I've got a '90 Legacy that the tach needle will drop to zero and the engine will start to miss at the same time. It's kind of like the worn distributor bushing action on the old EA81/82 series, but obviously a Legacy doesn't have a distributor. It only lasts for a couple of seconds, then goes away. Does anybody have any ideas what the problem might be? I'm thinking maybe a bad connection or possibly a bare wire going to the tach. Can someone explain how the tach wiring ties into the ignition system? I'm wide open for a cure here, folks.

possibly an IAC valve sticking.

I assume all other electrical items are OK when this happens? If not it could be an alternator, ground or other more general issue.

might be your coil pack or the ignitor thinger for the coil pack.

Legacy 777, ya got me, what's an IAC valve, and how do I check it? Texan, yes, it only effects the engine. Hondasucks, how do I check the coil pack and ignitor thinger? By the way, the Check Engine Light almost never comes on, so I can't use that to help track down the problem. Thanks for your input.

Idle air control valve.

 

I don't have the testing procedure in front of me right now, but you can test it.

How can the idle air control valve effect the tach? Just so we're both clear, the engine RPMs don't actually drop significantly. The tach needle drops clear to zero, and the engine develops a slight miss. Please explain the connection between the idle air control valve and the tach. I'm not doubting your word, just trying to understand how it works. Thanks.

does the tach bouncing around coincide with the actual engine rpms?

No, it doesn't. When the car acts up, the tach needle drops right to zero, just like you shut the engine off, and the engine will start missing, but it doesn't quit running. For example, I can be cruising down the road at 55 mph, and when the tach/engine does it's thing, the engine will start a miss I can hear, but it will still hold 55 mph, and the tach needle stays at zero. So it's not a real bad miss, it just loses a little power, and it comes and goes, only lasting for a few seconds at a time. That's about the best I can describe it in writing!

 

Just had a thought: does the crank position sensor provide the signal for the tach? I just had to replace one of those on my '95 Legacy. The '90 does start hard on occasion, too.

 

Thanks again.

Sounds like both the cam and crank -shaft sensors give up simultaneously. They provide the ECU with the same info, namely engine RPM, so if one fails it will be ignored and usually trigger the Check Engine light.

 

Having dismantled my dashboard a number of times due to blown bulbs, I am quite sure that the tach runs off info from the ECU.

The tach on the combination meter comes from ECU. There's a lead that is the "engine tachometer output"

 

Assuming the combination meter is good, the wiring between the combination meter and ECU are good, and the ECU is not on the fritz, yeah I'd probably suspect the crank & cam sensors like setright said.

I was talking to my daughter some more (it's her car) about the symptoms. She says the tach drops to zero, the gas gauge drops to empty, the check engine light comes on, and the engine starts missing (whew!). After a few seconds, it will straighten up, and everything goes back to normal. That sounds like a ground problem to me, rather than cam/crank sensors. The most obvious would be the negative battery cable, I'll check it out as soon as I can. Could there be a bad ground connection to the dash? If so, where would I look in the wiring for that? I'm having a tough time relating the engine miss to the dash problems. Maybe a bad ground wire going to the ECU? Do you have any other thoughts?

 

Thanks again.

OK, I cleaned the negative terminal and where the ground wire hooks to the engine and body, made no difference. Legacy777, you suggested possibly the ECU itself, and the more I work on this, the more I'm inclined to agree. Is there any way to test the ECU? I hate to buy a new one and find out that's not it!

ummm.....not really.....you might want to try and find a used one at a junkyard with a return policy that if it doesn't fix your problem, you can return it and get a full refund.

Since the problem triggers a CEL according to your daughter, the fault code is now stored in the computer's memory. Might be the time to have someoche check what it is...

Since you stated the gas gauge goes down to empty also when this happens it may be a power supply problem. I wouldn't think the ECU is in the circuit for the gas gauge but I can't say that for sure. It could be though that they have a common power connection somehow. Perhaps the power to the fuel pump is causing this to happen.

but I would start with the battery connections and then go to the ignition switch.

The switch controls those circuits and is a wearing item.

Does anything trigger it like a bump?

I was checking it out some more this morning. I found that when the tach drops to zero, a sharp blow to the dash turns it back on again. Tells me there's a bad connection in the dash somewhere. Can you guys tell me how to take the gauge cluster out, or whatever I have to do to check the wires behind the dash?

This problem may be caused by a failing power relay contact. I don't have a manual to help with this sorry to say. You may be able to to pin this down by taping lightly on suspected areas with a screwdriver handle while looking for gliches in the operation. I would purchase a service manual for the car that has some good wiring prints. A good service manual is "priceless" as they say.