Hi Everyone,

I just picked up an '85 300TD which needs a lot of love. One of these problems is that the tachometer doesn't work. I thought I traced the problem to a bad tachometer amplifier, however they changed over to the over-voltage protection relay for 1985. I checked the fuse which was good then I tested the OVP with ideas from this guide. With the relay slightly out, I tested the continuity of posts 30 and 87. I didn't get a reading but I did get a 12V at each post. In addition, I took out the relay and put in a jumper wire and still that didn't work. I'm not sure where to go from here. Everyone seems to point to the OVP as culprit. Is there any other symptoms of a bad OVP? I heard the A/C compressor won't engage (which is true), but the rest of my climate control doesn't work either. Any guidance would be greatly appreciated!

If you have access to external 12 vdc you can try and energize the coil of the relay and listen for the contacts. I purchased one from this site, (relay) however it had one too many pins which isn't needed (I think it was the normally open contact) anyway cut it off and plug it in. I installed a ty-wrap around mine to hole it in. That relay does a few of other functions.

Pull the relay identify pins 5 and 6. Put +12 on 6 and gnd on pin 5 if no click its no good.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Tach sensor on the 85 is on the bell housing of the tranny which goes to the egr computer located passenger footwell behind kick-panel before going to the tach in the cluster. Common problen in 85 300D is water coming in through egr harness, corroding connector pins rendering tach and AC (and egr) in-op. You can by pass the egr computer so tach sensor drives the tach in cluster directly by putting in a pair of jumpers in the connector. There is a post with pics showing how it's done. Search and you will find it.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The only valid way to check an OVP is to replace it with a new part. Doesn't matter what the manuals say. To explain: The OVP provides three functions:

- Over current protection provided by the fuse. The fuse can be checked and replaced if necessary, but the construction of the unit is physically very weak, and the fuse contacts tend to break and bend internally. For units with two fuses, the design is idiotic, with the two fuses in parallel. What this means is that if the contacts for one fuse are internally damaged, the other fuse will start to blow...very confusing.

- Relay switching. This can be checked like any other relay, as long as the fuse contacts aren't internally compromised. But it won't tell you whether the OVP is actually providing over voltage protection.

- Over voltage protection. This is the key function of the OVP. It's implemented using a zener diode or an MOV to shunt high voltages to ground. The problem is that the semiconductor device can fail while the relay continues to function, appearing normal for all intents and purposes. There is no way to test the function short of sending a 200V spike through the contacts, which may itself cause the unit to fail. With a bad Zener or MOV the device won't snub a voltage spike, which could cause a big problem.

My advice is if you suspect the OVP, don't over think it. Just get a new one. If it doesn't resolve the problem, keep the old one for spare, you'll eventually need it.

The OVP feeds surge protected power to the engine electronics and the ABS system. It doesn't directly affect HVAC, but the engine electronics usually feeds an engine speed signal to the Klima. Depending on what generation of electronics you have, other downstream systems affected can include EGR, engine idle control and tachometer, all of which require a speed signal. On some cars, the speed signal even affects the outdoor temperature display.

Thanks everyone. I try testing the relay tomorrow and see if it clicks. I'll also study the EGR computer and see if I can bypass it.

I agree with mxfrank just get another ovp relay fixed my non working tach. on my 85.

Well, in an unfortunate turn of events... the car just failed PA inspection because of rotten frame rails.

I wonder if that could be repaired with some level of confedence?

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Actually, yes. After talking with a bunch of people, I'm taking it to my welder this week to have it fixed. So I'm back to figuring out all these wiring gremlins worse than both BMWs I've owned.

To return to the topic of the tach, I fear it might be a bad EGR controller. I was able to test my a/c yesterday and the compressor clicked on with the ovp in place. I also have a grimy intake and rough idle which are two more symptoms. I'm going to finish testing the OVP first to check it off first.

OVP clicked and checked out. I'm trying to troubleshoot the EGR controller now

I'm not sure if you found this discussion but I have it bookmarked for my Cali wagon just in case.

http://www.peachparts.com/shopforum/diesel-discussion/341652-quick-fix-1985-w123-tach-c-after-egr-failure.html

If possible can you take pictures before and after of the frame rust and repair?

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Thanks Voll, I've been using this as my guide. I've tested the RPM pick up at the firewall connector and at the EGR computer and both have healthy continuity and VAC. Later in that thread, it explains to jump the EGR connectors for the tach. I did that but the tach still remains at 0. So I have input directly past the EGR, at least with the jumpers in place. Is there anything between the tach and the EGR controller? Is it safe to assume that the gauge itself is the culprit?

However, here's a contrasting diagnosis: my A/C doesn't work and I just tested the Klima relay. The full diagnosis can be found on this site. I'll just quote the important bit here:

"So, connect 15 to 87 to engage the clutch, +12V between KL and 31 is the signal then turn the switch on without turning the car on. If, the compressor clutch engages either the Klima is bad or its signal inputs are not working properly... one input signal you can test is the Tach signal. The Klima relay is connected to the EGR controller (that's how it gets the RPM information, I suppose)... This probably means that the relay will not work if the EGR Controller has failed; a clue might be loss of tachometer function."

So my compressor clutch engages under the procedure, indicating a faulty Klima relay or bad RPM readings from the EGR controller. Now the EGR controller doesn't have any evidence of water damage. The connectors are in excellent shape. All the other discussions I've read either point to the OVP or the EGR controller and the problem is solved. It doesn't seem I've been as lucky

Sure, I started another thread here: http://www.peachparts.com/shopforum/body-repair/379294-frame-rail-repair-behind-front-tires-85-300td.html#post3616471