Electrical Problem Immitates Crank Sensor Going Out

[Black91SHO]

1991 MTX SHO - 112K

Symptoms: While driving, suddenly (and randomly), the engine drops out, like it's been turned off and right back on briefly, or like someone's cut the power for a split second, just like when the crank sensor's going out. One oddity is that the low coolant light will stay on or go off regardless of the overflow tank level, and sometimes, I've noticed that the low coolant light will come on, the right afterward, the engine drops out. The low coolant light is always on when the engine drops out.

Likely Cause: The previous owner put a Gen 2 starter on the car, which means he had one rewire part of the harness to skip the starter relay. Last year, the shoddy electrical splicing began to short and caused some things to go out, like the starter and other little things. We replaced the starter and fixed the shoddy wiring so it wouldn't short anymore (kept the Gen 2 starter setup). Worked fine for a while, but eventually started having little issues again here and there, like one of the seat motors going out and this crank sensor problem. I unplugged the seat motors, and replaced the crank sensor. The problem persists.

It's not throwing any codes, but I plan to check the codes again today to make sure none have popped up.

Any ideas what could cause this or where the short might be?

- Brian

 

[Devin]

Thinking it could be thermal expansion. Are there any commonalities between incidents?

 

[Black91SHO]

It's usually while driving. I think one time I might have seen it happen while the car was sitting still, idling, but it happens so fast, it's hard to spot if you're just idling. It's a quick drop of idle then back again along with maybe the low coolant light coming on or another light on the dash, but just for a second. Not sure which lights come on when it happens. But I sat idling for a long time today and never saw it happen.

It doesn't happen at any particular RPM, just while driving. Doesn't happen that often either, usually. I drove about an hour today and it didn't happen once that I can remember. I could be out driving and it might happen two or three times in 10 minutes, or not happen at all. The power just drops suddenly, like the car's been turned off then right back on again. You kind of jut forward, because you don't expect the sudden lack of power if you're accelerating. Weird sensation.

There is a strange issue with the RPMs, though I don't know if it's related. The RPMs are showing higher than they actually are. And sometimes, the car will not drop the idle all the way down--it might sit at 2,000 rpms for a little bit, then drop to its normal of 1,200 (normal for it registering incorrectly, I mean). When I disconnect the speed sensor cable, it corrects the problem. I changed the speed sensor just to eliminate it as a possibility. Didn't fix it. I think that's all of the oddities.

 

[luigisho]

Lots of weird places to start but I am a simple caveman and would return the starter setup and wiring to stock config. Interesting ideas on where to look from Perry (pjtoledo) in this old post http://www.shoforum.com/index.php?t...un-cant-get-codes-solved.123791/#post-1370683. Maybe some good leads to check out?
Is it possible that it's as simple as there may indeed a small coolant leak and coolant is dripping on the crank sensor?

 

[Black91SHO]

Thanks, man. I'll check out that thread.

Yeah, I had the same idea, to rewire the stock starter back in. I wish I knew why the previous owner decided on the gen 2 starter setup in the first place.

Nah, can't be a coolant leak or we would've seen the coolant all over the crank sensor when replacing it recently. Also, the coolant light randomly comes on, regardless of water level in the overflow, so there's definitely a short somewhere in-line to it and I might be bunched together with the wires causing the crank sensor to flip out. Not entirely sure. This setup all worked fine for a while, but eventually shorted and caused all these problems. What a shame...such a nice car with nearly $10k worth of track mods...

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[Black91SHO]

After looking at that thread, my suspicions are more confirmed...that the wiring harness has the crank sensor and coolant sensor together and they're shorting out, causing the problem. Awesome. Thanks, man. That's a huge help.

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[sperold]

I had a similar experience years ago, but the difference was the chimes would come on after the hic-cup, like you left the key in the ignition and then opened the door. All your other symptoms were the same.
That issue was solved when I took the cladding off the steering column and noticed a burnt spot in the huge connectors that are on the side of the column. I think the adjustable wheel location movement had a role in it.

 

[Black91SHO]

I had a similar experience years ago, but the difference was the chimes would come on after the hic-cup, like you left the key in the ignition and then opened the door. All your other symptoms were the same.
That issue was solved when I took the cladding off the steering column and noticed a burnt spot in the huge connectors that are on the side of the column. I think the adjustable wheel location movement had a role in it.

Interesting. So you were getting the low coolant light turning on randomly, or sometimes turning on and then the engine would drop out right afterward?

 

[sperold]

In my experience, the blip in the engine would occur and a lot of the lights and the chime would come on. The low coolant light would come on as well, but my experience is that light comes on more often than the others even when there are no issues.

I think a lot of power goes through the ignition switch (the light coloured part buried in the column, not the thing with the key sticking out) and that means wires and things get hot, and the switch itself can be an issue.

I would concentrate on the column to start things off, but like I said, it is easy to see as the part will be scorched, and the switch will be discoloured if it is the culprit.

 

[Black91SHO]

Thanks, man. I'll check it out first.

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[rubydist]

I have seen the coolant sensor short out from the junk buildup on the sensor from the crap in the overflow tank. I would start by taking it out and cleaning it carefully and then taking a good look at it to see if there is any obvious damage to the sensor.

 

[Black91SHO]

Yeah, I know the sensor is gunked up a bit. I'll have to clean it off tomorrow. Thanks.

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[black92]

I have a very similar issue. I'll be driving along and the engine just skips a beat from time to time. Lots of good recommendations.

 

[Black91SHO]

It probably is just your crank sensor going out, but it wouldn't hurt to check for other causes, I suppose.

My other show had a seat motor going out but showed no symptoms. It caused a short making the crank sensor to drop out just like what I described with this car. Replaced the seat motors and it fixed it, but I'd already replaced the crank sensor unnecessarily. Sucks when that happens.

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