RPM/Engine speed sensor

[dcraigk]

I ordered a new sensor because I read that it controls the tach (not working) and the tach controls the AC compressor (on an '85). I would love to see if the AC works because it's been over 100 here.

I tested this connection per the service manual (now I can't find it) and it was not within spec.

So my question is. Where is the other end? Some people say it's on the transmission, others say it's at the front by the crank/vacuum pump. Now I'm not sure I ordered the correct part.

[dcraigk]

I found it. On my car if you look down between the oil filter housing and the valve cover you can see it. Unfortunately the new part did not give me a tach.

[Sev]

1985 300D CA Emissions Tachometer repair

[dcraigk]

I've read that before, thank you. Since I tested and replaced the thing on the transmission and the tach still doesn't work, I'm going to replace the OVP this week and see what happens.

When I got the car 2 weeks ago the fuse was blown on the OVP.

[kuene]

don't want to hijack a thread - but does any of this apply to 82 federal cars? I think my tach amp is fine (I've replaced the original unit already). But I still get intermittent tach operation and I can't figure it out. Doesn't seem to be at all related to the tach op amp. (With the old one, I could improve the situation, at least temporarily, by stuffing little bits of paper towel, et. under the cover to increase pressure).

[David Wilson]

If that tach is like a W124 tach, you might have good luck to replace the capacitors on the tach board. They go bad with time and heat. Cheap fix for me. couple of threads on it

[dcraigk]

I guess I'll have to try that next because a new RPM sensor and new OVP relay didn't fix my tach.

The clock isn't working either. I wonder if they can be related? I need to see if the clock is getting power.

[BillGrissom]

As far as checking the AC compressor, you could try bypassing the Klima relay to actuate the clutch yourself. Most straightforward would be to apply 12 V to the two lugs on the compressor, using insulated spade lugs to be safe. I recall a post showing which pins on the Klima relay (or refer to schematic). Those with earlier cars are easier - just pull the AC relay and jumper terminals 30 to 87 w/ a thick ~12 awg wire. The AC relay is on the driver's inner fender (under black cover). The other relay is for the radiator fan. I label w/ a Sharpie after figuring out which.

Re could a bad tach signal stop AC engaging, I would think not since I think it disables the AC at high rpm so might be OK w/ a 0 rpm reading, but have never tried.

[dcraigk]

I read that a bad OVP will prevent the tach from working and if the tach is showing 0 the AC compressor will not function.

I also tested the RPM sensor per the repair manual. It was not functioning correctly so I replaced that too. All of these are supposed to be related.

I think if I unplug the tach, I should have power at the plug with the engine running.

[Diseasel300]

You've covered everything but the Tach amplifier. If that's gone south, your tach won't work and your Klima relay won't be receiving the RPM pulses. The tach is just a readout, it doesn't have any impact on whether or not the Klima relay receives the RPM signal, that comes from the Tach amp.

Later vehicles with the OM60x engines have the RPM pulses generated in the EDS system instead of a separate tach amp.

[dcraigk]

As I understand, 1985 doesn't have a tach amp.

[mach4]

Originally Posted by dcraigk

As I understand, 1985 doesn't have a tach amp.

True

[Diseasel300]

What replaced the tach amp then? Does the 1985 have EDS? There's some sort of electronic gizmo in there somewhere that takes the pulses from the crankshaft sensor and turns it into something useful.

[funola]

85 does have a tach amp. it is a circuit in the EGR computer. The tach amp output signal on the 85 is totally different than prior years. It's a square wave instead of sine, much higher voltage, and higher frequency. The 85 tach amp could be bypassed with jumpers since the magnetic pickup outputs a 4 V sine wave that is sufficient to drive the tach in the dash.

Your problem may very well be water in the EGR computer connector, which can short out the tach signal and blow the OVP fuse if the corrosion is bad enough around certain pins.

[Diseasel300]

So it is the EGR computer in the '85 that needs to be looked into then. Question for those who know: If the EGR computer is bypassed as suggested above, will the Klima relay still function, or is it looking for that square-wave signal generated by the EGR computer? Important info since the OP wants to get the A/C working