tachometer and engine management

[Backcountry_Ranger]

88 ranger, 2.9, 4x4, XLT, supercab, 270k miles, Sierra Nevada mountains, CA; 95% highway, 5% offroad, lots of snow (except this year)

The tachometer has started jumping around intermittently, up to +/- 1,000 rpm. Tends to happen on uphills, but not always; the tach behaves about 99% of the time. These fluctuations are rapid-- sort of like what you'd see with a failing speedometer cable. At first, it's just the tach needle, but if it continues, the engine begins to lose some power. At it's worst, I need to head back downhill, then all is fine.

Pulled codes (OBDI) KOEO: 67, 87, 95
KOER: none

The KOEO codes are: 67, switch stuff, like neutral switch, clutch switch, AC clutch
87: fuel pump relay
95: thermactor air system or fuel pump monitor

The relay, connector, and wiring were a corroded mess that needed changing out anyway. Done, no improvement.

I visually checked the switches and wiring, but didn't do continuity checks yet. I was particularly interested to see if the neutral switch was well-seated-- it was, and the wiring looked good.

Any ideas? Thanks.

 

[jhammel85]

I've had it happen when I had a TFI module dying out. Your truck hasn't died on youlately right?

I would be driving my truck and the tack would jump wildly for maybe a second or two. 5 mins after that, it would just die and not start until it cooled off. The typical TFI Failure.

So, I guess if I were you, I would check to make sure that connection and all other ignition related electronics are in good order. I know you've already looked but that's just my recommendation.

 

[Backcountry_Ranger]

That's interesting. No, the truck hasn't died on me lately. But-- I've had a lot of TFI issues. The past three summers, I've either had TFI modules fail (checked with resistance tests), or had a breakdown of the conductive grease. This happened each time while climbing 6000' at 100 deg F temp (in the valley). In those cases I didn't notice any tach issues, but the truck would die completely a couple of minutes after it started struggling, and I'd need to wait up to an hour to get cool enough to get it going again.

I'll check and re-grease that module. Thanks, and we'll see what happens.

 

[adsm08]

I agree something is up in your management system.

The tach signal gets routed through the PCM to the gauge. The PCM controls all that other stuff. But the PCM shouldn't act up like that. I'd say go with TFI as well. I usually carry around a spare anyway, just in case.

 

[Backcountry_Ranger]

The TFI connector wasn't seated very well, so maybe . . . I'm going to see what happens on the next drive, then grease and check resistance on the TFI if there is still goofiness.

 

[jhammel85]

Great to hear! I hope thats it...

BTW...Is that original engine mileage on that truck?

 

[Backcountry_Ranger]

It's the original engine, but it was rebuilt with World Products heads at 140k. Uh-oh, I've put about another 140 on it now . . .

 

[Backcountry_Ranger]

Hey, jhammel85 and adsm08, I think you guys were correct on the TFI module. It's been nine days since getting that TFI module connector seated better. No problems since doing that. I think you called it-- thanks.