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Had a coolant leak that turned out to be heater hose under the intake manifold. Removed the intake manifold to deal with it, since I was already planning on doing the oil separator anyway.
Went fine, for the most part, although the inline heater valve fitting broke off; I had to put in a piece of copper pipe to get back on the road, while I'm getting a replacement. So, full heat for now, but I can put on vent and close them all.
Apart from that, everything else is fine, but the tach is acting odd. At idle, it sits at zero. I know it didn't used to, although I can't say I specifically looked at it at idle, recently. It seems like it indicates high when I accelerate, but I haven't had it on the highway yet, to see if it levels out at the expected 2700 rpm @ 120 km/hr.
Now, before I started the job, I didn't do what I often see recommended - disconnecting the negative battery terminal. I did have numerous cables disconnected - ECT, injectors, TPS - but never had the ignition on during the job.
One other thing. I found that the jacket for the crank position sensor cable was torn, which I wrapped with electrical tape (didn't seem damaged inside). (Car has been konking out lately; could relate, I suppose). Don't think that this would relate though.
Anyway, I don't see that anything I did, should have affected the tach. I exercised the coil connections, but no change. I've seen suggestions that instrument panel / connection might cause such things, but I don't see why the service I did would bring this on.
thanks,
David
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David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto
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posted by
someone claiming to be rjk
on
Sun Oct 10 07:11 CST 2010 [ RELATED]
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Replace the CPS. When the jacket opens, grime gets inside & things only get worse. Wrapping it with tape may keep new grime out but doesn't clean out what is already in there. You can be assured that coolant leak deposited moisture inside the cable as well. Only a matter of time until it fails.
If that does not help out the tach, then look at the instrument cluster board to tach contacts, they are probably oxidized.
I wouldn't worry about the battery not being disconnected during your earlier repair.
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Dear David,
Hope you're well. You provided the key evidence. The crank sensor needs to be replaced. Once the outer insulation fails, moisture inevitably gets to the wiring and causes: (a) intermittent - and ultimately, total - failure(s) to start and (b) less frequently, aberrant tach readings.
Via the engine computer, the crank sensor sends a signal to the fuel pump, that the engine is turning. Without this signal, the fuel pumps shut down. This is a safety "feature". In an accident that cut a fuel line - but stopped the engine
- the loss of signal from the crank sensor shuts down the fuel pump(s), preventing petrol from being sprayed into the engine bay or elsewhere. This shut-down thus limits fire danger.
Simply replace the crank sensor. Your tach will read correctly and the intermittent start failures will go away.
See the FAQs for a detailed description of how to replace the crank sensor. It is not especially hard or time consuming. If the sensor does not at once come free from the transmission bell housing, DO NOT use brute force on it. Rather, rotate it a tad (10 degrees, either way) to try to work it loose. Do not rock it from side to side: if the sensor snaps, and part drops into the bell housing, the tranny has to be removed, to extract the broken piece.
Hope this helps.
Yours faithfully,
Spook
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Thanks Spook, rjk.
I agree that I should replace the CPS (hell, I've had a new one sitting in a box for years), but I'm still not convinced that this will necessarily cure my tach problem. The CPS connects to the ignition CPU. This in turn drives the power stage, which in turn drives the coil and tach.
The car appears to be otherwise running perfectly. This suggests to me that:
- the signal at the output of the power stage is fine
- at least for now, the CPS is generating an acceptable pulse in order to achieve that acceptable power stage signal
- that this is more a downstream indicator problem
So I tend to lean to suspecting the cluster, etc., but since the CPS definitely needs replacing, when I get ambitious again (these days, this car is eating up my leisure time!), I'll replace the CPS and post back as to whether it resolves the tach issue.
Thanks,
--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto
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Go figure.
Drove the car the next day - tach problem is no longer occurring. Idling above 500 now, as before.
I didn't do anything in the interim, so I have no idea why this would have resolved itself.
Thanks,
--
David Armstrong - '86 240(350k km?), '93 940T(270k km), '89 240(parts source for others) near Toronto
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Dear David,
Hope you're well. I, too, suspect the problem will recur.
The moisture, that shorted the crank sensor's signal, may have evaporated, allowing normal operation to resume.
Your having taped-over the failed insulation has bought you some time. For what it is worth, I don't recall a lot of posts reporting tachometer instrument malfunctions.
I'd get a replacement crank sensor and do the repair. When the sensor fails, the car stops. This could be in your driveway. Or, it could be on a highway, as you're trying to merge into fast-moving traffic.
I'd not take such risk, when the repair part's cost is about $40 and the repair takes perhaps 20 minutes.
Hope this helps.
Yours faithfully,
Spook
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I suspect it is not resolved, only intermittent.
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1999 E320 1994 F150 1989 560 SL 1986 560 SL (deceased) 1988 300 TE (departed) 1994 945T (daughter's) 1988 244DL (fallow) 1986 242Ti (fallow) 1968 GT500KR (under restoration)
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See other reply; finger problems
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