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87 YJ won't go over 55 mph

87 YJ won't go over 55 mph
cdlongjr

cdlongjr

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Location
Newark, De
Vehicle(s)
1983 Jeep CJ-5, 1987 Wrangler YJ, 1974 J-10
My YJ has the AMC 258 i6 / 4.2l with a B&M TH-350 auto Transmission . As some are aware, she'll only reach 2100 rpm under a load. She's very slow on accelleration. She'll rev over 4K in park. I have 32" tires on aluminum wheels, & a 4" suspension lift. I had a guy tell me the other day it may be because of my gearing. Does this make sense? If so, would someone please explain to me in plain english what this guy is talking about? Engine acts like it's governed, only reaching 2100 rpm.
 
Yesterday I was talking with a 70's muscle car nut. He thinks the problem is in the B&M auto tranny. Either the valve body is set wrong or the converter is slipping. Make sense to anyone?
 
I guess that could be, what are your RPM's at 55? What is your tire size and gear ratio?
 
2100 rpms @ 55 mph. 32" tires.

Nutter by-pass has not been done, unless it was done under the dash. Removed wires from the entire loom. Orange & purple from the dist are twisted together the entire lenght of the harness. Orange wire from the ign module into the female side of the plug looks like it changes to a lime green w/ black tracer @ the male side of the plug. Is the lime green wire the one I should cut & tie the orange wire from the dist to?

Rear is open. I jacked up the driver's side, marked the tire & u joint. Rotated the 32" tire 2 revolutions, (like I was told to do one the 5). The u joint turned 3 times, 3:07 gears?

I noticed a popping sound coming out the exhaust pipe while I was checking rotation of the driveshaft.
 
Last edited:
cdlongjr said:
2100 rpms @ 55 mph. 32" tires.

Nutter by-pass has not been done, unless it was done under the dash. Removed wires from the entire loom. Orange & purple from the dist are twisted together the entire lenght of the harness. Orange wire from the ign module into the female side of the plug looks like it changes to a lime green w/ black tracer @ the male side of the plug. Is the lime green wire the one I should cut & tie the orange wire from the dist to?

Rear is open. I jacked up the driver's side, marked the tire & u joint. Rotated the 32" tire 2 revolutions, (like I was told to do one the 5). The u joint turned 3 times, 3:07 gears?

I noticed a popping sound coming out the exhaust pipe while I was checking rotation of the driveshaft.
Click to expand...

My dad has a 86 GMC surburban with a brand new 350 but the tranny is Auto 4 speed with Overdrive the TH700R i think with 140xxx miles on the tranny and the only time that acts up is at 55-60 mph and it will still cruise to 80 but my neighbor(retired mechanic) thinks the torque converter is going out, but it is not causing the engine or RPMS to stay at a certain rpm or speed.
 
PO put the tranny in. B&M can't tell me anything. They say it's not theirs. Tag riveted to case says:
B&M Transmissions
Chatsworth, California
Serial # 26888
Skid plate is in the way to read the numbers in the 2 circles on the driver's side of the case. Between the 2 circles is the number 2 over the capital letter T
2
T
 
Does this thing have some wires going to the Transmission from the dissy? If it dosnt than it dosnt have a lock up torque converter. That means that it has a 1:1 third gear or drive.

What does it do when you do a manual up shift start at first than go to second that drive? Will the rpms go to 3k in first and second?
 
At 55 mph with a 3.08 gear, 32" tires and a 1:1 final ratio you should be running about 1800 rpm (+- 10%). -->> Engine RPM Calculator

If your running 2100 rpm - it sounds like the converter is slipping - but I feel you should still be able to reach a higher MPH... Does the engine bog?
 
Yes, the engine bogs. Acceleration is very slow. MC2100 has 121 ventris. I swapped the MC2100 off the J-10 & it made no difference. Sometimes the engine tries to die then I have to pump the accelerator to get the engine to come back to life. Sometimes she just dies. Will pick up some speed down hill. Tranny fluid isn't burnt. Nothing on Transmission except the serial number.
Where should the wires from the tranny be coming from? There are a couple of wires, but I think they are back-up lites & neutral start,(which doesn't work). Manual upshift- 1st-2100 rpms. 2nd-3000 rpms, D-2100 rpms.
I did find out that the lime green wire w/ black tracer is the "orange wire" to do the Nutter by-pass. It's a very small wire so I think I'll use a 16 ga wire in it's place.
 
It could be the Torque converter slipping. It also could be the bands are out of adjustment and allowing the clutch pacts to slip. It has been a long time since I had a 350 Transmission apart.
 
If it was the tranny, wouldn't the fluid smell burnt? The fluid does not smell burnt.
 
She's running. ECU is bad. Found the correct wire colors & did the Nutter by-pass, then power timed the engine.
 
Took forever. So much for taking her to a repair shop.
 
Glad to hear you got her runen good.:chug:
 
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