct, and the issue lies elsewhere.


Have you traced the wires going to the tach to make sure all of them are correct?

Do you even know if it's a military tachometer and not an aftermarket tach the previous owner installed?


Every piece of information you have provided all points towards the tachometer giving a bad reading.